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Nov 20, 2025
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doc: correct concurrency wording in test() documentation #60773

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doc: correct concurrency wording in test() documentation
azadgupta1 Nov 18, 2025
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docs: update concurrency wording per reviewer suggestion
azadgupta1 Nov 18, 2025
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4 changes: 2 additions & 2 deletions doc/api/test.md
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Original file line number Diff line number Diff line change
Expand Up @@ -1619,7 +1619,7 @@ changes:
* `options` {Object} Configuration options for the test. The following
properties are supported:
* `concurrency` {number|boolean} If a number is provided,
then that many tests would run in parallel within the application thread.
then that many tests would run concurrently within the test process.
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@aduh95 aduh95 Nov 18, 2025

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  • Clarifies that concurrency is achieved using child processes, not threads

I don't think it clarifies, specifying concurrency only affects the current thread, IMO talking about "process" muddies the water.

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@JakobJingleheimer JakobJingleheimer Nov 18, 2025

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It's also not always correct: isolation none runs the tests in the main process (instead of a dedicated test process).

"Thread" is indeed wrong though, and that affects shared globals, so maybe that should be corrected. I think there may be a circumstance where it uses threads instead of processes though, but that may not be the case (anymore?). I'd have to check.

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@aduh95 aduh95 Nov 18, 2025

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Is there any case where test({ concurrency: true }, …) would result in creating multiple threads / processes? I don't think that's a thing, I feel like we're talking about different things.

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@JakobJingleheimer JakobJingleheimer Nov 18, 2025 •
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Is there any case where test({ concurrency: true }, …) would result in creating multiple threads / processes?

Processes: no, never (each test file gets its own process though)

Threads: I believe it uses Promise.allSettled when concurrency: true (when concurrency: number, I think it uses threads)

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@aduh95 aduh95 Nov 18, 2025

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Using Promise.allSettled would make more sense, because that runs in the same thread. Test files written in JS are no different for other JS files: if making a JS file multi-threaded was as easy as setting a boolean value, we would not reserve it only to tests.

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@Ethan-Arrowood Ethan-Arrowood Nov 18, 2025

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I think its just a queue of promises

node/lib/internal/test_runner/test.js

Lines 989 to 1000 in 0b6ae6d

// If there is enough available concurrency to run the test now, then do
// it. Otherwise, return a Promise to the caller and mark the test as
// pending for later execution.
this.parent.unfinishedSubtests.add(this);
this.reporter.enqueue(this.nesting, this.loc, this.name, this.reportedType);
if (this.root.harness.buildPromise || !this.parent.hasConcurrency()) {
const deferred = PromiseWithResolvers();
deferred.test = this;
this.parent.addPendingSubtest(deferred);
return deferred.promise;
}

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@Ethan-Arrowood Ethan-Arrowood Nov 18, 2025

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And to be clear we are explicitly talking about the concurrency option for a suite or test function. Such as suite('foo', { concurrency: number | boolean }, () => {}); or test('bar', { concurrency: number | boolean }, () => {});

The concurrency option for run() or the CLI args does use processes.

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@JakobJingleheimer JakobJingleheimer Nov 18, 2025

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And to be clear we are explicitly talking about the concurrency option for a suite or test function.

I know 🙂

It would probably be easiest to find the original PR that introduced it (there was specific discussion around Boolean vs number, so the answer will be there).

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@aduh95 aduh95 Nov 18, 2025

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IMO if we think talking about "thread" is bad because there's only one thread, it makes no sense to replace it with "process", for the same reason. Maybe we can find an alternative wording, e.g.:

Suggested change
then that many tests would run concurrently within the test process.
then that many tests would run asynchronously (they are still managed by the single-threaded event loop).

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@Ethan-Arrowood Ethan-Arrowood Nov 18, 2025

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IMO if we think talking about "thread" is bad because there's only one thread, it makes no sense to replace it with "process", for the same reason.

I better understand your reasoning now. I think your proposed wording is much clearer.

If `true`, all scheduled asynchronous tests run concurrently within the
thread. If `false`, only one test runs at a time.
If unspecified, subtests inherit this value from their parent.
Expand Down Expand Up @@ -3882,7 +3882,7 @@ changes:
* `options` {Object} Configuration options for the subtest. The following
properties are supported:
* `concurrency` {number|boolean|null} If a number is provided,
then that many tests would run in parallel within the application thread.
then that many tests would run concurrently within the test process.
If `true`, it would run all subtests in parallel.
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@aduh95 aduh95 Nov 18, 2025 •
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We should probably also get rid of the other mentions of "parallel" (non blocking, can also happen in a follow up PR)

Suggested change
If `true`, it would run all subtests in parallel.
If `true`, it would run all subtests asynchronously.

or

Suggested change
If `true`, it would run all subtests in parallel.
If `true`, it would run all subtests at once.

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@Ethan-Arrowood Ethan-Arrowood Nov 18, 2025

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I prefer "asynchronously"

If `false`, it would only run one test at a time.
If unspecified, subtests inherit this value from their parent.
Expand Down
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