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<!DOCTYPE html><html lang="en-US"><meta charset="utf-8">
<meta content="width=device-width,initial-scale=1,shrink-to-fit=no" name="viewport">
<meta content="#3c790a" name="theme-color">
<title>Changes to the Encoding Standard to support the Stream Standard</title>
<link href="https://resources.whatwg.org/standard.css" rel="stylesheet">
<link href="https://resources.whatwg.org/bikeshed.css" rel="stylesheet">
<link href="https://resources.whatwg.org/logo-encoding.svg" rel="icon">
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<div class="head">
<h1>Changes to the <a href="https://encoding.spec.whatwg.org">Encoding
Standard</a> to support the <a href="https://streams.spec.whatwg.org/">Stream
Standard</a></h1>
<dl>
<dt>On Github:
<dd><a href="https://github.com/ricea/encoding-streams">Repository</a>
(<a href="https://github.com/ricea/encoding-streams/issues">open issues</a>)
</dl>
</div>
<h3>In 8.1, Interface <code title="">TextDecoder</code>:</h3>
<p class="addition">In the IDL, add
<p><pre class="idl highlight def">
[<a class="nv idl-code" data-link-type="extended-attribute" href="https://heycam.github.io/webidl/#SameObject" id="ref-for-SameObject">SameObject</a>] <span class="kt">readonly</span> <span class="kt">attribute</span> <a class="n" data-link-type="idl-name" href="https://streams.spec.whatwg.org/#rs-class" id="ref-for-rs-class">ReadableStream</a> <a class="nv idl-code" data-link-type="attribute" data-readonly="" data-type="ReadableStream" href="#dom-textdecoder-readable" id="ref-for-dom-textdecoder-readable">readable</a>;
[<a class="nv idl-code" data-link-type="extended-attribute" href="https://heycam.github.io/webidl/#SameObject" id="ref-for-SameObject①">SameObject</a>] <span class="kt">readonly</span> <span class="kt">attribute</span> <a class="n" data-link-type="idl-name" href="https://streams.spec.whatwg.org/#ws-class" id="ref-for-ws-class">WritableStream</a> <a class="nv idl-code" data-link-type="attribute" data-readonly="" data-type="WritableStream" href="#dom-textdecoder-writable" id="ref-for-dom-textdecoder-writable">writable</a>;
</pre>
<p class="addition">to the TextDecoder interface.
<p class="addition">In the paragraph following the IDL, insert ", and <dfn data-dfn-for="TextDecoder" data-dfn-type="dfn" data-noexport="" id="textdecoder-transform">transform<a class="self-link" href="#textdecoder-transform"></a></dfn>." at the end of the sentence.
<p class="addition">To the DOM intro note, add
<p><dl class="domintro">
<dt><code><var>decoder</var> . <a class="idl-code" data-link-type="attribute" href="#dom-textdecoder-readable">readable</a></code>
<dd>
<p>Returns a <a data-link-type="dfn" href="https://streams.spec.whatwg.org/#readable-stream" id="ref-for-readable-stream">readable stream</a> whose <a data-link-type="dfn" href="https://streams.spec.whatwg.org/#chunk" id="ref-for-chunk">chunks</a> are strings resulting from running <a data-link-type="dfn" href="https://encoding.spec.whatwg.org/#textdecoder-encoding" id="ref-for-textdecoder-encoding②">encoding</a>’s <a data-link-type="dfn" href="https://encoding.spec.whatwg.org/#decoder" id="ref-for-decoder①①">decoder</a> on the chunks written to <code class="idl"><a data-link-type="idl" href="#dom-textdecoder-writable" id="ref-for-dom-textdecoder-writable①">writable</a></code>. </p>
<dt><code><var>decoder</var> . <a class="idl-code" data-link-type="attribute" href="#dom-textdecoder-writable">writable</a></code>
<dd>
<p>Returns a <a data-link-type="dfn" href="https://streams.spec.whatwg.org/#writable-stream" id="ref-for-writable-stream">writable stream</a> which accepts <code class="idl"><a data-link-type="idl" href="https://heycam.github.io/webidl/#BufferSource" id="ref-for-BufferSource①">BufferSource</a></code> chunks and runs them through <a data-link-type="dfn" href="https://encoding.spec.whatwg.org/#textdecoder-encoding" id="ref-for-textdecoder-encoding③">encoding</a>’s <a data-link-type="dfn" href="https://encoding.spec.whatwg.org/#decoder" id="ref-for-decoder①②">decoder</a> before making them available to <code class="idl"><a data-link-type="idl" href="#dom-textdecoder-readable" id="ref-for-dom-textdecoder-readable②">readable</a></code>. </p>
<p>Typically this will be used via the <code class="idl"><a data-link-type="idl" href="https://streams.spec.whatwg.org/#rs-pipe-through" id="ref-for-rs-pipe-through">pipeThrough()</a></code> method on a <code class="idl"><a data-link-type="idl" href="https://streams.spec.whatwg.org/#rs-class" id="ref-for-rs-class①">ReadableStream</a></code> source. </p>
<pre class="example" id="example-textdecode-writable"><a class="self-link" href="#example-textdecode-writable"></a><code class="lang-javascript highlight"><span class="kd">var</span> decoder <span class="o">=</span> <span class="k">new</span> TextDecoder<span class="p">(</span>encoding<span class="p">);</span>
byteReadable
<span class="p">.</span>pipeThrough<span class="p">(</span>decoder<span class="p">)</span>
<span class="p">.</span>pipeTo<span class="p">(</span>textWritable<span class="p">);</span></code></pre>
<p>If the <a data-link-type="dfn" href="https://encoding.spec.whatwg.org/#textdecoder-error-mode" id="ref-for-textdecoder-error-mode①">error mode</a> is "<code>fatal</code>" and <a data-link-type="dfn" href="https://encoding.spec.whatwg.org/#textdecoder-encoding" id="ref-for-textdecoder-encoding④">encoding</a>’s <a data-link-type="dfn" href="https://encoding.spec.whatwg.org/#decoder" id="ref-for-decoder①③">decoder</a> returns <a data-link-type="dfn" href="https://encoding.spec.whatwg.org/#error" id="ref-for-error④">error</a>, both <code class="idl"><a data-link-type="idl" href="#dom-textdecoder-readable" id="ref-for-dom-textdecoder-readable③">readable</a></code> and <code class="idl"><a data-link-type="idl" href="#dom-textdecoder-writable" id="ref-for-dom-textdecoder-writable③">writable</a></code> will be errored with a <code class="idl"><a data-link-type="idl" href="https://heycam.github.io/webidl/#exceptiondef-typeerror" id="ref-for-exceptiondef-typeerror">TypeError</a></code>. </p>
</dl>
<p class="addition">To the <dfn class="idl-code" data-dfn-for="TextDecoder" data-dfn-type="constructor" data-export="" id="dom-textdecoder"><code>TextDecoder(<var>label</var>, <var>options</var>)</code><a class="self-link" href="#dom-textdecoder"></a></dfn> constructor steps, before the final step, add:
<ol start="7">
<li>
<p>Let <var>decForTransform</var> be a new <code class="idl"><a data-link-type="idl" href="https://encoding.spec.whatwg.org/#textdecoder" id="ref-for-textdecoder⑤">TextDecoder</a></code> object. </p>
<li>
<p>Set <var>decForTransform</var>’s <a data-link-type="dfn" href="https://encoding.spec.whatwg.org/#textdecoder-encoding" id="ref-for-textdecoder-encoding⑧">encoding</a> to <var>encoding</var>. </p>
<li>
<p>Set <var>decForTransform</var>’s <a data-link-type="dfn" href="https://encoding.spec.whatwg.org/#textdecoder-error-mode" id="ref-for-textdecoder-error-mode④">error mode</a> to <var>dec</var>’s <a data-link-type="dfn" href="https://encoding.spec.whatwg.org/#textdecoder-error-mode" id="ref-for-textdecoder-error-mode⑤">error mode</a>. </p>
<li>
<p>Set <var>decForTransform</var>’s <a data-link-type="dfn" href="https://encoding.spec.whatwg.org/#textdecoder-ignore-bom-flag" id="ref-for-textdecoder-ignore-bom-flag③">ignore BOM flag</a> to <var>dec</var>’s <a data-link-type="dfn" href="https://encoding.spec.whatwg.org/#textdecoder-ignore-bom-flag" id="ref-for-textdecoder-ignore-bom-flag④">ignore BOM flag</a>. </p>
<li>
<p>Set <var>decForTransform</var>’s <a data-link-type="dfn" href="https://encoding.spec.whatwg.org/#textdecoder-decoder" id="ref-for-textdecoder-decoder">decoder</a> to a new <a data-link-type="dfn" href="https://encoding.spec.whatwg.org/#decoder" id="ref-for-decoder①⑥">decoder</a> for <var>decForTransform</var>’s <a data-link-type="dfn" href="https://encoding.spec.whatwg.org/#textdecoder-encoding" id="ref-for-textdecoder-encoding⑨">encoding</a>, and set <var>decForTransform</var>’s <a data-link-type="dfn" href="https://encoding.spec.whatwg.org/#textdecoder-stream" id="ref-for-textdecoder-stream">stream</a> to a new <a data-link-type="dfn" href="https://encoding.spec.whatwg.org/#concept-stream" id="ref-for-concept-stream①④">stream</a>. </p>
<p class="note" role="note">For simplicity, <var>dec</var> and <var>decForTransform</var> have redundant
members. However, the <a data-link-type="dfn" href="https://encoding.spec.whatwg.org/#textdecoder-bom-seen-flag" id="ref-for-textdecoder-bom-seen-flag③">BOM seen flag</a>, <a data-link-type="dfn" href="https://encoding.spec.whatwg.org/#textdecoder-do-not-flush-flag" id="ref-for-textdecoder-do-not-flush-flag">do not flush
flag</a>, and <a data-link-type="dfn" href="#textdecoder-transform" id="ref-for-textdecoder-transform">transform</a> are unused in <var>decForTransform</var>, and <a data-link-type="dfn" href="https://encoding.spec.whatwg.org/#textdecoder-encoding" id="ref-for-textdecoder-encoding①⓪">encoding</a>, <a data-link-type="dfn" href="https://encoding.spec.whatwg.org/#textdecoder-ignore-bom-flag" id="ref-for-textdecoder-ignore-bom-flag⑤">ignore BOM flag</a> and <a data-link-type="dfn" href="https://encoding.spec.whatwg.org/#textdecoder-error-mode" id="ref-for-textdecoder-error-mode⑥">error mode</a> are identical to <var>dec</var>. It is not necessary for
implementations to duplicate these member fields. </p>
<li>
<p>Let <var>startAlgorithm</var> be an algorithm that takes no arguments and returns nothing. </p>
<li>
<p>Let <var>transformAlgorithm</var> be an algorithm which takes a <var>chunk</var> argument
and runs the <a data-link-type="dfn" href="#concept-td-decode-and-enqueue" id="ref-for-concept-td-decode-and-enqueue">decode and enqueue a chunk</a> algorithm with <var>decForTransform</var> and <var>chunk</var>. </p>
<li>
<p>Let <var>flushAlgorithm</var> be an algorithm which takes no arguments and runs the <a data-link-type="dfn" href="#concept-td-flush-and-enqueue" id="ref-for-concept-td-flush-and-enqueue">flush
and enqueue</a> algorithm with <var>decForTransform</var>. </p>
<li>
<p>Let <var>transform</var> be the result of calling <a data-link-type="abstract-op" href="https://streams.spec.whatwg.org/#create-transform-stream" id="ref-for-create-transform-stream">CreateTransformStream</a>(<var>startAlgorithm</var>, <var>transformAlgorithm</var>, <var>flushAlgorithm</var>). </p>
<li>
<p>Set <var>dec</var>’s <a data-link-type="dfn" href="#textdecoder-transform" id="ref-for-textdecoder-transform①">transform</a> to <var>transform</var>. </p>
</ol>
<p class="addition">After the description of the ignoreBOM attribute, add:
<p>The <dfn class="idl-code" data-dfn-for="TextDecoder" data-dfn-type="attribute" data-export="" id="dom-textdecoder-readable"><code>readable</code><a class="self-link" href="#dom-textdecoder-readable"></a></dfn> attribute’s getter must return <a data-link-type="dfn" href="#textdecoder-transform" id="ref-for-textdecoder-transform①">transform</a>.[[readable]]. </p>
<p>The <dfn class="idl-code" data-dfn-for="TextDecoder" data-dfn-type="attribute" data-export="" id="dom-textdecoder-writable"><code>writable</code><a class="self-link" href="#dom-textdecoder-writable"></a></dfn> attribute’s getter must return <a data-link-type="dfn" href="#textdecoder-transform" id="ref-for-textdecoder-transform②">transform</a>.[[writable]]. </p>
<p class="addition">After the definition of the decode() method, add:
<p>The <dfn class="dfn-paneled" data-dfn-type="dfn" data-noexport="" id="concept-td-decode-and-enqueue">decode and enqueue a chunk</dfn> algorithm, given a <code class="idl"><a data-link-type="idl" href="https://encoding.spec.whatwg.org/#textdecoder" id="ref-for-textdecoder⑥">TextDecoder</a></code> <var>decForTransform</var> and a <var>chunk</var>, runs these steps: </p>
<ol>
<li>
<p>If <a data-link-type="dfn" href="https://tc39.github.io/ecma262/#sec-ecmascript-data-types-and-values" id="ref-for-sec-ecmascript-data-types-and-values">Type</a>(<var>chunk</var>) is not Object, or <var>chunk</var> does not have an
[[ArrayBufferData]] internal slot, or <a data-link-type="abstract-op" href="https://tc39.github.io/ecma262/#sec-isdetachedbuffer" id="ref-for-sec-isdetachedbuffer">IsDetachedBuffer</a>(<var>chunk</var>) is
true, or <a data-link-type="abstract-op" href="https://tc39.github.io/ecma262/#sec-issharedarraybuffer" id="ref-for-sec-issharedarraybuffer">IsSharedArrayBuffer</a>(<var>chunk</var>) is true, then return a new
promise rejected with a <code class="idl"><a data-link-type="idl" href="https://heycam.github.io/webidl/#exceptiondef-typeerror" id="ref-for-exceptiondef-typeerror③">TypeError</a></code> exception. </p>
<li>
<p><a data-link-type="dfn" href="https://encoding.spec.whatwg.org/#concept-stream-push" id="ref-for-concept-stream-push③">Push</a> a <a data-link-type="dfn" href="https://heycam.github.io/webidl/#dfn-get-buffer-source-copy" id="ref-for-dfn-get-buffer-source-copy①">copy of</a> <var>chunk</var> to <var>decForTransform</var>’s <a data-link-type="dfn" href="https://encoding.spec.whatwg.org/#textdecoder-stream" id="ref-for-textdecoder-stream⑤">stream</a>. </p>
<li>
<p>Let <var>controller</var> be <var>decForTransform</var>’s <a data-link-type="dfn" href="#textdecoder-transform" id="ref-for-textdecoder-transform④">transform</a>.[[transformStreamController]]. </p>
<li>
<p>Let <var>output</var> be a new <a data-link-type="dfn" href="https://encoding.spec.whatwg.org/#concept-stream" id="ref-for-concept-stream①⑦">stream</a>. </p>
<li>
<p>While true, run these steps: </p>
<ol>
<li>
<p>Let <var>token</var> be the result of <a data-link-type="dfn" href="https://encoding.spec.whatwg.org/#concept-stream-read" id="ref-for-concept-stream-read⑥">reading</a> from <var>decForTransform</var>’s <a data-link-type="dfn" href="https://encoding.spec.whatwg.org/#textdecoder-stream" id="ref-for-textdecoder-stream⑥">stream</a>. </p>
<li>
<p>If <var>token</var> is <a data-link-type="dfn" href="https://encoding.spec.whatwg.org/#end-of-stream" id="ref-for-end-of-stream⑥">end-of-stream</a>, then run these steps: </p>
<ol>
<li>
<p>Let <var>outputChunk</var> be <var>output</var>, <a data-link-type="dfn" href="https://encoding.spec.whatwg.org/#concept-td-serialize" id="ref-for-concept-td-serialize②">serialized</a>. </p>
<li>
<p>Call <a data-link-type="abstract-op" href="https://streams.spec.whatwg.org/#transform-stream-default-controller-enqueue" id="ref-for-transform-stream-default-controller-enqueue">TransformStreamDefaultControllerEnqueue</a>(<var>controller</var>, <var>outputChunk</var>). </p>
<li>
<p>Return a new promise resolved with undefined. </p>
</ol>
<li>
<p>Let <var>result</var> be the result of <a data-link-type="dfn" href="https://encoding.spec.whatwg.org/#concept-encoding-process" id="ref-for-concept-encoding-process②">processing</a> <var>token</var> for <var>decForTransform</var>’s <a data-link-type="dfn" href="https://encoding.spec.whatwg.org/#textdecoder-decoder" id="ref-for-textdecoder-decoder④">decoder</a>, <var>decForTransform</var>’s <a data-link-type="dfn" href="https://encoding.spec.whatwg.org/#textdecoder-stream" id="ref-for-textdecoder-stream⑦">stream</a>, <var>output</var>, and <var>decForTransform</var>’s <a data-link-type="dfn" href="https://encoding.spec.whatwg.org/#textdecoder-error-mode" id="ref-for-textdecoder-error-mode⑨">error mode</a>. </p>
<li>
<p>If <var>result</var> is <a data-link-type="dfn" href="https://encoding.spec.whatwg.org/#error" id="ref-for-error⑦">error</a>, then return a new promise rejected with a <code class="idl"><a data-link-type="idl" href="https://heycam.github.io/webidl/#exceptiondef-typeerror" id="ref-for-exceptiondef-typeerror④">TypeError</a></code> exception. </p>
</ol>
</ol>
<p>The <dfn class="dfn-paneled" data-dfn-type="dfn" data-noexport="" id="concept-td-flush-and-enqueue">flush and enqueue</dfn> algorithm, which handles the end
of data from the input <code class="idl"><a data-link-type="idl" href="https://streams.spec.whatwg.org/#rs-class" id="ref-for-rs-class②">ReadableStream</a></code>, given a <code class="idl"><a data-link-type="idl" href="https://encoding.spec.whatwg.org/#textdecoder" id="ref-for-textdecoder⑦">TextDecoder</a></code> <var>decForTransform</var>, runs
these steps: </p>
<ol>
<li>
<p>Let <var>output</var> be a new <a data-link-type="dfn" href="https://encoding.spec.whatwg.org/#concept-stream" id="ref-for-concept-stream①⑧">stream</a>. </p>
<li>
<p>Let <var>result</var> be the result of <a data-link-type="dfn" href="https://encoding.spec.whatwg.org/#concept-encoding-process" id="ref-for-concept-encoding-process③">processing</a> <a data-link-type="dfn" href="https://encoding.spec.whatwg.org/#end-of-stream" id="ref-for-end-of-stream⑦">end-of-stream</a> for <var>decForTransform</var>’s <a data-link-type="dfn" href="https://encoding.spec.whatwg.org/#textdecoder-decoder" id="ref-for-textdecoder-decoder⑤">decoder</a> and <var>decForTransform</var>’s <a data-link-type="dfn" href="https://encoding.spec.whatwg.org/#textdecoder-stream" id="ref-for-textdecoder-stream⑧">stream</a>, <var>output</var>, and <var>decForTransform</var>’s <a data-link-type="dfn" href="https://encoding.spec.whatwg.org/#textdecoder-error-mode" id="ref-for-textdecoder-error-mode①⓪">error mode</a>. </p>
<li>
<p>If <var>result</var> is <a data-link-type="dfn" href="https://encoding.spec.whatwg.org/#finished" id="ref-for-finished②">finished</a>, then run these steps: </p>
<ol>
<li>
<p>Let <var>outputChunk</var> be <var>output</var>, <a data-link-type="dfn" href="https://encoding.spec.whatwg.org/#concept-td-serialize" id="ref-for-concept-td-serialize③">serialized</a>. </p>
<li>
<p>Let <var>controller</var> be <var>decForTransform</var>’s <a data-link-type="dfn" href="#textdecoder-transform" id="ref-for-textdecoder-transform⑤">transform</a>.[[transformStreamController]]. </p>
<li>
<p>Call <a data-link-type="abstract-op" href="https://streams.spec.whatwg.org/#transform-stream-default-controller-enqueue" id="ref-for-transform-stream-default-controller-enqueue①">TransformStreamDefaultControllerEnqueue</a>(<var>controller</var>, <var>outputChunk</var>). </p>
<li>
<p>Return a new promise resolved with undefined. </p>
</ol>
<li>
<p>Otherwise, return a new promise rejected with a <code class="idl"><a data-link-type="idl" href="https://heycam.github.io/webidl/#exceptiondef-typeerror" id="ref-for-exceptiondef-typeerror⑤">TypeError</a></code> exception. </p>
</ol>
<h3>In 8.2 Interface <code title="">TextEncoder</code>, add:</h3>
<p class="addition">In the IDL, add
<p><pre class="idl highlight def">
[<a class="nv idl-code" data-link-type="extended-attribute" href="https://heycam.github.io/webidl/#SameObject" id="ref-for-SameObject②">SameObject</a>] <span class="kt">readonly</span> <span class="kt">attribute</span> <a class="n" data-link-type="idl-name" href="https://streams.spec.whatwg.org/#rs-class" id="ref-for-rs-class③">ReadableStream</a> <a class="nv idl-code" data-link-type="attribute" data-readonly="" data-type="ReadableStream" href="#dom-textencoder-readable" id="ref-for-dom-textencoder-readable">readable</a>;
[<a class="nv idl-code" data-link-type="extended-attribute" href="https://heycam.github.io/webidl/#SameObject" id="ref-for-SameObject③">SameObject</a>] <span class="kt">readonly</span> <span class="kt">attribute</span> <a class="n" data-link-type="idl-name" href="https://streams.spec.whatwg.org/#ws-class" id="ref-for-ws-class①">WritableStream</a> <a class="nv idl-code" data-link-type="attribute" data-readonly="" data-type="WritableStream" href="#dom-textencoder-writable" id="ref-for-dom-textencoder-writable">writable</a>;
</pre>
<p class="addition">to the TextEncoder interface.
<p class="addition">In the paragraph following the IDL, insert " and <dfn data-dfn-for="TextEncoder" data-dfn-type="dfn" data-noexport="" id="textencoder-transform">transform<a class="self-link" href="#textencoder-transform"></a></dfn> and <dfn data-dfn-for="TextEncoder" data-dfn-type="dfn" data-lt="pending high surrogate" data-noexport="" id="textencoder-pending-high-surrogate">pending high
surrogate<a class="self-link" href="#textencoder-pending-high-surrogate"></a></dfn> (initially null)" at the end of the sentence.
<p class="addition">To the DOM intro note, add
<p><dl class="domintro">
<dt><code><var>encoder</var> . <a class="idl-code" data-link-type="attribute" href="#dom-textencoder-readable">readable</a></code>
<dd>
<p>Returns a <a data-link-type="dfn" href="https://streams.spec.whatwg.org/#readable-stream" id="ref-for-readable-stream①">readable stream</a> whose <a data-link-type="dfn" href="https://streams.spec.whatwg.org/#chunk" id="ref-for-chunk①">chunks</a> are <code class="idl"><a data-link-type="idl" href="https://heycam.github.io/webidl/#idl-Uint8Array" id="ref-for-idl-Uint8Array②">Uint8Array</a></code>s resulting from running <a data-link-type="dfn" href="https://encoding.spec.whatwg.org/#utf-8" id="ref-for-utf-8①⑤">UTF-8</a>’s <a data-link-type="dfn" href="https://encoding.spec.whatwg.org/#encoder" id="ref-for-encoder⑨">encoder</a> on the chunks written to <code class="idl"><a data-link-type="idl" href="#dom-textencoder-writable" id="ref-for-dom-textencoder-writable①">writable</a></code>. </p>
<dt><code><var>encoder</var> . <a class="idl-code" data-link-type="attribute" href="#dom-textencoder-writable" id="ref-for-dom-textencoder-writable②">writable</a></code>
<dd>
<p>Returns a <a data-link-type="dfn" href="https://streams.spec.whatwg.org/#writable-stream" id="ref-for-writable-stream①">writable stream</a> which accepts string chunks and runs them through <a data-link-type="dfn" href="https://encoding.spec.whatwg.org/#utf-8" id="ref-for-utf-8①⑥">UTF-8</a>’s <a data-link-type="dfn" href="https://encoding.spec.whatwg.org/#encoder" id="ref-for-encoder①⓪">encoder</a> before making them available to <code class="idl"><a data-link-type="idl" href="#dom-textencoder-readable" id="ref-for-dom-textencoder-readable②">readable</a></code>. </p>
<p>Typically this will be used via the <code class="idl"><a data-link-type="idl" href="https://streams.spec.whatwg.org/#rs-pipe-through" id="ref-for-rs-pipe-through①">pipeThrough()</a></code> method on a <code class="idl"><a data-link-type="idl" href="https://streams.spec.whatwg.org/#rs-class" id="ref-for-rs-class④">ReadableStream</a></code> source. </p>
<pre class="example" id="example-textencode-writable"><a class="self-link" href="#example-textencode-writable"></a><code class="lang-javascript highlight">textReadable
<span class="p">.</span>pipeThrough<span class="p">(</span><span class="k">new</span> TextEncoder<span class="p">())</span>
<span class="p">.</span>pipeTo<span class="p">(</span>byteWritable<span class="p">);</span></code></pre>
</dl>
<p class="addition">To the <dfn class="idl-code" data-dfn-for="TextEncoder" data-dfn-type="constructor" data-export="" id="dom-textencoder"><code>TextEncoder()</code><a class="self-link" href="#dom-textencoder"></a></dfn> constructor steps, before the final step, add:
<ol start="3">
<li>
<p>Let <var>encForTransform</var> be a new <code class="idl"><a data-link-type="idl" href="https://encoding.spec.whatwg.org/#textencoder" id="ref-for-textencoder⑥">TextEncoder</a></code> object. </p>
<li>
<p>Set <var>encForTransform</var>’s <a data-link-type="dfn" href="https://encoding.spec.whatwg.org/#textencoder-encoder" id="ref-for-textencoder-encoder①">encoder</a> to <a data-link-type="dfn" href="https://encoding.spec.whatwg.org/#utf-8" id="ref-for-utf-8①⑨">UTF-8</a>’s <a data-link-type="dfn" href="https://encoding.spec.whatwg.org/#encoder" id="ref-for-encoder①③">encoder</a>. </p>
<p class="note" role="note">For simplicity, <var>enc</var> and <var>encForTransform</var> have the same
members. However, <a data-link-type="dfn" href="#textencoder-transform" id="ref-for-textencoder-transform">transform</a> is not used by <var>encForTransform</var> and <a data-link-type="dfn" href="#textencoder-pending-high-surrogate" id="ref-for-textencoder-pending-high-surrogate">pending high surrogate</a> is not used by <var>enc</var>. It is not necessary
for implementations to store these unused member fields. </p>
<li>
<p>Let <var>startAlgorithm</var> be an algorithm that takes no arguments and returns nothing. </p>
<li>
<p>Let <var>transformAlgorithm</var> be an algorithm which takes a <var>chunk</var> argument
and runs the <a data-link-type="dfn" href="#concept-te-encode-and-enqueue" id="ref-for-concept-te-encode-and-enqueue">encode and enqueue a chunk</a> algorithm with <var>encForTransform</var> and <var>chunk</var>. </p>
<li>
<p>Let <var>flushAlgorithm</var> be an algorithm which runs the <a data-link-type="dfn" href="#concept-te-encode-and-flush" id="ref-for-concept-te-encode-and-flush">encode and flush</a> algorithm with <var>encForTransform</var>. </p>
<li>
<p>Let <var>transform</var> be the result of calling <a data-link-type="abstract-op" href="https://streams.spec.whatwg.org/#create-transform-stream" id="ref-for-create-transform-stream①">CreateTransformStream</a>(<var>startAlgorithm</var>, <var>transformAlgorithm</var>, <var>flushAlgorithm</var>). </p>
<li>
<p>Set <var>enc</var>’s <a data-link-type="dfn" href="#textencoder-transform" id="ref-for-textencoder-transform①">transform</a> to <var>transform</var>. </p>
</ol>
<p class="addition">After the description of the encoding attribute, add:
<p>The <dfn class="idl-code" data-dfn-for="TextEncoder" data-dfn-type="attribute" data-export="" id="dom-textencoder-readable"><code>readable</code><a class="self-link" href="#dom-textencoder-readable"></a></dfn> attribute’s getter must return <a data-link-type="dfn" href="#textencoder-transform" id="ref-for-textencoder-transform①">transform</a>.[[readable]]. </p>
<p>The <dfn class="idl-code" data-dfn-for="TextEncoder" data-dfn-type="attribute" data-export="" id="dom-textencoder-writable"><code>writable</code><a class="self-link" href="#dom-textencoder-writable"></a></dfn> attribute’s getter must return <a data-link-type="dfn" href="#textencoder-transform" id="ref-for-textencoder-transform②">transform</a>.[[writable]]. </p>
<p class="addition">After the definition of the encode() method, add:
<p>The <dfn class="dfn-paneled" data-dfn-type="dfn" data-noexport="" id="concept-te-encode-and-enqueue">encode and enqueue a chunk</dfn> algorithm, given a <code class="idl"><a data-link-type="idl" href="https://encoding.spec.whatwg.org/#textencoder" id="ref-for-textencoder⑦">TextEncoder</a></code> <var>encForTransform</var> and <var>chunk</var>, runs these steps: </p>
<ol>
<li>
<p>Let <var>input</var> be the result of <a data-link-type="dfn" href="https://heycam.github.io/webidl/#dfn-convert-ecmascript-to-idl-value" id="ref-for-dfn-convert-ecmascript-to-idl-value">converting</a> <var>chunk</var> to a <code class="idl"><a data-link-type="idl" href="https://heycam.github.io/webidl/#idl-DOMString" id="ref-for-idl-DOMString③">DOMString</a></code>. If this throws an exception, then return a promise rejected
with that exception. </p>
<li>
<p>Convert <var>input</var> to a <a data-link-type="dfn" href="https://encoding.spec.whatwg.org/#concept-stream" id="ref-for-concept-stream②①">stream</a>. </p>
<li>
<p>Let <var>output</var> be a new <a data-link-type="dfn" href="https://encoding.spec.whatwg.org/#concept-stream" id="ref-for-concept-stream②②">stream</a>. </p>
<li>
<p>Let <var>controller</var> be <var>encForTransform</var>’s <a data-link-type="dfn" href="#textencoder-transform" id="ref-for-textencoder-transform④">transform</a>.[[transformStreamController]]. </p>
<li>
<p>While true, run these steps: </p>
<ol>
<li>
<p>Let <var>token</var> be the result of <a data-link-type="dfn" href="https://encoding.spec.whatwg.org/#concept-stream-read" id="ref-for-concept-stream-read⑧">reading</a> from <var>input</var>. </p>
<li>
<p>If <var>token</var> is <a data-link-type="dfn" href="https://encoding.spec.whatwg.org/#end-of-stream" id="ref-for-end-of-stream⑧">end-of-stream</a>, then run these steps: </p>
<ol>
<li>
<p>Convert <var>output</var> into a byte sequence. </p>
<li>
<p>Let <var>chunk</var> be a <code class="idl"><a data-link-type="idl" href="https://heycam.github.io/webidl/#idl-Uint8Array" id="ref-for-idl-Uint8Array④">Uint8Array</a></code> object wrapping an <code class="idl"><a data-link-type="idl" href="https://heycam.github.io/webidl/#idl-ArrayBuffer" id="ref-for-idl-ArrayBuffer③">ArrayBuffer</a></code> containing <var>output</var>. </p>
<li>
<p>Call <a data-link-type="abstract-op" href="https://streams.spec.whatwg.org/#transform-stream-default-controller-enqueue" id="ref-for-transform-stream-default-controller-enqueue②">TransformStreamDefaultControllerEnqueue</a>(<var>controller</var>, <var>chunk</var>). </p>
<li>
<p>Return a new promise resolved with undefined. </p>
</ol>
<li>
<p>Let <var>result</var> be the result of executing the <a data-link-type="dfn" href="#concept-te-convert-code-unit-to-scalar-value" id="ref-for-concept-te-convert-code-unit-to-scalar-value">convert code unit to scalar
value</a> algorithm with <var>encForTransform</var>, <var>token</var> and <var>input</var>. </p>
<li>
<p>If <var>result</var> is not <a data-link-type="dfn" href="https://encoding.spec.whatwg.org/#continue" id="ref-for-continue③">continue</a>, then <a data-link-type="dfn" href="https://encoding.spec.whatwg.org/#concept-encoding-process" id="ref-for-concept-encoding-process⑤">process</a> <var>result</var> for <a data-link-type="dfn" href="https://encoding.spec.whatwg.org/#textencoder-encoder" id="ref-for-textencoder-encoder③">encoder</a>, <var>input</var>, <var>output</var>. </p>
</ol>
</ol>
<p>The <dfn class="dfn-paneled" data-dfn-type="dfn" data-noexport="" id="concept-te-convert-code-unit-to-scalar-value">convert code unit to scalar value</dfn> algorithm, given a <code class="idl"><a data-link-type="idl" href="https://encoding.spec.whatwg.org/#textencoder" id="ref-for-textencoder⑧">TextEncoder</a></code> <var>encForTransform</var>, <var>token</var> and <var>input</var> stream, runs these steps: </p>
<ol>
<li>
<p>If <var>encForTransform</var>’s <a data-link-type="dfn" href="#textencoder-pending-high-surrogate" id="ref-for-textencoder-pending-high-surrogate①">pending high surrogate</a> is non-null, then run these
steps: </p>
<ol>
<li>
<p>Let <var>high surrogate</var> be <var>encForTransform</var>’s <a data-link-type="dfn" href="#textencoder-pending-high-surrogate" id="ref-for-textencoder-pending-high-surrogate②">pending high
surrogate</a>. </p>
<li>
<p>Set <var>encForTransform</var>’s <a data-link-type="dfn" href="#textencoder-pending-high-surrogate" id="ref-for-textencoder-pending-high-surrogate③">pending high surrogate</a> to null. </p>
<li>
<p>If <var>token</var> is in the range U+DC00 to U+DFFF, inclusive, then return a code point
whose value is 0x10000 + ((<var>high surrogate</var> − 0xD800) << 10) +
(<var>token</var> − 0xDC00). </p>
<li>
<p><a data-link-type="dfn" href="https://encoding.spec.whatwg.org/#concept-stream-prepend" id="ref-for-concept-stream-prepend④">Prepend</a> <var>token</var> to <var>input</var>. </p>
<li>
<p>Return U+FFFD. </p>
</ol>
<li>
<p>If <var>token</var> is in the range U+D800 to U+DBFF, inclusive, then set <a data-link-type="dfn" href="#textencoder-pending-high-surrogate" id="ref-for-textencoder-pending-high-surrogate④">pending high
surrogate</a> to <var>token</var> and return <a data-link-type="dfn" href="https://encoding.spec.whatwg.org/#continue" id="ref-for-continue④">continue</a>. </p>
<li>
<p>If <var>token</var> is in the range U+DC00 to U+DFFF, inclusive, then return U+FFFD. </p>
<li>
<p>Return <var>token</var>. </p>
</ol>
<p class="note" role="note">This is equivalent to the "<a data-link-type="dfn" href="https://infra.spec.whatwg.org/#javascript-string-convert" id="ref-for-javascript-string-convert">convert</a> a <a data-link-type="dfn" href="https://infra.spec.whatwg.org/#javascript-string" id="ref-for-javascript-string">JavaScript string</a> into a <a data-link-type="dfn" href="https://infra.spec.whatwg.org/#scalar-value-string" id="ref-for-scalar-value-string">scalar
value string</a>" algorithm from the Infra Standard, but allows for surrogate pairs that are split
between strings. </p>
<p>The <dfn class="dfn-paneled" data-dfn-type="dfn" data-noexport="" id="concept-te-encode-and-flush">encode and flush</dfn> algorithm, given a <code class="idl"><a data-link-type="idl" href="https://encoding.spec.whatwg.org/#textencoder" id="ref-for-textencoder⑨">TextEncoder</a></code> <var>encForTransform</var>, runs these steps: </p>
<ol>
<li>
<p>If <var>encForTransform</var>’s <a data-link-type="dfn" href="#textencoder-pending-high-surrogate" id="ref-for-textencoder-pending-high-surrogate⑤">pending high surrogate</a> is non-null, then run these
steps: </p>
<ol>
<li>
<p>Let <var>controller</var> be <var>encForTransform</var>’s <a data-link-type="dfn" href="#textencoder-transform" id="ref-for-textencoder-transform⑤">transform</a>.[[transformStreamController]]. </p>
<li>
<p>Let <var>output</var> be the byte sequence 0xEF 0xBF 0xBD. </p>
<p class="note" role="note">This is the replacement character U+FFFD encoded as UTF-8. </p>
<li>
<p>Let <var>chunk</var> be a <code class="idl"><a data-link-type="idl" href="https://heycam.github.io/webidl/#idl-Uint8Array" id="ref-for-idl-Uint8Array⑤">Uint8Array</a></code> object wrapping an <code class="idl"><a data-link-type="idl" href="https://heycam.github.io/webidl/#idl-ArrayBuffer" id="ref-for-idl-ArrayBuffer④">ArrayBuffer</a></code> containing <var>output</var>. </p>
<li>
<p>Call <a data-link-type="abstract-op" href="https://streams.spec.whatwg.org/#transform-stream-default-controller-enqueue" id="ref-for-transform-stream-default-controller-enqueue③">TransformStreamDefaultControllerEnqueue</a>(<var>controller</var>, <var>chunk</var>). </p>
</ol>
<li>
<p>Return a new promise resolved with undefined. </p>
</ol>