Revision 938251 of Generator comprehensions

<div class="warning"><strong>Non-standard. Do not use!</strong><br />
The generator comprehensions is non-standard, and it's unlikely to be added to ECMAScript,&nbsp; For future-facing usages, consider using {{jsxref("Statements/function*", "generator", "", 1)}}.</div>

<div>{{jsSidebar("Operators")}}</div>

<p>The <strong>generator comprehension</strong> syntax is a JavaScript expression which allows you to quickly assemble a new generator function based on an existing iterable object. Comprehensions exist in many programming languages.</p>

<p>See <a href="#Differences_to_the_older_JS1.7.2FJS1.8_comprehensions">below</a> for differences to the old generator expression syntax in SpiderMonkey, based on proposals for ECMAScript 4.</p>

<h2 id="Syntax">Syntax</h2>

<pre class="syntaxbox">
(for (x of iterable) x)
(for (x of iterable) if (condition) x)
(for (x of iterable) for (y of iterable) x + y)
</pre>

<h2 id="Description">Description</h2>

<p>Inside generator comprehensions, these two kinds of components are allowed:</p>

<ul>
 <li>{{jsxref("Statements/for...of", "for...of")}} and</li>
 <li>{{jsxref("Statements/if...else", "if")}}</li>
</ul>

<p>The <code>for-of</code> iteration is always the first component. Multiple <code>for-of</code> iterations or if statements are allowed.</p>

<h2 id="Examples">Examples</h2>

<h3 id="Simple_generator_comprehensions">Simple generator comprehensions</h3>

<pre class="brush:js">
(for (i of [ 1, 2, 3 ]) i*i );
// generator function which yields 1, 4, and 9

[...(for (i of [ 1, 2, 3 ]) i*i )];
// [1, 4, 9]

var abc = [ "A", "B", "C" ];
(for (letters of abc) letters.toLowerCase());
// generator function which yields "a", "b", and "c"
</pre>

<h3 id="Generator_comprehensions_with_if_statement">Generator comprehensions with if statement</h3>

<pre class="brush: js">
var years = [ 1954, 1974, 1990, 2006, 2010, 2014 ];

(for (year of years) if (year &gt; 2000) year);
// generator function which yields 2006, 2010, and 2014

(for (year of years) if (year &gt; 2000) if(year &lt; 2010) year);
// generator function which yields 2006, the same as below:

(for (year of years) if (year &gt; 2000 &amp;&amp; year &lt; 2010) year);
// generator function which yields 2006
</pre>

<h3 id="Generator_comprehensions_compared_to_generator_function">Generator comprehensions compared to generator function</h3>

<p>An easy way to understand generator comprehension syntax, is to compare it with the generator function.</p>

<p>Example 1: Simple generator.</p>

<pre class="brush: js">
var numbers = [ 1, 2, 3 ];

// Generator function
(function*() {
  for (let i of numbers) {
    yield i * i;
  }
})()

// Generator comprehension
(for (i of numbers) i*i );

// Result: both return a generator which yields [ 1, 4, 9 ]
</pre>

<p>Example 2: Using <code>if</code> in generator.</p>

<pre class="brush: js">
var numbers = [ 1, 2, 3 ];

// Generator function
(function*() {
  for (let i of numbers) {
    if (i &lt; 3) {
      yield i * 1;
    }
  }
})()

// Generator comprehension
(for (i of numbers) if (i &lt; 3) i);

// Result: both return a generator which yields [ 1, 2 ]</pre>

<h2 id="Specifications">Specifications</h2>

<p>{{WhyNoSpecStart}} Generator comprehensions were initially in the ECMAScript 6 draft, but got removed in revision 27 (August 2014). Please see older revisions of ES6 for specification semantics. {{WhyNoSpecEnd}}</p>

<h2 id="Browser_compatibility">Browser compatibility</h2>

<p>{{CompatibilityTable}}</p>

<div id="compat-desktop">
<table class="compat-table">
 <tbody>
  <tr>
   <th>Feature</th>
   <th>Chrome</th>
   <th>Firefox (Gecko)</th>
   <th>Internet Explorer</th>
   <th>Opera</th>
   <th>Safari</th>
  </tr>
  <tr>
   <td>Basic support</td>
   <td>{{CompatNo}}</td>
   <td>{{ CompatGeckoDesktop("30") }}</td>
   <td>{{CompatNo}}</td>
   <td>{{CompatNo}}</td>
   <td>{{CompatNo}}</td>
  </tr>
 </tbody>
</table>
</div>

<div id="compat-mobile">
<table class="compat-table">
 <tbody>
  <tr>
   <th>Feature</th>
   <th>Android</th>
   <th>Chrome for Android</th>
   <th>Firefox Mobile (Gecko)</th>
   <th>IE Mobile</th>
   <th>Opera Mobile</th>
   <th>Safari Mobile</th>
  </tr>
  <tr>
   <td>Basic support</td>
   <td>{{CompatNo}}</td>
   <td>{{CompatNo}}</td>
   <td>{{ CompatGeckoMobile("30") }}</td>
   <td>{{CompatNo}}</td>
   <td>{{CompatNo}}</td>
   <td>{{CompatNo}}</td>
  </tr>
 </tbody>
</table>
</div>

<h2 id="SpiderMonkey-specific_implementation_notes">SpiderMonkey-specific implementation notes</h2>

<ul>
 <li>{{jsxref("Statements/let", "let")}} as an identifier is not supported as <code>let</code> is currently only available to JS version 1.7 and XUL scripts tags.</li>
 <li>Destructuring in comprehensions is not supported yet ({{bug(980828)}}).</li>
</ul>

<h2 id="Differences_to_the_older_JS1.7JS1.8_comprehensions">Differences to the older JS1.7/JS1.8 comprehensions</h2>

<ul>
 <li>ES7 comprehensions create one scope per "for" node instead of the comprehension as a whole.
  <ul>
   <li>Old: <code>[...(()=&gt;x for (x of [0, 1, 2]))][1]() // 2</code></li>
   <li>New: <code>[...(for (x of [0, 1, 2]) ()=&gt;x)][1]() // 1, each iteration creates a fresh binding for x. </code></li>
  </ul>
 </li>
 <li>ES7 comprehensions start with "for" instead of the assignment expression.
  <ul>
   <li>Old: <code>(i * 2 for (i of numbers))</code></li>
   <li>New: <code>(for (i of numbers) i * 2)</code></li>
  </ul>
 </li>
 <li>ES7 comprehensions can have multiple <code>if</code> and <code>for</code> components.</li>
 <li>ES7 comprehensions only work with <code>{{jsxref("Statements/for...of", "for...of")}}</code> and not with <code>{{jsxref("Statements/for...in", "for...in")}}</code> iterations.</li>
</ul>

<h2 id="See_also">See also</h2>

<ul>
 <li>{{jsxref("Statements/for...of", "for...of")}}</li>
 <li>{{jsxref("Operators/Array_comprehensions", "Array comprehensions")}}</li>
</ul>