Split out some algorithms and define how to obtain 1024 bytes

fetch.bs

@@ -2950,9 +2950,10 @@ and a <a for=/>response</a> <var>response</var>, is to run these steps:
  false.
  <!-- TODO Integrate https://wicg.github.io/background-fetch/#validate-a-partial-response into Fetch -->
 
- <li><p>Wait for 1024 bytes of <var>response</var>'s <a for=response>body</a> or end-of-file,
- whichever comes first and let <var>bytes</var> be those bytes.
- <!-- TODO Obtaining these bytes needs to be defined in terms of a transform stream. -->
+ <li><p>Let <var>bytes</var> be the result of running
+ <a>obtain a copy of the first 1024 bytes of response</a> given <var>response</var>.
+
+ <li><p>If <var>bytes</var> is failure, then return false.
 
  <li>
   <p>If the <a>audio or video type pattern matching algorithm</a> given <var>bytes</var> does not
@@ -2990,6 +2991,79 @@ and a <a for=/>response</a> <var>response</var>, is to run these steps:
  <li><p>If <var>mimeType</var>'s <a for="MIME type">essence</a> <a for=string>starts with</a>
  "<code>audio/</code>", "<code>image/</code>", or "<code>video/</code>", then return false.
 
+ <li><p>Return <a>determine if response is JavaScript and not JSON</a> given <var>response</var>.
+</ol>
+
+<hr>
+
+<p>To <dfn>obtain a copy of the first 1024 bytes of response</dfn>, given a <a for=/>response</a>
+<var>response</var>, run these  steps:
+
+<ol>
+ <li><p>Let <var>first1024Bytes</var> be null.
+
+ <li>
+  <p><a for=/>In parallel</a>:
+
+  <ol>
+   <li><p>Let <var>bytes</var> be the empty <a for=/>byte sequence</a>.
+
+   <li><p>Let <var>transformStream</var> be a new {{TransformStream}}.
+
+   <li>
+    <p>Let <var>transformAlgorithm</var> given a <var>chunk</var> be these steps:
+
+    <ol>
+     <li><p><a for=ReadableStream>Enqueue</a> <var>chunk</var> in <var>transformStream</var>.
+
+     <li>
+      <p>If <var>first1024Bytes</var> is null, then:
+
+      <ol>
+       <li><p>Let <var>chunkBytes</var> be
+       <a lt="get a copy of the bytes held by the buffer source">a copy of the bytes held by</a>
+       <var>chunk</var>.
+
+       <li><p>Append <var>chunkBytes</var> to <var>bytes</var>.
+
+       <li>
+        <p>If <var>bytes</var>'s <a for="byte sequencue">length</a> is greater than 1024, then:
+
+        <ol>
+         <li><p>Truncate <var>bytes</var> from the end so that it only contains 1024 bytes.
+
+         <li><p>Set <var>first1024Bytes</var> to <var>bytes</var>.
+        </ol>
+      </ol>
+    </ol>
+
+   <li><p>Let <var>flushAlgorithm</var> be this step: if <var>first1024Bytes</var> is null, then set
+   <var>first1024Bytes</var> to <var>bytes</var>.
+
+   <li><p><a for=TransformStream>Set up</a> <var>transformStream</var> with
+   <a for="TransformStream/set up"><i>transformAlgorithm</i></a> set to
+   <var>transformAlgorithm</var> and <a for="TransformStream/set up"><i>flushAlgorithm</i></a> set
+   to <var>flushAlgorithm</var>.
+
+   <li><p>Set <var>response</var>'s <a for=response>body</a>'s <a for=body>stream</a> to the result
+   of <var>response</var>'s <a for=response>body</a>'s <a for=body>stream</a>
+   <a for=TransformStream>piped through</a> <var>transformStream</var>.
+  </ol>
+
+ <li><p>Wait until <var>first1024Bytes</var> is non-null or <var>response</var>'s
+ <a for=response>body</a>'s <a for=body>stream</a> is <a for=ReadableStream>errored</a>.
+
+ <li><p>If <var>first1024Bytes</var> is null, then return failure.
+
+ <li>Return <var>first1024Bytes</var>.
+</ol>
+
+<hr>
+
+<p>To <dfn>determine if response is JavaScript and not JSON</dfn> given a <a for=/>response</a>
+<var>response</var>, run these steps:</p>
+
+<ol>
  <li><p>Let <var>responseBodyBytes</var> be null.
 
  <li>
@@ -3013,8 +3087,11 @@ and a <a for=/>response</a> <var>response</var>, is to run these steps:
 
  <li><p><a for=/>Assert</a>: <var>responseBodyBytes</var> is a <a for=/>byte sequence</a>.
 
- <li><p>If <a>parse JSON bytes to a JavaScript value</a> given <var>responseBodyBytes</var> does not
- throw, then return false. If it throws, catch the exception and ignore it.
+ <li>
+  <p>If <a>parse JSON bytes to a JavaScript value</a> given <var>responseBodyBytes</var> does not
+  throw, then return false. If it throws, catch the exception and ignore it.
+
+  <p class=note>If there is an exception, <var>response</var> is not JSON. If there is not, it is.
 
  <li><p>Let <var>sourceText</var> be the result of <a for=/>decoding</a>
  <var>responseBodyBytes</var> given <var>request</var>'s