spec.bs

<pre class="metadata">
Title: Observable
Shortname: observable
Repository: WICG/observable
Inline Github Issues: true
Group: WICG
Status: CG-DRAFT
Level: 1
URL: https://wicg.github.io/observable/
Boilerplate: omit conformance, omit feedback-header
Editor: Dominic Farolino, Google https://www.google.com/, domfarolino@gmail.com, https://domfarolino.com
Abstract: The Observable API provides a composable, ergonomic way of handling an asynchronous stream of events
!Participate: <a href="https://github.com/WICG/observable">GitHub WICG/observable</a> (<a href="https://github.com/WICG/observable/issues/new">new issue</a>, <a href="https://github.com/WICG/observable/issues?state=open">open issues</a>)
!Commits: <a href="https://github.com/WICG/observable/commits/master/spec.bs">GitHub spec.bs commits</a>
Complain About: accidental-2119 yes, missing-example-ids yes
Indent: 2
Default Biblio Status: current
Markup Shorthands: markdown yes
Assume Explicit For: yes
WPT Display: open
</pre>

<pre class="link-defaults">
</pre>
<pre class="anchors">
urlPrefix: https://tc39.es/ecma262/#; spec: ECMASCRIPT
  type: dfn
    text: current realm
    text: Object; url: sec-object-type
    text: normal completion; url: sec-completion-record-specification-type
    text: NormalCompletion; url: sec-normalcompletion
    text: throw completion; url: sec-completion-record-specification-type
    text: Iterator Record; url: sec-iterator-records
    url: sec-returnifabrupt-shorthands
      text: ?
      text: !
  type: abstract-op
    text: Type; url: sec-ecmascript-data-types-and-values
urlPrefix: https://dom.spec.whatwg.org; spec: DOM
  type: dfn
    for: event listener
      text: type; url: event-listener-type
      text: capture; url: event-listener-capture
      text: passive; url: event-listener-passive
      text: once; url: event-listener-once
      text: signal; url: event-listener-signal
    for: AbortSignal
      text: dependent signals; url: abortsignal-dependent-signals
      text: signal abort; url:abortsignal-signal-abort
      text: abort reason; url:abortsignal-abort-reason
urlPrefix: https://webidl.spec.whatwg.org; spec: WEBIDL
  type: dfn
    text: a promise rejected with
</pre>

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<h2 id=introduction>Introduction</h2>

*This section is non-normative.*

<h2 id=core-infrastructure>Core infrastructure</h2>

<h3 id=subscriber-api>The {{Subscriber}} interface</h3>

<xmp class=idl>
[Exposed=*]
interface Subscriber {
  undefined next(any value);
  undefined error(any error);
  undefined complete();
  undefined addTeardown(VoidFunction teardown);

  // True after the Subscriber is created, up until either
  // complete()/error() are invoked, or the subscriber unsubscribes. Inside
  // complete()/error(), this attribute is true.
  readonly attribute boolean active;

  readonly attribute AbortSignal signal;
};
</xmp>

Each {{Subscriber}} has an [=ordered set=] of <dfn for=Subscriber>internal observers</dfn>,
initially empty.

Each {{Subscriber}} has a <dfn for=Subscriber>teardown callbacks</dfn>, which is a [=list=] of
{{VoidFunction}}s, initially empty.

Each {{Subscriber}} has a <dfn for=Subscriber>subscription controller</dfn>, which is an
{{AbortController}}.

Each {{Subscriber}} has a <dfn for=Subscriber>active</dfn> boolean, initially true.

Note: This is a bookkeeping variable to ensure that a {{Subscriber}} never calls any of the
callbacks it owns after it has been [=close a subscription|closed=].

The <dfn attribute for=Subscriber><code>active</code></dfn> getter steps are to return [=this=]'s
[=Subscriber/active=] boolean.

The <dfn attribute for=Subscriber><code>signal</code></dfn> getter steps are to return [=this=]'s
[=Subscriber/subscription controller=]'s [=AbortController/signal=].

<div algorithm>
  The <dfn for=Subscriber method><code>next(|value|)</code></dfn> method steps are:

    1. If [=this=]'s [=Subscriber/active=] is false, then return.

    1. If [=this=]'s [=relevant global object=] is a {{Window}} object, and its [=associated
       Document=] is not [=Document/fully active=], then return.

    1. [=set/For each=] |observer| of [=this=]'s [=Subscriber/internal observers=]:

       1. Run |observer|'s [=internal observer/next steps=] given |value|.

          [=Assert=]: No <a spec=webidl lt="an exception was thrown">exception was thrown</a>.

          <div class=note>
            <p>Note: No exception can be thrown here because in the case where the
            [=Subscriber/internal observer=]'s [=internal observer/next steps=] is just a wrapper
            around a script-provided callback, the <a href=#process-observer>process observer</a>
            steps take care to wrap these callbacks in logic that, when invoking them, catches any
            exceptions, and reports them to the global.</p>

            <p>When the [=internal observer/next steps=] is a spec algorithm, those steps take care
            to not throw any exceptions outside of itself, to appease this assert.</p>
          </div>
</div>

<div algorithm>
  The <dfn for=Subscriber method><code>error(|error|)</code></dfn> method steps are:

    1. If [=this=]'s [=Subscriber/active=] is false, [=report the exception=] |error|, then return.

    1. If [=this=]'s [=relevant global object=] is a {{Window}} object, and its [=associated
       Document=] is not [=Document/fully active=], then return.

    1. [=close a subscription|Close=] [=this=].

    1. [=set/For each=] |observer| of [=this=]'s [=Subscriber/internal observers=]:

       1. Run |observer|'s [=internal observer/error steps=] given |error|.

          [=Assert=]: No <a spec=webidl lt="an exception was thrown">exception was thrown</a>.

          Note: See the documentation in {{Subscriber/next()}} for details on why this is true.
</div>

<div algorithm>
  The <dfn for=Subscriber method><code>complete()</code></dfn> method steps are:

    1. If [=this=]'s [=Subscriber/active=] is false, then return.

    1. If [=this=]'s [=relevant global object=] is a {{Window}} object, and its [=associated
       Document=] is not [=Document/fully active=], then return.

    1. [=close a subscription|Close=] [=this=].

    1. [=set/For each=] |observer| of [=this=]'s [=Subscriber/internal observers=]:

       1. Run |observer|'s [=internal observer/complete steps=].

          [=Assert=]: No <a spec=webidl lt="an exception was thrown">exception was thrown</a>.

          Note: See the documentation in {{Subscriber/next()}} for details on why this is true.
</div>

<div algorithm>
  The <dfn for=Subscriber method><code>addTeardown(|teardown|)</code></dfn> method steps are:

    1. If [=this=]'s [=relevant global object=] is a {{Window}} object, and its [=associated
       Document=] is not [=Document/fully active=], then return.

    1. If [=this=]'s [=Subscriber/active=] is true, then [=list/append=] |teardown| to [=this=]'s
       [=Subscriber/teardown callbacks=] list.

    1. Otherwise, [=invoke=] |teardown|.

       If <a spec=webidl lt="an exception was thrown">an exception |E| was thrown</a>, then [=report
       the exception=] |E|.
</div>

<div algorithm>
  To <dfn>close a subscription</dfn> given a {{Subscriber}} |subscriber|, and an optional {{any}}
  |reason|, run these steps:

    1. If |subscriber|'s [=Subscriber/active=] is false, then return.

       <div class=note>
         <p>This guards against re-entrant invocation, which can happen in the "producer-initiated"
         unsubscription case. Consider the following example:</p>
         <div class=example id=re-entrant-close>
           <pre highlight=js>
const outerController = new AbortController();
const observable = new Observable(subscriber =&gt; {
  subscriber.addTeardown(() =&gt; {
    // 2.) This teardown executes inside the "Close" algorithm, while it's
    //     running. Aborting the downstream signal run its abort algorithms,
    //     one of which is the currently-running "Close" algorithm.
    outerController.abort();
  });

  // 1.) This immediately invokes the "Close" algorithm, which
  //     sets subscriber.active to false.
  subscriber.complete();
});

observable.subscribe({}, {signal: outerController.signal});
           </pre>
         </div>
       </div>

    1. Set |subscriber|'s [=Subscriber/active=] boolean to false.

    1. [=AbortSignal/Signal abort=] |subscriber|'s [=Subscriber/subscription controller=] with
       |reason|, if it is given.

    1. [=list/For each=] |teardown| of |subscriber|'s [=Subscriber/teardown callbacks=] sorted in
       reverse insertion order:

       1. If |subscriber|'s [=relevant global object=] is a {{Window}} object, and its [=associated
          Document=] is not [=Document/fully active=], then abort these steps.

          Note: This step runs repeatedly because each |teardown| could result in the above
          {{Document}} becoming inactive.

       1. [=Invoke=] |teardown|.

          If <a spec=webidl lt="an exception was thrown">an exception |E| was thrown</a>, then
          [=report the exception=] |E|.
</div>

<h3 id=observable-api>The {{Observable}} interface</h3>

<xmp class=idl>
// SubscribeCallback is where the Observable "creator's" code lives. It's
// called when subscribe() is called, to set up a new subscription.
callback SubscribeCallback = undefined (Subscriber subscriber);
callback ObservableSubscriptionCallback = undefined (any value);

dictionary SubscriptionObserver {
  ObservableSubscriptionCallback next;
  ObservableSubscriptionCallback error;
  VoidFunction complete;
};

callback ObservableInspectorAbortHandler = undefined (any value);

dictionary ObservableInspector {
  ObservableSubscriptionCallback next;
  ObservableSubscriptionCallback error;
  VoidFunction complete;

  VoidFunction subscribe;
  ObservableInspectorAbortHandler abort;
};

typedef (ObservableSubscriptionCallback or SubscriptionObserver) ObserverUnion;
typedef (ObservableSubscriptionCallback or ObservableInspector) ObservableInspectorUnion;

dictionary SubscribeOptions {
  AbortSignal signal;
};

callback Predicate = boolean (any value, unsigned long long index);
callback Reducer = any (any accumulator, any currentValue, unsigned long long index);
callback Mapper = any (any value, unsigned long long index);
// Differs from Mapper only in return type, since this callback is exclusively
// used to visit each element in a sequence, not transform it.
callback Visitor = undefined (any value, unsigned long long index);

// This callback returns an `any` that must convert into an `Observable`, via
// the `Observable` conversion semantics.
callback CatchCallback = any (any value);

[Exposed=*]
interface Observable {
  constructor(SubscribeCallback callback);
  undefined subscribe(optional ObserverUnion observer = {}, optional SubscribeOptions options = {});

  // Constructs a native Observable from value if it's any of the following:
  //   - Observable
  //   - AsyncIterable
  //   - Iterable
  //   - Promise
  static Observable from(any value);

  // Observable-returning operators. See "Operators" section in the spec.
  //
  // takeUntil() can consume promises, iterables, async iterables, and other
  // observables.
  Observable takeUntil(any value);
  Observable map(Mapper mapper);
  Observable filter(Predicate predicate);
  Observable take(unsigned long long amount);
  Observable drop(unsigned long long amount);
  Observable flatMap(Mapper mapper);
  Observable switchMap(Mapper mapper);
  Observable inspect(optional ObservableInspectorUnion inspectorUnion = {});
  Observable catch(CatchCallback callback);
  Observable finally(VoidFunction callback);

  // Promise-returning operators.
  Promise<sequence<any>> toArray(optional SubscribeOptions options = {});
  Promise<undefined> forEach(Visitor callback, optional SubscribeOptions options = {});
  Promise<boolean> every(Predicate predicate, optional SubscribeOptions options = {});
  Promise<any> first(optional SubscribeOptions options = {});
  Promise<any> last(optional SubscribeOptions options = {});
  Promise<any> find(Predicate predicate, optional SubscribeOptions options = {});
  Promise<boolean> some(Predicate predicate, optional SubscribeOptions options = {});
  Promise<any> reduce(Reducer reducer, optional any initialValue, optional SubscribeOptions options = {});
};
</xmp>

Each {{Observable}} has a <dfn for=Observable>subscribe callback</dfn>, which is a
{{SubscribeCallback}} or a set of steps that take in a {{Subscriber}}.

Each {{Observable}} has a <dfn for=Observable>weak subscriber</dfn>, which is a weak reference to a
{{Subscriber}}-or-null, initially null.

Note: The "union" of these types is to support both {{Observable}}s created by JavaScript (that are
always constructed with a {{SubscribeCallback}}), and natively-constructed {{Observable}} objects
(whose [=Observable/subscribe callback=] could be an arbitrary set of native steps, not a JavaScript
callback). The return value of {{EventTarget/when()}} is an example of the latter.

<div algorithm>
  The <dfn for=Observable constructor lt="Observable(callback)"><code>new
  Observable(|callback|)</code></dfn> constructor steps are:

    1. Set [=this=]'s [=Observable/subscribe callback=] to |callback|.

      Note: This callback will get invoked later when {{Observable/subscribe()}} is called.
</div>

<div algorithm>
  The <dfn for=Observable method><code>subscribe(|observer|, |options|)</code></dfn> method steps
  are:

    1. <a for=Observable lt="subscribe to an Observable">Subscribe</a> to [=this=] given |observer|
       and |options|.
</div>

<h4 id=observable-supporting-concepts>Supporting concepts</h4>

<div algorithm>
  The <dfn>default error algorithm</dfn> is an algorithm that takes an {{any}} |error|, and runs
  these steps:

    1. [=Report the exception=] |error|.

  Note: We pull this default out separately so that every place in this specification that natively
  <a for=Observable lt="subscribe to an Observable">subscribes</a> to an {{Observable}} (i.e.,
  subscribes from spec prose, not going through the {{Observable/subscribe()}} method) doesn't have
  to redundantly define these steps.
</div>

An <dfn>internal observer</dfn> is a [=struct=] with the following [=struct/items=]:

<dl dfn-for="internal observer">
  : <dfn>next steps</dfn>
  :: An algorithm that takes a single parameter of type {{any}}. Initially, these steps do nothing.

  : <dfn>error steps</dfn>
  :: An algorithm that takes a single parameter of type {{any}}. Initially, the [=default error
     algorithm=].

  : <dfn>complete steps</dfn>
  :: An algorithm with no parameters. Initially, these steps do nothing.
</dl>

<div class=note>
  <p>The [=internal observer=] [=struct=] is used to mirror the {{SubscriptionObserver/next}},
  {{SubscriptionObserver/error}}, and {{SubscriptionObserver/complete}} [=callback functions=]. For
  any {{Observable}} that is subscribed by JavaScript via the {{Observable/subscribe()}} method,
  these algorithm "steps" will just be a wrapper around [=invoking=] the corresponding
  {{SubscriptionObserver/next}}, {{SubscriptionObserver/error}}, and
  {{SubscriptionObserver/complete}} [=callback functions=] provided by script.</p>

  <p>But when internal spec prose (not user script) <a for=Observable lt="subscribe to an
  Observable">subscribes</a> to an {{Observable}}, these "steps" are arbitrary spec algorithms that
  are not provided via an {{ObserverUnion}} packed with Web IDL [=callback functions=]. See the
  [[#promise-returning-operators]] that make use of this, for example.</p>
</div>

<div algorithm>
  To <dfn for=Observable>convert to an Observable</dfn> an {{any}} |value|, run these steps:

  Note: We split this algorithm out from the Web IDL {{Observable/from()}} method, so that spec
  prose can <a for=Observable lt="convert to an observable">convert</a> values to without going
  through the Web IDL bindings.

    1. If [$Type$](|value|) is not [=Object=], [=exception/throw=] a {{TypeError}}.

       Note: This prevents primitive types from being coerced into iterables (e.g., String). See
       discussion in <a href=https://github.com/WICG/observable/issues/125>WICG/observable#125</a>.

    1. <i id=from-observable-conversion><b>From Observable</b></i>: If |value|'s [=specific type=]
       is an {{Observable}}, then return |value|.

    1. <i id=from-async-iterable-conversion><b>From async iterable</b></i>: Let
       |asyncIteratorMethod| be [=?=] [$GetMethod$](|value|, {{%Symbol.asyncIterator%}}).

       Note: We use [$GetMethod$] instead of [$GetIterator$] because we're only probing for async
       iterator protocol support, and we don't want to throw if it's not implemented.
       [$GetIterator$] throws errors in BOTH of the following cases: (a) no iterator protocol is
       implemented, (b) an iterator protocol is implemented, but isn't callable or its getter
       throws. [$GetMethod$] lets us ONLY throw in the latter case.

    1. If |asyncIteratorMethod|'s is undefined or null, then jump to the step labeled <a
       href=#from-iterable-conversion>From iterable</a>.

    1. Let |nextAlgorithm| be the following steps, given a {{Subscriber}} |subscriber| and an
       [=Iterator Record=] |iteratorRecord|:

         1. If |subscriber|'s [=Subscriber/subscription controller=]'s [=AbortController/signal=] is
            [=AbortSignal/aborted=], then return.

         1. Let |nextPromise| be a {{Promise}}-or-undefined, initially undefined.

         1. Let |nextCompletion| be [$IteratorNext$](|iteratorRecord|).

            Note: We use [$IteratorNext$] here instead of [$IteratorStepValue$], because
            [$IteratorStepValue$] expects the iterator's `next()` method to return an object that
            can immediately be inspected for a value, whereas in the async iterator case, `next()`
            is expected to return a Promise/thenable (which we wrap in a Promise and react to to get
            that value).

         1. If |nextCompletion| is a [=throw completion=], then:

            1. [=Assert=]: |iteratorRecord|'s \[[Done]] is true.

            1. Set |nextPromise| to [=a promise rejected with=] |nextRecord|'s \[[Value]].

         1. Otherwise, if |nextRecord| is [=normal completion=], then set |nextPromise| to [=a
            promise resolved with=] |nextRecord|'s \[[Value]].

            Note: This is done in case |nextRecord|'s \[[Value]] is not *itself* already a
            {{Promise}}.

         1. [=promise/React=] to |nextPromise|:

             * If |nextPromise| was fulfilled with value |iteratorResult|, then:

               1. If [$Type$](|iteratorResult|) is not Object, then run |subscriber|'s
                  {{Subscriber/error()}} method with a {{TypeError}} and abort these steps.

               1. Let |done| be [$IteratorComplete$](|iteratorResult|).

               1. If |done| is a [=throw completion=], then run |subscriber|'s
                  {{Subscriber/error()}} method with |done|'s \[[Value]] and abort these steps.

               1. If |done|'s \[[Value]] is true, then run |subscriber|'s {{Subscriber/complete()}}
                  and abort these steps.

               1. Let |value| be [$IteratorValue$](|iteratorResult|).

               1. If |value| is a [=throw completion=], then run |subscriber|'s
                  {{Subscriber/error()}} method with |value|'s \[[Value]] and abort these steps.

               1. Run |subscriber|'s {{Subscriber/next()}} given |value|'s \[[Value]].

               1. Run |nextAlgorithm| given |subscriber| and |iteratorRecord|.

             * If |nextPromise| was rejected with reason |r|, then run |subscriber|'s
               {{Subscriber/error()}} method given |r|.

    1. Return a [=new=] {{Observable}} whose [=Observable/subscribe callback=] is an algorithm that
       takes a {{Subscriber}} |subscriber| and does the following:

       1. If |subscriber|'s [=Subscriber/subscription controller=]'s [=AbortController/signal=] is
          [=AbortSignal/aborted=], then return.

       1. Let |iteratorRecordCompletion| be [$GetIterator$](|value|, async).

          Note: This both re-invokes any {{%Symbol.asyncIterator%}} method getters on |value|—note
          that whether this is desirable is an extreme corner case, but it matches test
          expectations; see <a href=https://github.com/WICG/observable/issues/127>issue#127</a> for
          discussion—and invokes the protocol itself to obtain an [=Iterator Record=].

       1. If |iteratorRecordCompletion| is a [=throw completion=], then run |subscriber|'s
          {{Subscriber/error()}} method with |iteratorRecordCompletion|'s \[[Value]] and abort these
          steps.

          Note: This means we invoke the {{Subscriber/error()}} method synchronously with respect to
          subscription, which is the only time this can happen for async iterables that are
          converted to {{Observable}}s. In all other cases, errors are propagated to the observer
          asynchronously, with microtask timing, by virtue of being wrapped in a rejected
          {{Promise}} that |nextAlgorithm| [=promise/reacts=] to. This synchronous-error-propagation
          behavior is consistent with language constructs, i.e., **for-await of** loops that invoke
          {{%Symbol.asyncIterator%}} and synchronously re-throw exceptions to catch blocks outside
          the loop, before any [$Await|Awaiting$] takes place.

       1. Let |iteratorRecord| be [=!=] |iteratorRecordCompletion|.

       1. [=Assert=]: |iteratorRecord| is an [=Iterator Record=].

       1. If |subscriber|'s [=Subscriber/subscription controller=]'s [=AbortController/signal=] is
          [=AbortSignal/aborted=], then return.

       1. [=AbortSignal/add|Add the following abort algorithm=] to |subscriber|'s
          [=Subscriber/subscription controller=]'s [=AbortController/signal=]:

          1. Run [$AsyncIteratorClose$](|iteratorRecord|, [=NormalCompletion=](|subscriber|'s
             [=Subscriber/subscription controller=]'s [=AbortSignal/abort reason=])).

       1. Run |nextAlgorithm| given |subscriber| and |iteratorRecord|.

    1. <i id=from-iterable-conversion><b>From iterable</b></i>: Let |iteratorMethod| be [=?=]
       [$GetMethod$](|value|, {{%Symbol.iterator%}}).

    1. If |iteratorMethod| is undefined, then jump to the step labeled <a
       href=#from-promise-conversion>From Promise</a>.

       Otherwise, return a [=new=] {{Observable}} whose [=Observable/subscribe callback=] is an
       algorithm that takes a {{Subscriber}} |subscriber| and does the following:

         1. If |subscriber|'s [=Subscriber/subscription controller=]'s [=AbortController/signal=] is
            [=AbortSignal/aborted=], then return.

         1. Let |iteratorRecordCompletion| be [$GetIterator$](|value|, sync).

         1. If |iteratorRecordCompletion| is a [=throw completion=], then run |subscriber|'s
            {{Subscriber/error()}} method, given |iteratorRecordCompletion|'s \[[Value]], and abort
            these steps.

         1. Let |iteratorRecord| be [=!=] |iteratorRecordCompletion|.

         1. If |subscriber|'s [=Subscriber/subscription controller=]'s [=AbortController/signal=] is
            [=AbortSignal/aborted=], then return.

         1. [=AbortSignal/add|Add the following abort algorithm=] to |subscriber|'s
            [=Subscriber/subscription controller=]'s [=AbortController/signal=]:

            1. Run [$IteratorClose$](|iteratorRecord|, [=NormalCompletion=](UNUSED)).

         1. [=iteration/While=] true:

            1. Let |next| be [$IteratorStepValue$](|iteratorRecord|).

            1. If |next| is a [=throw completion=], then run |subscriber|'s {{Subscriber/error()}}
               method, given |next|'s \[[Value]], and [=iteration/break=].

            1. Set |next| to [=!=] to |next|.

            1. If |next| is done, then:

               1. [=Assert=]: |iteratorRecord|'s \[[Done]] is true.

               2. Run |subscriber|'s {{Subscriber/complete()}}.

               3. Return.

            1. Run |subscriber|'s {{Subscriber/next()}} given |next|.

            1. If |subscriber|'s [=Subscriber/subscription controller=]'s [=AbortController/signal=]
               is [=AbortSignal/aborted=], then [=iteration/break=].

    1. <i id=from-promise-conversion><b>From Promise</b></i>: If [$IsPromise$](|value|) is true,
       then:

       1. Return a [=new=] {{Observable}} whose [=Observable/subscribe callback=] is an algorithm
          that takes a {{Subscriber}} |subscriber| and does the following:

          1. [=promise/React=] to |value|:

             1. If |value| was fulfilled with value |v|, then:

                1. Run |subscriber|'s {{Subscriber/next()}} method, given |v|.

                1. Run |subscriber|'s {{Subscriber/complete()}} method.

             1. If |value| was rejected with reason |r|, then run |subscriber|'s
                {{Subscriber/error()}} method, given |r|.

    1. [=exception/Throw=] a {{TypeError}}.

<wpt>
  /dom/observable/tentative/observable-from.any.js
</wpt>
</div>

<div algorithm>
  To <dfn for=Observable>subscribe to an {{Observable}}</dfn> given an
  {{ObserverUnion}}-or-[=internal observer=] |observer|, and a {{SubscribeOptions}} |options|, run
  these steps:

  Note: We split this algorithm out from the Web IDL {{Observable/subscribe()}} method, so that spec
  prose can <a for=Observable lt="subscribe to an Observable">subscribe</a> to an {{Observable}}
  without going through the Web IDL bindings. See <a
  href=https://github.com/w3c/IntersectionObserver/issues/464>w3c/IntersectionObserver#464</a> for
  similar context, where "internal" prose <span class=allow-2119>must</span> not go through Web IDL
  bindings on objects whose properties could be mutated by JavaScript. See
  [[#promise-returning-operators]] for usage of this.

    1. If [=this=]'s [=relevant global object=] is a {{Window}} object, and its [=associated
       Document=] is not [=Document/fully active=], then return.

    1. Let |internal observer| be a new [=internal observer=].

    1. Process |observer| as follows:
       <ol id=process-observer>
         <li>
           <dl class="switch">
             <dt>If |observer| is an {{ObservableSubscriptionCallback}}</dt>
             <dd>Set |internal observer|'s [=internal observer/next steps=] to these steps that take
             an {{any}} |value|:

                 1. [=Invoke=] |observer| with |value|.

                    If <a spec=webidl lt="an exception was thrown">an exception |E| was thrown</a>,
                    then [=report the exception=] |E|.
             </dd>

             <dt>If |observer| is a {{SubscriptionObserver}}</dt>
             <dd>
               1. If |observer|'s {{SubscriptionObserver/next}} [=map/exists=], then set |internal
                  observer|'s [=internal observer/next steps=] to these steps that take an {{any}}
                  |value|:

                  1. [=Invoke=] |observer|'s {{SubscriptionObserver/next}} with |value|.

                     If <a spec=webidl lt="an exception was thrown">an exception |E| was thrown</a>,
                     then [=report the exception=] |E|.

               1. If |observer|'s {{SubscriptionObserver/error}} [=map/exists=], then set |internal
                  observer|'s [=internal observer/error steps=] to these steps that take an {{any}}
                  |error|:

                  1. [=Invoke=] |observer|'s {{SubscriptionObserver/error}} with |error|.

                     If <a spec=webidl lt="an exception was thrown">an exception |E| was thrown</a>,
                     then [=report the exception=] |E|.

               1. If |observer|'s {{SubscriptionObserver/complete}} [=map/exists=], then set
                  |internal observer|'s [=internal observer/complete steps=] to these steps:

                  1. [=Invoke=] |observer|'s {{SubscriptionObserver/complete}}.

                     If <a spec=webidl lt="an exception was thrown">an exception |E| was thrown</a>,
                     then [=report the exception=] |E|.
             </dd>

             <dt>If |observer| is an [=internal observer=]</dt>
             <dd>Set |internal observer| to |observer|.</dd>
           </dl>
         </li>
       </ol>

    1. [=Assert=]: |internal observer|'s [=internal observer/error steps=] is either the [=default
       error algorithm=], or an algorithm that [=invokes=] the provided
       {{SubscriptionObserver/error}} [=callback function=].

    1. If [=this=]'s [=Observable/weak subscriber=] is not null and [=this=]'s [=Observable/weak
       subscriber=]'s [=Subscriber/active=] is true:

       1. Let |subscriber| be [=this=]'s [=Observable/weak subscriber=].

       1. [=set/Append=] |internal observer| to |subscriber|'s [=Subscriber/internal observers=].

       1. If |options|'s {{SubscribeOptions/signal}} [=map/exists=], then:

          1. If |options|'s {{SubscribeOptions/signal}} is [=AbortSignal/aborted=], then
             [=set/remove=] |internal observer| from |subscriber|'s [=Subscriber/internal
             observers=].

          1. Otherwise, [=AbortSignal/add|add the following abort algorithm=] to |options|'s
             {{SubscribeOptions/signal}}:

             1. If |subscriber|'s [=Subscriber/active=] is false, then abort these steps.

             1. [=set/Remove=] |internal observer| from |subscriber|'s [=Subscriber/internal
                observers=].

             1. If |subscriber|'s [=Subscriber/internal observers=] is [=set/empty=], then [=close a
                subscription|close=] |subscriber| with |options|'s {{SubscribeOptions/signal}}'s
                [=AbortSignal/abort reason=].

       1. Return.

    1. Let |subscriber| be a [=new=] {{Subscriber}}.

    1. [=set/Append=] |internal observer| to |subscriber|'s [=Subscriber/internal observers=].

    1. Set [=this=]'s [=Observable/weak subscriber=] to |subscriber|.

    1. If |options|'s {{SubscribeOptions/signal}} [=map/exists=], then:

       1. If |options|'s {{SubscribeOptions/signal}} is [=AbortSignal/aborted=], then [=close a
          subscription|close=] |subscriber| given |options|'s {{SubscribeOptions/signal}}
          [=AbortSignal/abort reason=].

       1. Otherwise, [=AbortSignal/add|add the following abort algorithm=] to |options|'s
          {{SubscribeOptions/signal}}:

          1. If |subscriber|'s [=Subscriber/active=] is false, then abort these steps.

          1. [=set/Remove=] |internal observer| from |subscriber|'s [=Subscriber/internal
             observers=].

          1. If |subscriber|'s [=Subscriber/internal observers=] is [=set/empty=], then [=close a
             subscription|close=] |subscriber| with |options|'s {{SubscribeOptions/signal}}'s
             [=AbortSignal/abort reason=].

    1. If [=this=]'s [=Observable/subscribe callback=] is a {{SubscribeCallback}}, [=invoke=] it
       with |subscriber|.

       If <a spec=webidl lt="an exception was thrown">an exception |E| was thrown</a>, call
       |subscriber|'s {{Subscriber/error()}} method with |E|.

    1. Otherwise, run the steps given by [=this=]'s [=Observable/subscribe callback=], given
       |subscriber|.
</div>

<wpt>
  /dom/observable/tentative/observable-constructor.any.js
  /dom/observable/tentative/observable-constructor.window.js
</wpt>


<h3 id=operators>Operators</h3>

For now, see
[https://github.com/wicg/observable#operators](https://github.com/wicg/observable#operators).

<h4 id=observable-from>{{Observable/from()}}</h4>

<div algorithm>
  The <dfn for=Observable method><code>from(|value|)</code></dfn> method steps are:

    1. Return the result of <a for=Observable lt="convert to an Observable">converting</a> |value|
       to an {{Observable}}. Rethrow any exceptions.
</div>

<h4 id=observable-returning-operators>{{Observable}}-returning operators</h4>

<div algorithm>
  The <dfn for=Observable method><code>takeUntil(|value|)</code></dfn> method steps are:

    1. Let |sourceObservable| be [=this=].

    1. Let |notifier| be the result of <a for=Observable lt="convert to an Observable">
       converting</a> |value| to an {{Observable}}.

    1. Let |observable| be a [=new=] {{Observable}} whose [=Observable/subscribe callback=] is an
       algorithm that takes a {{Subscriber}} |subscriber| and does the following:

       <div class=note>
         Note that this method involves <a for=Observable lt="subscribe to an
         Observable">Subscribing</a> to two {{Observable}}s: (1) |notifier|, and (2)
         |sourceObservable|. We "unsubscribe" from **both** of them in the following situations:

           1. |notifier| starts emitting values (either "next" or "error"). In this case, we
              unsubscribe from |notifier| since we got all we need from it, and no longer need it to
              keep producing values. We also unsubscribe from |sourceObservable|, because it no
              longer needs to produce values that get plumbed through this method's returned
              |observable|, because we're manually ending the subscription to |observable|, since
              |notifier| finally produced a value.

           1. |sourceObservable| either {{Subscriber/error()}}s or {{Subscriber/complete()}}s
              itself. In this case, we unsubscribe from |notifier| since we no longer need to listen
              for values it emits in order to determine when |observable| can stop mirroring values
              from |sourceObservable| (since |sourceObservable| ran to completion by itself).
              Unsubscribing from |sourceObservable| isn't necessary, since its subscription has been
              exhausted by itself.
       </div>

       1. Let |notifierObserver| be a new [=internal observer=], initialized as follows:

          : [=internal observer/next steps=]
          :: Run |subscriber|'s {{Subscriber/complete()}} method.

             Note: This will "unsubscribe" from |sourceObservable|, if it has been subscribed to by
             this point. This is because |sourceObservable| is subscribed to with the "outer"
             |subscriber|'s [=Subscriber/subscription controller=]'s [=AbortController/signal=] as
             an input signal, and that signal will get [=AbortSignal/signal abort|aborted=] when the
             "outer" |subscriber|'s {{Subscriber/complete()}} is called above (and below).

          : [=internal observer/error steps=]
          :: Run |subscriber|'s {{Subscriber/complete()}} method.

          Note: We do not specify [=internal observer/complete steps=], because if the |notifier|
          {{Observable}} completes itself, we do not need to complete the |subscriber| associated
          with the |observable| returned from this method. Rather, the |observable| will continue to
          mirror |sourceObservable| uninterrupted.

       1. Let |options| be a new {{SubscribeOptions}} whose {{SubscribeOptions/signal}} is
          |subscriber|'s [=Subscriber/subscription controller=]'s [=AbortController/signal=].

       1. <a for=Observable lt="subscribe to an Observable">Subscribe</a> to |notifier| given
          |notifierObserver| and |options|.

       1. If |subscriber|'s [=Subscriber/active=] is false, then return.

          Note: This means that |sourceObservable|'s [=Observable/subscribe callback=] will not even
          get invoked once, if |notifier| synchronously emits a value. If |notifier| only
          "completes" synchronously though (without emitting a "next" or "error" value), then
          |subscriber|'s [=Subscriber/active=] will still be true, and we proceed to subscribe to
          |sourceObservable|, which |observable| will mirror uninterrupted.

       1. Let |sourceObserver| be a new [=internal observer=], initialized as follows:

          : [=internal observer/next steps=]
          :: Run |subscriber|'s {{Subscriber/next()}} method, given the passed in <var
             ignore>value</var>.

          : [=internal observer/error steps=]
          :: Run |subscriber|'s {{Subscriber/error()}} method, given the passed in <var
             ignore>error</var>.

          : [=internal observer/complete steps=]
          :: Run |subscriber|'s {{Subscriber/complete()}} method.

          Note: |sourceObserver| is mostly a pass-through, mirroring everything that
          |sourceObservable| emits, with the exception of having the ability to unsubscribe from the
          |notifier| {{Observable}} in the case where |sourceObservable| is exhausted before
          |notifier| emits anything.

       1. <a for=Observable lt="subscribe to an Observable">Subscribe</a> to |sourceObservable|
          given |sourceObserver| and |options|.

    1. Return |observable|.

  <wpt>
    /dom/observable/tentative/observable-takeUntil.any.js
    /dom/observable/tentative/observable-takeUntil.window.js
  </wpt>
</div>

<div algorithm>
  The <dfn for=Observable method><code>map(|mapper|)</code></dfn> method steps are:

    1. Let |sourceObservable| be [=this=].

    1. Let |observable| be a [=new=] {{Observable}} whose [=Observable/subscribe callback=] is an
       algorithm that takes a {{Subscriber}} |subscriber| and does the following:

       1. Let |idx| be an {{unsigned long long}}, initially 0.

       1. Let |sourceObserver| be a new [=internal observer=], initialized as follows:

          : [=internal observer/next steps=]
          :: 1. [=Invoke=] |mapper| with the passed in <var ignore>value</var>, and |idx|, and let
                |mappedValue| be the returned value.

                If <a spec=webidl lt="an exception was thrown">an exception |E| was thrown</a>, then
                run |subscriber|'s {{Subscriber/error()}} method, given |E|, and abort these steps.

             1. Increment |idx|.

             1. Run |subscriber|'s {{Subscriber/next()}} method, given |mappedValue|.

          : [=internal observer/error steps=]
          :: Run |subscriber|'s {{Subscriber/error()}} method, given the passed in <var
             ignore>error</var>.

          : [=internal observer/complete steps=]
          :: Run |subscriber|'s {{Subscriber/complete()}} method.

       1. Let |options| be a new {{SubscribeOptions}} whose {{SubscribeOptions/signal}} is
          |subscriber|'s [=Subscriber/subscription controller=]'s [=AbortController/signal=].

       1. <a for=Observable lt="subscribe to an Observable">Subscribe</a> to |sourceObservable|
          given |sourceObserver| and |options|.

    1. Return |observable|.

  <wpt>
    /dom/observable/tentative/observable-map.any.js
    /dom/observable/tentative/observable-map.window.js
  </wpt>
</div>

<div algorithm>
  The <dfn for=Observable method><code>filter(|predicate|)</code></dfn> method steps are:

    1. Let |sourceObservable| be [=this=].

    1. Let |observable| be a [=new=] {{Observable}} whose [=Observable/subscribe callback=] is an
       algorithm that takes a {{Subscriber}} |subscriber| and does the following:

       1. Let |idx| be an {{unsigned long long}}, initially 0.

       1. Let |sourceObserver| be a new [=internal observer=], initialized as follows:

          : [=internal observer/next steps=]
          :: 1. [=Invoke=] |predicate| with the passed in |value| and |idx|, and let |matches| be
                the returned value.

                If <a spec=webidl lt="an exception was thrown">an exception |E| was thrown</a>, then
                run |subscriber|'s {{Subscriber/error()}} method, given |E|, and abort these steps.

             1. Set |idx| to |idx| + 1.

             1. If |matches| is true, then run |subscriber|'s {{Subscriber/next()}} method, given
                |value|.

          : [=internal observer/error steps=]
          :: Run |subscriber|'s {{Subscriber/error()}} method, given the passed in <var
             ignore>error</var>.

          : [=internal observer/complete steps=]
          :: Run |subscriber|'s {{Subscriber/complete()}} method.

       1. Let |options| be a new {{SubscribeOptions}} whose {{SubscribeOptions/signal}} is
          |subscriber|'s [=Subscriber/subscription controller=]'s [=AbortController/signal=].

       1. <a for=Observable lt="subscribe to an Observable">Subscribe</a> to |sourceObservable|
          given |sourceObserver| and |options|.

    1. Return |observable|.

  <wpt>
    /dom/observable/tentative/observable-filter.any.js
  </wpt>
</div>

<div algorithm>
  The <dfn for=Observable method><code>take(|amount|)</code></dfn> method steps are:

    1. Let |sourceObservable| be [=this=].

    1. Let |observable| be a [=new=] {{Observable}} whose [=Observable/subscribe callback=] is an
       algorithm that takes a {{Subscriber}} |subscriber| and does the following:

       1. Let |remaining| be |amount|.

       1. If |remaining| is 0, then run |subscriber|'s {{Subscriber/complete()}} method and abort
          these steps.

       1. Let |sourceObserver| be a new [=internal observer=], initialized as follows:

          : [=internal observer/next steps=]
          :: 1. Run |subscriber|'s {{Subscriber/next()}} method with the passed in <var
                ignore>value</var>.

             1. Decrement |remaining|.

             1. If |remaining| is 0, then run |subscriber|'s {{Subscriber/complete()}} method.

          : [=internal observer/error steps=]
          :: Run |subscriber|'s {{Subscriber/error()}} method, given the passed in <var
             ignore>error</var>.

          : [=internal observer/complete steps=]
          :: Run |subscriber|'s {{Subscriber/complete()}} method.

       1. Let |options| be a new {{SubscribeOptions}} whose {{SubscribeOptions/signal}} is
          |subscriber|'s [=Subscriber/subscription controller=]'s [=AbortController/signal=].

       1. <a for=Observable lt="subscribe to an Observable">Subscribe</a> to |sourceObservable|
          given |sourceObserver| and |options|.

    1. Return |observable|.

  <wpt>
    /dom/observable/tentative/observable-take.any.js
  </wpt>
</div>

<div algorithm>
  The <dfn for=Observable method><code>drop(|amount|)</code></dfn> method steps are:

    1. Let |sourceObservable| be [=this=].

    1. Let |observable| be a [=new=] {{Observable}} whose [=Observable/subscribe callback=] is an
       algorithm that takes a {{Subscriber}} |subscriber| and does the following:

       1. Let |remaining| be |amount|.

       1. Let |sourceObserver| be a new [=internal observer=], initialized as follows:

          : [=internal observer/next steps=]
          :: 1. If |remaining| is &gt; 0, then decrement |remaining| and abort these steps.

             1. [=Assert=]: |remaining| is 0.

             1. Run |subscriber|'s {{Subscriber/next()}} method with the passed in <var
                ignore>value</var>.

          : [=internal observer/error steps=]
          :: Run |subscriber|'s {{Subscriber/error()}} method, given the passed in <var
             ignore>error</var>.

          : [=internal observer/complete steps=]
          :: Run |subscriber|'s {{Subscriber/complete()}} method.

       1. Let |options| be a new {{SubscribeOptions}} whose {{SubscribeOptions/signal}} is
          |subscriber|'s [=Subscriber/subscription controller=]'s [=AbortController/signal=].

       1. <a for=Observable lt="subscribe to an Observable">Subscribe</a> to |sourceObservable|
          given |sourceObserver| and |options|.

    1. Return |observable|.

  <wpt>
    /dom/observable/tentative/observable-drop.any.js
  </wpt>
</div>

<div algorithm>
  The <dfn for=Observable method><code>flatMap(|mapper|)</code></dfn> method steps are:

    1. Let |sourceObservable| be [=this=].

    1. Let |observable| be a [=new=] {{Observable}} whose [=Observable/subscribe callback=] is an
       algorithm that takes a {{Subscriber}} |subscriber| and does the following:

       1. Let |idx| be an {{unsigned long long}}, initially 0.

       1. Let |outerSubscriptionHasCompleted| to a [=boolean=], initially false.

       1. Let |queue| be a new [=list=] of {{any}} values, initially empty.

          Note: This |queue| is used to store any {{Observable}}s emitted by |sourceObservable|,
          while |observable| is currently subscribed to an {{Observable}} emitted earlier by
          |sourceObservable| that has not yet been exhausted.

       1. Let |activeInnerSubscription| be a [=boolean=], initially false.

       1. Let |sourceObserver| be a new [=internal observer=], initialized as follows:

          : [=internal observer/next steps=]
          :: 1. If |activeInnerSubscription| is true, then:

                1. [=list/Append=] |value| to |queue|.

                   Note: This |value| will eventually be processed once the {{Observable}} that is
                   currently subscribed-to (as indicated by |activeInnerSubscription|) is exhausted.

             1. Otherwise:

                1. Set |activeInnerSubscription| to true.

                1. Run the [=flatmap process next value steps=] with |value|, |subscriber|,
                   |mapper|, and <b>references</b> to all of the following: |queue|,
                   |activeInnerSubscription|, |outerSubscriptionHasCompleted|, and |idx|.

                   <div class=note>
                     <p>Note: This [=flatmap process next value steps=] will subscribe to the
                     {{Observable}} derived from |value| (if one such can be derived) and keep
                     processing values from it until its subscription becomes inactive (either by
                     error or completion). If this "inner" {{Observable}} completes, then the
                     processing steps will recursively invoke themselves with the next {{any}} in
                     |queue|.</p>

                     <p>If no such value [=list/exists=], then the processing steps will terminate,
                     <b>unsetting</b> |activeInnerSubscription|, so that future values emitted from
                     |sourceObservable| are processed correctly.</p>
                   </div>

          : [=internal observer/error steps=]
          :: Run |subscriber|'s {{Subscriber/error()}} method, given the passed in <var
             ignore>error</var>.

          : [=internal observer/complete steps=]
          :: 1. Set |outerSubscriptionHasCompleted| to true.

                Note: If |activeInnerSubscription| is true, then the below step will *not* complete
                |subscriber|. In that case, the [=flatmap process next value steps=] will be
                responsible for completing |subscriber| when |queue| is [=list/empty=], after the
                "inner" subscription becomes inactive.

             1. If |activeInnerSubscription| is false and |queue| is [=list/empty=], run
                |subscriber|'s {{Subscriber/complete()}} method.

       1. Let |options| be a new {{SubscribeOptions}} whose {{SubscribeOptions/signal}} is
          |subscriber|'s [=Subscriber/subscription controller=]'s [=AbortController/signal=].

       1. <a for=Observable lt="subscribe to an Observable">Subscribe</a> to |sourceObservable|
          given |sourceObserver| and |options|.

    1. Return |observable|.
</div>

<div algorithm>
  The <dfn>flatmap process next value steps</dfn>, given an {{any}} |value|, a {{Subscriber}}
  |subscriber|, a {{Mapper}} |mapper|, and <b>references</b> to all of the following: a [=list=] of
  {{any}} values |queue|, a [=boolean=] |activeInnerSubscription|, a [=boolean=]
  |outerSubscriptionHasCompleted|, and an {{unsigned long long}} |idx|:

    1. Let |mappedResult| be the result of [=invoking=] |mapper| with |value| and |idx|.

       If <a spec=webidl lt="an exception was thrown">an exception |E| was thrown</a>, then run
       |subscriber|'s {{Subscriber/error()}} method, given |E|, and abort these steps.

    1. Set |idx| to |idx| + 1.

    1. Let |innerObservable| be the result of calling {{Observable/from()}} with |mappedResult|.

       If <a spec=webidl lt="an exception was thrown">an exception |E| was thrown</a>, then run
       |subscriber|'s {{Subscriber/error()}} method, given |E|, and abort these steps.

       Issue: We shouldn't invoke {{Observable/from()}} directly. Rather, we should call some
       internal algorithm that passes-back the exceptions for us to handle properly here, since we
       want to pipe them to |subscriber|.

     1. Let |innerObserver| be a new [=internal observer=], initialized as follows:

        : [=internal observer/next steps=]
        :: Run |subscriber|'s {{Subscriber/next()}} method, given the passed in |value|.

        : [=internal observer/error steps=]
        :: Run |subscriber|'s {{Subscriber/error()}} method, given the passed in <var
           ignore>error</var>.

        : [=internal observer/complete steps=]
        :: 1. If |queue| is not empty, then:

              1. Let |nextValue| be the first item in |queue|; [=list/remove=] remove this item from
                 |queue|.

              1. Run [=flatmap process next value steps=] given |nextValue|, |subscriber|, |mapper|,
                 and <b>references</b> to |queue| and |activeInnerSubscription|.

           1. Otherwise:

              1. Set |activeInnerSubscription| to false.

                 Note: Because |activeInnerSubscription| is a reference, this has the effect of
                 ensuring that all subsequent values emitted from the "outer" {{Observable}} (called
                 <var ignore>sourceObservable</var>.

              1. If |outerSubscriptionHasCompleted| is true, run |subscriber|'s
                 {{Subscriber/complete()}} method.

                 Note: This means the "outer" {{Observable}} has already completed, but did not
                 proceed to complete |subscriber| yet because there was at least one more pending
                 "inner" {{Observable}} (i.e., |innerObservable|) that had already been queued and
                 had not yet completed. Until right now!

     1. Let |innerOptions| be a new {{SubscribeOptions}} whose {{SubscribeOptions/signal}} is
        |subscriber|'s [=Subscriber/subscription controller=]'s [=AbortController/signal=].

     1. <a for=Observable lt="subscribe to an Observable">Subscribe</a> to |innerObservable| given
        |innerObserver| and |innerOptions|.
</div>

<div algorithm>
  The <dfn for=Observable method><code>switchMap(|mapper|)</code></dfn> method steps are:

    1. Let |sourceObservable| be [=this=].

    1. Let |observable| be a [=new=] {{Observable}} whose [=Observable/subscribe callback=] is an
       algorithm that takes a {{Subscriber}} |subscriber| and does the following:

       1. Let |idx| be an {{unsigned long long}}, initially 0.

       1. Let |outerSubscriptionHasCompleted| be a [=boolean=], initially false.

       1. Let |activeInnerAbortController| be an {{AbortController}}-or-null, initially null.

          Note: This {{AbortController}} is assigned to a new {{AbortController}} only by this
          algorithm's <a href=#switchmap-next-steps>next steps</a> (below), and only assigned to
          null by the [=switchmap process next value steps=], when the "inner" {{Observable}} either
          completes or errors. This variable is used as a marker for whether there is currently an
          active "inner" subscription. The <a href=#switchmap-complete-steps>complete steps</a>
          below care about this, because if |sourceObservable| completes while there is an active
          "inner" subscription, we do not immediately complete |subscriber|. In that case,
          |subscriber|'s completion becomes blocked on the "inner" subscription's completion.

       1. Let |sourceObserver| be a new [=internal observer=], initialized as follows:

          : <dt id=switchmap-next-steps>[=internal observer/next steps=]</dt>
          :: 1. If |activeInnerAbortController| is not null, then [=AbortController/signal abort=]
                |activeInnerAbortController|.

                Note: This "unsubscribes" from the "inner" {{Observable}} that was derived from the
                value that was <i>last</i> pushed from |sourceObservable|. Then we immediately
                subscribe to the <i>new</i> {{Observable}} that we're about to derive from |value|,
                i.e., the <i>most-recently</i> pushed value from |sourceObservable|.

             1. Set |activeInnerAbortController| to a [=new=] {{AbortController}}.

             1. Run the [=switchmap process next value steps=] with |value|, |subscriber|, |mapper|,
                and <b>references</b> to all of the following: |activeInnerAbortController|,
                |outerSubscriptionHasCompleted|, and |idx|.

                Note: The [=switchmap process next value steps=] will subscribe to the
                {{Observable}} derived from |value| (if one such can be derived) and keep processing
                values from it until either (1) its subscription becomes inactive (either by error
                or completion), or (2) |activeInnerAbortController| <a
                href=#switchmap-next-steps>gets aborted</a>, due to |sourceObservable| pushing
                another <i>newer</i> value that will replace the current "inner" subscription.

          : [=internal observer/error steps=]
          :: Run |subscriber|'s {{Subscriber/error()}} method, given the passed in <var
             ignore>error</var>.

          : <dt id=switchmap-complete-steps>[=internal observer/complete steps=]</dt>
          :: 1. Set |outerSubscriptionHasCompleted| to true.

                Note: If |activeInnerAbortController| is not null, then we don't immediately
                complete |subscriber|. Instead, the [=switchmap process next value steps=] will
                complete |subscriber| when the inner subscription finally completes itself.

             1. If |activeInnerAbortController| is null, run |subscriber|'s
                {{Subscriber/complete()}} method.

       1. Let |options| be a new {{SubscribeOptions}} whose {{SubscribeOptions/signal}} is
          |subscriber|'s [=Subscriber/subscription controller=]'s [=AbortController/signal=].

       1. <a for=Observable lt="subscribe to an Observable">Subscribe</a> to |sourceObservable|
          given |sourceObserver| and |options|.

    1. Return |observable|.
</div>

<div algorithm>
  The <dfn>switchmap process next value steps</dfn>, given an {{any}} |value|, a {{Subscriber}}
  |subscriber|, a {{Mapper}} |mapper|, and <b>references</b> to all of the following: an
  {{AbortController}} |activeInnerAbortController|, a [=boolean=] |outerSubscriptionHasCompleted|,
  and an {{unsigned long long}} |idx| are to run these steps:

    1. Let |mappedResult| be the result of [=invoking=] |mapper| with |value| and |idx|.

       If <a spec=webidl lt="an exception was thrown">an exception |E| was thrown</a>, then run
       |subscriber|'s {{Subscriber/error()}} method, given |E|, and abort these steps.

    1. Set |idx| to |idx| + 1.

    1. Let |innerObservable| be the result of calling {{Observable/from()}} with |mappedResult|.

       If <a spec=webidl lt="an exception was thrown">an exception |E| was thrown</a>, then run
       |subscriber|'s {{Subscriber/error()}} method, given |E|, and abort these steps.

     1. Let |innerObserver| be a new [=internal observer=], initialized as follows:

        : [=internal observer/next steps=]
        :: Run |subscriber|'s {{Subscriber/next()}} method, given the passed in |value|.

        : [=internal observer/error steps=]
        :: Run |subscriber|'s {{Subscriber/error()}} method, given the passed in <var
           ignore>error</var>.

           Note: We don't have to set |activeInnerAbortController| to null here, to signal to the
           {{Observable/switchMap()}} method steps above that the inner "subscription" has been
           canceled. That's because calling |subscriber|'s {{Subscriber/error()}} method already
           unsubscribes from the "outer" source Observable, so it will not be able to push any more
           values to the {{Observable/switchMap()}} internal observer.

        : [=internal observer/complete steps=]
        :: 1. If |outerSubscriptionHasCompleted| is true, run |subscriber|'s
              {{Subscriber/complete()}} method.

           1. Otherwise, set |activeInnerAbortController| to null.

              Note: Because this variable is a reference, it signals to the <a
              href=#switchmap-complete-steps>switchMap complete steps</a> that there is no active
              inner subscription.

     1. Let |innerOptions| be a new {{SubscribeOptions}} whose {{SubscribeOptions/signal}} is the
        result of [=creating a dependent abort signal=] from the list
        «|activeInnerAbortController|'s [=AbortController/signal=], |subscriber|'s
        [=Subscriber/subscription controller=]'s [=AbortController/signal=]», using {{AbortSignal}},
        and the [=current realm=].

     1. <a for=Observable lt="subscribe to an Observable">Subscribe</a> to |innerObservable| given
        |innerObserver| and |innerOptions|.
</div>

<div algorithm>
  The <dfn for=Observable method><code>inspect(|inspectorUnion|)</code></dfn> method steps are:

    1. Let |subscribe callback| be a {{VoidFunction}}-or-null, initially null.

    1. Let |next callback| be a {{ObservableSubscriptionCallback}}-or-null, initially null.

    1. Let |error callback| be a {{ObservableSubscriptionCallback}}-or-null, initially null.

    1. Let |complete callback| be a {{VoidFunction}}-or-null, initially null.

    1. Let |abort callback| be a {{ObservableInspectorAbortHandler}}-or-null, initially null.

    1. Process |inspectorUnion| as follows:
         <dl class="switch">
           <dt>If |inspectorUnion| is an {{ObservableSubscriptionCallback}}</dt>
           <dd>
             1. Set |next callback| to |inspectorUnion|.

           <dt>If |inspectorUnion| is an {{ObservableInspector}}</dt>
           <dd>
             1. If {{ObservableInspector/subscribe}} [=map/exists=] in |inspectorUnion|, then set
                |subscribe callback| to it.

             1. If {{ObservableInspector/next}} [=map/exists=] in |inspectorUnion|, then set |next
                callback| to it.

             1. If {{ObservableInspector/error}} [=map/exists=] in |inspectorUnion|, then set |error
                callback| to it.

             1. If {{ObservableInspector/complete}} [=map/exists=] in |inspectorUnion|, then set
                |complete callback| to it.

             1. If {{ObservableInspector/abort}} [=map/exists=] in |inspectorUnion|, then set |abort
                callback| to it.
           </dd>
         </dl>

    1. Let |sourceObservable| be [=this=].

    1. Let |observable| be a [=new=] {{Observable}} whose [=Observable/subscribe callback=] is an
       algorithm that takes a {{Subscriber}} |subscriber| and does the following:

       1. If |subscribe callback| is not null, then [=invoke=] it.

          If <a spec=webidl lt="an exception was thrown">an exception |E| was thrown</a>, then run
          |subscriber|'s {{Subscriber/error()}} method, given |E|, and abort these steps.

          Note: The result of this is that |sourceObservable| is never subscribed to.

       1. If |abort callback| is not null, then [=AbortSignal/add|add the following abort
          algorithm=] to |subscriber|'s [=Subscriber/subscription controller=]'s
          [=AbortController/signal=]:

          1. [=Invoke=] |abort callback| with |subscriber|'s [=Subscriber/subscription
             controller=]'s [=AbortController/signal=]'s [=AbortSignal/abort reason=].

             If <a spec=webidl lt="an exception was thrown">an exception |E| was thrown</a>, then
             [=report the exception=] |E|.

       1. Let |sourceObserver| be a new [=internal observer=], initialized as follows:

          : [=internal observer/next steps=]
          :: 1. If |next callback| is not null, then [=invoke=] |next callback| with the passed in
                |value|.

                If <a spec=webidl lt="an exception was thrown">an exception |E| was thrown</a>,
                then:

                  1. [=AbortSignal/Remove=] |abort callback| from |subscriber|'s
                     [=Subscriber/subscription controller=]'s [=AbortController/signal=].

                     Note: This step is important, because the |abort callback| is only meant to be
                     called for *consumer-initiated* unsubscriptions. When the producer terminates
                     the subscription (via |subscriber|'s {{Subscriber/error()}} or
                     {{Subscriber/complete()}} methods) like below, we have to ensure that |abort
                     callback| is not run.

                     Issue: This matches Chromium's implementation, but consider holding a reference
                     to the originally-passed-in {{SubscribeOptions}}'s {{SubscribeOptions/signal}}
                     and just invoking |abort callback| when *it* aborts. The result is likely the
                     same, but needs investigation.

                  1. Run |subscriber|'s {{Subscriber/error()}} method, given |E|, and abort these
                     steps.

             1. Run |subscriber|'s {{Subscriber/next()}} method with the passed in |value|.

          : [=internal observer/error steps=]
          :: 1. [=AbortSignal/Remove=] |abort callback| from |subscriber|'s
                [=Subscriber/subscription controller=]'s [=AbortController/signal=].

             1. If |error callback| is not null, then [=invoke=] |error callback| given the passed
                in |error|.

                If <a spec=webidl lt="an exception was thrown">an exception |E| was thrown</a>, then
                run |subscriber|'s {{Subscriber/error()}} method, given |E|, and abort these steps.

             1. Run |subscriber|'s {{Subscriber/error()}} method, given the passed in |error|.

          : [=internal observer/complete steps=]
          :: 1. [=AbortSignal/Remove=] |abort callback| from |subscriber|'s
                [=Subscriber/subscription controller=]'s [=AbortController/signal=].

             1. If |complete callback| is not null, then [=invoke=] |complete callback|.

                If <a spec=webidl lt="an exception was thrown">an exception |E| was thrown</a>, then
                run |subscriber|'s {{Subscriber/error()}} method, given |E|, and abort these steps.

             1. Run |subscriber|'s {{Subscriber/complete()}} method.

       1. Let |options| be a new {{SubscribeOptions}} whose {{SubscribeOptions/signal}} is
          |subscriber|'s [=Subscriber/subscription controller=]'s [=AbortController/signal=].

       1. <a for=Observable lt="subscribe to an Observable">Subscribe</a> to |sourceObservable|
          given |sourceObserver| and |options|.

    1. Return |observable|.

  <wpt>
    /dom/observable/tentative/observable-inspect.any.js
  </wpt>
</div>

<div algorithm>
  The <dfn for=Observable method><code>catch(|callback|)</code></dfn> method steps are:

    1. Let |sourceObservable| be [=this=].

    1. Let |observable| be a [=new=] {{Observable}} whose [=Observable/subscribe callback=] is an
       algorithm that takes a {{Subscriber}} |subscriber| and does the following:

       1. Let |sourceObserver| be a new [=internal observer=], initialized as follows:

          : [=internal observer/next steps=]
          :: Run |subscriber|'s {{Subscriber/next()}} method, given the passed in <var
             ignore>value</var>.

          : [=internal observer/error steps=]
          ::
             1. [=Invoke=] |callback| with the passed in <var ignore>error</var>. Let |result| be
                the returned value.

                If <a spec=webidl lt="an exception was thrown">an exception |E| was thrown</a>, then
                run |subscriber|'s {{Subscriber/error()}} with |E|, and abort these steps.

             1. Let |innerObservable| be the result of calling {{Observable/from()}} with |result|.

                If <a spec=webidl lt="an exception was thrown">an exception |E| was thrown</a>, then
                run |subscriber|'s {{Subscriber/error()}} method, given |E|, and abort these steps.

                Issue: We shouldn't invoke {{Observable/from()}} directly. Rather, we should call
                some internal algorithm that passes-back the exceptions for us to handle properly
                here, since we want to pipe them to |subscriber|.

             1. Let |innerObserver| be a new [=internal observer=], initialized as follows:

                : [=internal observer/next steps=]
                :: Run |subscriber|'s {{Subscriber/next()}} method, given the passed in <var
                   ignore>value</var>.

                : [=internal observer/error steps=]
                :: Run |subscriber|'s {{Subscriber/error()}} method, given the passed in <var
                   ignore>error</var>.

                : [=internal observer/complete steps=]
                :: Run |subscriber|'s {{Subscriber/complete()}} method.

             1. Let |innerOptions| be a new {{SubscribeOptions}} whose {{SubscribeOptions/signal}}
                is |subscriber|'s [=Subscriber/subscription controller=]'s
                [=AbortController/signal=].

             1. <a for=Observable lt="subscribe to an Observable">Subscribe</a> to |innerObservable|
                given |innerObserver| and |innerOptions|.

                Note: We're free to subscribe to |innerObservable| here without first
                "unsubscribing" from |sourceObservable|, and without fear that |sourceObservable|
                will keep emitting values, because all of this is happening inside of the [=internal
                observer/error steps=] associated with |sourceObservable|. This means
                |sourceObservable| has already completed its subscription and will no longer produce
                any values, and we are free to safely switch our source of values to
                |innerObservable|.

          : [=internal observer/complete steps=]
          :: Run |subscriber|'s {{Subscriber/complete()}} method.

       1. Let |options| be a new {{SubscribeOptions}} whose {{SubscribeOptions/signal}} is
          |subscriber|'s [=Subscriber/subscription controller=]'s [=AbortController/signal=].

       1. <a for=Observable lt="subscribe to an Observable">Subscribe</a> to |sourceObservable|
          given |sourceObserver| and |options|.

    1. Return |observable|.
</div>

<div algorithm>
  The <dfn for=Observable method><code>finally(|callback|)</code></dfn> method steps are:

    1. Let |sourceObservable| be [=this=].

    1. Let |observable| be a [=new=] {{Observable}} whose [=Observable/subscribe callback=] is an
       algorithm that takes a {{Subscriber}} |subscriber| and does the following:

       1. Run |subscriber|'s {{Subscriber/addTeardown()}} method with |callback|.

       1. Let |sourceObserver| be a new [=internal observer=], initialized as follows:

          : [=internal observer/next steps=]
          :: Run |subscriber|'s {{Subscriber/next()}} method, given the passed in <var
             ignore>value</var>.

          : [=internal observer/error steps=]
          :: 1. Run |subscriber|'s {{Subscriber/error()}} method, given the passed in <var
                ignore>error</var>.

          : [=internal observer/complete steps=]
          :: 1. Run |subscriber|'s {{Subscriber/complete()}} method.

       1. Let |options| be a new {{SubscribeOptions}} whose {{SubscribeOptions/signal}} is
          |subscriber|'s [=Subscriber/subscription controller=]'s [=AbortController/signal=].

       1. <a for=Observable lt="subscribe to an Observable">Subscribe</a> to |sourceObservable|
          given |sourceObserver| and |options|.

    1. Return |observable|.
</div>


<h4 id=promise-returning-operators>{{Promise}}-returning operators</h4>

<div algorithm>
  The <dfn for=Observable method><code>toArray(|options|)</code></dfn> method steps are:

    1. Let |p| [=a new promise=].

    1. If |options|'s {{SubscribeOptions/signal}} is not null:

       1. If |options|'s {{SubscribeOptions/signal}} is [=AbortSignal/aborted=], then:

          1. [=Reject=] |p| with |options|'s {{SubscribeOptions/signal}}'s [=AbortSignal/abort
             reason=].

          1. Return |p|.

       1. [=AbortSignal/add|Add the following abort algorithm=] to |options|'s
          {{SubscribeOptions/signal}}:

          1. [=Reject=] |p| with |options|'s {{SubscribeOptions/signal}}'s [=AbortSignal/abort
             reason=].

          Note: All we have to do here is [=reject=] |p|. Note that the subscription to [=this=]
          {{Observable}} will also be closed automatically, since the "inner" Subscriber gets
          [=close a subscription|closed=] in response to |options|'s {{SubscribeOptions/signal}}
          getting [=AbortSignal/signal abort=].

    1. Let |values| be a new [=list=].

    1. Let |observer| be a new [=internal observer=], initialized as follows:

       : [=internal observer/next steps=]
       :: [=list/Append=] the passed in <var ignore>value</var> to |values|.

       : [=internal observer/error steps=]
       :: [=Reject=] |p| with the passed in <var ignore>error</var>.

       : [=internal observer/complete steps=]
       :: [=Resolve=] |p| with |values|.

    1. <a for=Observable lt="subscribe to an Observable">Subscribe</a> to [=this=] given |observer|
       and |options|.

    1. Return |p|.

  <wpt>
    /dom/observable/tentative/observable-toArray.any.js
  </wpt>
</div>

<div algorithm>
  The <dfn for=Observable method><code>forEach(|callback|, |options|)</code></dfn> method steps are:

    1. Let |p| [=a new promise=].

    1. Let |visitor callback controller| be a [=new=] {{AbortController}}.

    1. Let |internal options| be a new {{SubscribeOptions}} whose {{SubscribeOptions/signal}} is the
       result of [=creating a dependent abort signal=] from the list «|visitor callback
       controller|'s [=AbortController/signal=], |options|'s {{SubscribeOptions/signal}} if
       non-null», using {{AbortSignal}}, and the [=current realm=].

       <div class=note>
         <p>Many trivial [=internal observers=] act as pass-throughs, and do not control the
         subscription to the {{Observable}} that they represent; that is, their [=internal
         observer/error steps=] and [=internal observer/complete steps=] are called when the
         subscription is terminated, and their [=internal observer/next steps=] simply pass some
         version of the given value along the chain.</p>

         <p>For this operator, however, the below |observer|'s [=internal observer/next steps=] are
         responsible for actually aborting the underlying subscription to [=this=], in the event
         that |callback| throws an exception. In that case, the {{SubscribeOptions}}'s
         {{SubscribeOptions/signal}} we pass through to "<a for=Observable lt="subscribe to an
         Observable">Subscribe to an <code>Observable</code></a>", needs to be a [=creating a
         dependent abort signal|dependent signal=] derived from |options|'s
         {{SubscribeOptions/signal}}, **and** the {{AbortSignal}} of an {{AbortController}} that the
         [=internal observer/next steps=] below has access to, and can [=AbortController/signal
         abort=] when needed.
       </div>

    1. If |internal options|'s {{SubscribeOptions/signal}} is [=AbortSignal/aborted=], then:

       1. [=Reject=] |p| with |internal options|'s {{SubscribeOptions/signal}}'s [=AbortSignal/abort
          reason=].

       1. Return |p|.

    1. [=AbortSignal/add|Add the following abort algorithm=] to |internal options|'s
       {{SubscribeOptions/signal}}:

       1. [=Reject=] |p| with |internal options|'s {{SubscribeOptions/signal}}'s [=AbortSignal/abort
          reason=].

          Note: The fact that rejection of |p| is tied to |internal options|'s
          {{SubscribeOptions/signal}}, and not |options|'s {{SubscribeOptions/signal}} means, that
          any [=microtasks=] [=queue a microtask|queued=] during the firing of |options|'s
          {{SubscribeOptions/signal}}'s {{AbortSignal/abort}} event will run before |p|'s rejection
          handler runs.

    1. Let |idx| be an {{unsigned long long}}, initially 0.

    1. Let |observer| be a new [=internal observer=], initialized as follows:

       : [=internal observer/next steps=]
       ::
          1. [=Invoke=] |callback| with the passed in <var ignore>value</var>, and |idx|.

             If <a spec=webidl lt="an exception was thrown">an exception |E| was thrown</a>, then
             [=reject=] |p| with |E|, and [=AbortController/signal abort=] |visitor callback
             controller| with |E|.

          1. Increment |idx|.

       : [=internal observer/error steps=]
       :: [=Reject=] |p| with the passed in <var ignore>error</var>.

       : [=internal observer/complete steps=]
       :: [=Resolve=] |p| with {{undefined}}.

    1. <a for=Observable lt="subscribe to an Observable">Subscribe</a> to [=this=] given |observer|
       and |internal options|.

    1. Return |p|.

  <wpt>
    /dom/observable/tentative/observable-forEach.any.js
  </wpt>
</div>

<div algorithm>
  The <dfn for=Observable method><code>every(|predicate|, |options|)</code></dfn> method steps are:

    1. Let |p| [=a new promise=].

    1. Let |controller| be a [=new=] {{AbortController}}.

    1. Let |internal options| be a new {{SubscribeOptions}} whose {{SubscribeOptions/signal}} is the
       result of [=creating a dependent abort signal=] from the list «|controller|'s
       [=AbortController/signal=], |options|'s {{SubscribeOptions/signal}} if non-null», using
       {{AbortSignal}}, and the [=current realm=].

    1. If |internal options|'s {{SubscribeOptions/signal}} is [=AbortSignal/aborted=], then:

       1. [=Reject=] |p| with |internal options|'s {{SubscribeOptions/signal}}'s [=AbortSignal/abort
          reason=].

       1. Return |p|.

    1. [=AbortSignal/add|Add the following abort algorithm=] to |internal options|'s
       {{SubscribeOptions/signal}}:

       1. [=Reject=] |p| with |internal options|'s {{SubscribeOptions/signal}}'s [=AbortSignal/abort
          reason=].

    1. Let |idx| be an {{unsigned long long}}, initially 0.

    1. Let |observer| be a new [=internal observer=], initialized as follows:

       : [=internal observer/next steps=]
       :: 1. [=Invoke=] |predicate| with the passed in <var ignore>value</var> and |idx|, and let
             |passed| be the returned value.

             If <a spec=webidl lt="an exception was thrown">an exception |E| was thrown</a>, then
             [=reject=] |p| with |E|, and [=AbortController/signal abort=] |controller| with |E|.

         1. Set |idx| to |idx| + 1.

         1. If |passed| is false, then [=resolve=] |p| with false, and [=AbortController/signal
            abort=] |controller|.

       : [=internal observer/error steps=]
       :: [=Reject=] |p| with the passed in <var ignore>error</var>.

       : [=internal observer/complete steps=]
       :: [=Resolve=] |p| with true.

    1. <a for=Observable lt="subscribe to an Observable">Subscribe</a> to [=this=] given |observer|
       and |internal options|.

    1. Return |p|.
</div>

<div algorithm>
  The <dfn for=Observable method><code>first(|options|)</code></dfn> method steps are:

    1. Let |p| [=a new promise=].

    1. Let |controller| be a [=new=] {{AbortController}}.

    1. Let |internal options| be a new {{SubscribeOptions}} whose {{SubscribeOptions/signal}} is the
       result of [=creating a dependent abort signal=] from the list «|controller|'s
       [=AbortController/signal=], |options|'s {{SubscribeOptions/signal}} if non-null», using
       {{AbortSignal}}, and the [=current realm=].

    1. If |internal options|'s {{SubscribeOptions/signal}} is [=AbortSignal/aborted=], then:

       1. [=Reject=] |p| with |internal options|'s {{SubscribeOptions/signal}}'s [=AbortSignal/abort
          reason=].

       1. Return |p|.

    1. [=AbortSignal/add|Add the following abort algorithm=] to |internal options|'s
       {{SubscribeOptions/signal}}:

       1. [=Reject=] |p| with |internal options|'s {{SubscribeOptions/signal}}'s [=AbortSignal/abort
          reason=].

    1. Let |internal observer| be a new [=internal observer=], initialized as follows:

       : [=internal observer/next steps=]
       :: 1. [=Resolve=] |p| with the passed in <var ignore>value</var>.

        1. [=AbortController/Signal abort=] |controller|.

       : [=internal observer/error steps=]
       :: [=Reject=] |p| with the passed in <var ignore>error</var>.

       : [=internal observer/complete steps=]
       :: [=Reject=] |p| with a new {{RangeError}}.

          Note: This is only reached when the source {{Observable}} completes *before* it emits a
          single value.

    1. <a for=Observable lt="subscribe to an Observable">Subscribe</a> to [=this=] given |internal
       observer| and |internal options|.

    1. Return |p|.

</div>

<div algorithm>
  The <dfn for=Observable method><code>last(|options|)</code></dfn> method steps are:

    1. Let |p| [=a new promise=].

    1. If |options|'s {{SubscribeOptions/signal}} is not null:

       1. If |options|'s {{SubscribeOptions/signal}} is [=AbortSignal/aborted=], then:

          1. [=Reject=] |p| with |options|'s {{SubscribeOptions/signal}}'s [=AbortSignal/abort
             reason=].

          1. Return |p|.

       1. [=AbortSignal/add|Add the following abort algorithm=] to |options|'s
          {{SubscribeOptions/signal}}:

          1. [=Reject=] |p| with |options|'s {{SubscribeOptions/signal}}'s [=AbortSignal/abort
             reason=].

    1. Let |lastValue| be an {{any}}-or-null, initially null.

    1. Let |hasLastValue| be a [=boolean=], initially false.

    1. Let |observer| be a new [=internal observer=], initialized as follows:

       : [=internal observer/next steps=]
       :: 1. Set |hasLastValue| to true.

        1. Set |lastValue| to the passed in <var ignore>value</var>.

       : [=internal observer/error steps=]
       :: [=Reject=] |p| with the passed in <var ignore>error</var>.

       : [=internal observer/complete steps=]
       :: 1. If |hasLastValue| is true, [=resolve=] |p| with |lastValue|.

          1. Otherwise, [=reject=] |p| with a new {{RangeError}}.

             Note: See the note in {{Observable/first()}}.

    1. <a for=Observable lt="subscribe to an Observable">Subscribe</a> to [=this=] given |observer|
       and |options|.

    1. Return |p|.
</div>

<div algorithm>
  The <dfn for=Observable method><code>find(|predicate|, |options|)</code></dfn> method steps are:

    1. Let |p| [=a new promise=].

    1. Let |controller| be a [=new=] {{AbortController}}.

    1. Let |internal options| be a new {{SubscribeOptions}} whose {{SubscribeOptions/signal}} is the
       result of [=creating a dependent abort signal=] from the list «|controller|'s
       [=AbortController/signal=], |options|'s {{SubscribeOptions/signal}} if non-null», using
       {{AbortSignal}}, and the [=current realm=].

    1. If |internal options|'s {{SubscribeOptions/signal}} is [=AbortSignal/aborted=], then:

       1. [=Reject=] |p| with |internal options|'s {{SubscribeOptions/signal}}'s [=AbortSignal/abort
          reason=].

       1. Return |p|.

    1. [=AbortSignal/add|Add the following abort algorithm=] to |internal options|'s
       {{SubscribeOptions/signal}}:

       1. [=Reject=] |p| with |internal options|'s {{SubscribeOptions/signal}}'s [=AbortSignal/abort
          reason=].

    1. Let |idx| be an {{unsigned long long}}, initially 0.

    1. Let |observer| be a new [=internal observer=], initialized as follows:

       : [=internal observer/next steps=]
       :: 1. [=Invoke=] |predicate| with the passed in |value| an |idx|, and let |passed| be the
             returned value.

             If <a spec=webidl lt="an exception was thrown">an exception |E| was thrown</a>, then
             [=reject=] |p| with |E|, and [=AbortController/signal abort=] |controller| with |E|.

         1. Set |idx| to |idx| + 1.

         1. If |passed| is true, then [=resolve=] |p| with |value|, and [=AbortController/signal
            abort=] |controller|.

       : [=internal observer/error steps=]
       :: [=Reject=] |p| with the passed in <var ignore>error</var>.

       : [=internal observer/complete steps=]
       :: [=Resolve=] |p| with {{undefined}}.

    1. <a for=Observable lt="subscribe to an Observable">Subscribe</a> to [=this=] given |observer|
       and |internal options|.

    1. Return |p|.
</div>

<div algorithm>
  The <dfn for=Observable method><code>some(|predicate|, |options|)</code></dfn> method steps are:

    1. Let |p| [=a new promise=].

    1. Let |controller| be a [=new=] {{AbortController}}.

    1. Let |internal options| be a new {{SubscribeOptions}} whose {{SubscribeOptions/signal}} is the
       result of [=creating a dependent abort signal=] from the list «|controller|'s
       [=AbortController/signal=], |options|'s {{SubscribeOptions/signal}} if non-null», using
       {{AbortSignal}}, and the [=current realm=].

    1. If |internal options|'s {{SubscribeOptions/signal}} is [=AbortSignal/aborted=], then:

       1. [=Reject=] |p| with |internal options|'s {{SubscribeOptions/signal}}'s [=AbortSignal/abort
          reason=].

       1. Return |p|.

    1. [=AbortSignal/add|Add the following abort algorithm=] to |internal options|'s
       {{SubscribeOptions/signal}}:

       1. [=Reject=] |p| with |internal options|'s {{SubscribeOptions/signal}}'s [=AbortSignal/abort
          reason=].

    1. Let |idx| be an {{unsigned long long}}, initially 0.

    1. Let |observer| be a new [=internal observer=], initialized as follows:

       : [=internal observer/next steps=]
       :: 1. [=Invoke=] |predicate| with the passed in <var ignore>value</var> and |idx|, and let
             |passed| be the returned value.

             If <a spec=webidl lt="an exception was thrown">an exception |E| was thrown</a>, then
             [=reject=] |p| with |E|, and [=AbortController/signal abort=] |controller| with |E|.

         1. Set |idx| to |idx| + 1.

         1. If |passed| is true, then [=resolve=] |p| with true, and [=AbortController/signal
            abort=] |controller|.

       : [=internal observer/error steps=]
       :: [=Reject=] |p| with the passed in <var ignore>error</var>.

       : [=internal observer/complete steps=]
       :: [=Resolve=] |p| with false.

    1. <a for=Observable lt="subscribe to an Observable">Subscribe</a> to [=this=] given |observer|
       and |internal options|.

    1. Return |p|.
</div>

<div algorithm>
  The <dfn for=Observable method><code>reduce(|reducer|, |initialValue|, |options|)</code></dfn>
  method steps are:

    1. Let |p| [=a new promise=].

    1. Let |controller| be a [=new=] {{AbortController}}.

    1. Let |internal options| be a new {{SubscribeOptions}} whose {{SubscribeOptions/signal}} is the
       result of [=creating a dependent abort signal=] from the list «|controller|'s
       [=AbortController/signal=], |options|'s {{SubscribeOptions/signal}} if non-null», using
       {{AbortSignal}}, and the [=current realm=].

    1. If |internal options|'s {{SubscribeOptions/signal}} is [=AbortSignal/aborted=], then:

       1. [=Reject=] |p| with |internal options|'s {{SubscribeOptions/signal}}'s [=AbortSignal/abort
          reason=].

       1. Return |p|.

    1. [=AbortSignal/add|Add the following abort algorithm=] to |internal options|'s
       {{SubscribeOptions/signal}}:

       1. [=Reject=] |p| with |internal options|'s {{SubscribeOptions/signal}}'s [=AbortSignal/abort
          reason=].

    1. Let |idx| be an {{unsigned long long}}, initially 0.

    1. Let |accumulator| be |initialValue| if it is given, and uninitialized otherwise.

    1. Let |observer| be a new [=internal observer=], initialized as follows:

       : [=internal observer/next steps=]
       ::
          1. If |accumulator| is uninitialized (meaning no |initialValue| was passed in), then set
             |accumulator| to the passed in |value|, set |idx| to |idx| + 1, and abort these steps.

             Note: This means that |reducer| will not be called with the first |value| that [=this=]
             produces set as the {{Reducer/currentValue}}. Rather, when the *second* value is
             eventually emitted, we will call |reducer| with *it* as the {{Reducer/currentValue}},
             and the first value (that we're saving here) as the {{Reducer/accumulator}}.

          1. [=Invoke=] |reducer| with |accumulator| as {{Reducer/accumulator}}, the passed in
             |value| as {{Reducer/currentValue}}, and |idx| as {{Reducer/index}}. Let |result| be
             the returned value.

             If <a spec=webidl lt="an exception was thrown">an exception |E| was thrown</a>, then
             [=reject=] |p| with |E|, and [=AbortController/signal abort=] |controller| with |E|.

          1. Set |idx| to |idx| + 1.

          1. Set |accumulator| to |result|.

       : [=internal observer/error steps=]
       :: [=Reject=] |p| with the passed in <var ignore>error</var>.

       : [=internal observer/complete steps=]
       :: 1. If |accumulator| is not "<code>unset</code>", then [=resolve=] |p| with |accumulator|.

          Otherwise, [=reject=] |p| with a {{TypeError}}.

    1. <a for=Observable lt="subscribe to an Observable">Subscribe</a> to [=this=] given |observer|
       and |internal options|.

    1. Return |p|.
</div>


<h2 id=event-target-integration>{{EventTarget}} integration</h2>

<pre class=idl>
dictionary ObservableEventListenerOptions {
  boolean capture = false;
  boolean passive;
};

partial interface EventTarget {
  Observable when(DOMString type, optional ObservableEventListenerOptions options = {});
};
</pre>

<div algorithm>
  The <dfn for=EventTarget method><code>when(|type|, |options|)</code></dfn> method steps are:

    1. If [=this=]'s [=relevant global object=] is a {{Window}} object, and its [=associated
       Document=] is not [=Document/fully active=], then return.

    1. Let |event target| be [=this=].

    1. Let |observable| be a [=new=] {{Observable}}, initialized as follows:

       : [=Observable/subscribe callback=]
       :: An algorithm that takes a {{Subscriber}} |subscriber| and runs these steps:

            1. If |event target| is null, abort these steps.

               Note: This is meant to capture the fact that |event target| can be garbage collected
               by the time this algorithm runs upon subscription.

            1. If |subscriber|'s [=Subscriber/subscription controller=]'s [=AbortController/signal=]
               is [=AbortSignal/aborted=], abort these steps.

            1. [=Add an event listener=] with |event target| and an [=event listener=] defined as
               follows:

               : [=event listener/type=]
               :: |type|

               : [=event listener/callback=]
               :: The result of creating a new Web IDL {{EventListener}} instance representing a
                  reference to a function of one argument of type {{Event}} |event|. This function
                  executes the [=observable event listener invoke algorithm=] given |subscriber| and
                  |event|.

               : [=event listener/capture=]
               :: |options|'s {{ObservableEventListenerOptions/capture}}

               : [=event listener/passive=]
               :: |options|'s {{ObservableEventListenerOptions/passive}}

               : [=event listener/once=]
               :: false

               : [=event listener/signal=]
               :: |subscriber|'s [=Subscriber/subscription controller=]'s [=AbortController/signal=]

                  Note: This ensures that the [=event listener=] is cleaned up when
                  [=Subscriber/subscription controller=]'s [=AbortController/signal=] is
                  [=AbortSignal/aborted=], regardless of an engine's ownership model.

    1. Return |observable|.
</div>

<div algorithm>
  The <dfn>observable event listener invoke algorithm</dfn> takes a {{Subscriber}} |subscriber| and
  an {{Event}} |event|, and runs these steps:

    1. Run |subscriber|'s {{Subscriber/next()}} method with |event|.
</div>

<wpt>
  /dom/observable/tentative/observable-event-target.any.js
  /dom/observable/tentative/observable-event-target.window.js
</wpt>


<h2 id=security-and-privacy>Security & Privacy Considerations</h2>

This material is being upstreamed from our explainer into this specification, and in the meantime
you can consult the following resources:

 * [TAG Security/Privacy
   Questionnaire](https://github.com/WICG/observable/blob/master/security-privacy-questionnaire.md)

<h2 id=acks>Acknowledgements</h2>

A special thanks to [Ben Lesh](https://benlesh.com/) for much of the design input for the
{{Observable}} API, and his many years of work maintaining userland Observable code that made this
contribution to the web platform possible.