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    <h2>What does <code>void 0</code> mean?</h2> <p><a href="https://developer.mozilla.org/en/JavaScript/Reference/Operators/Special_Operators/void_Operator" rel="noreferrer"><code>void</code><sup>[MDN]</sup></a> is a prefix keyword that takes one argument and always returns <code>undefined</code>.</p> <p><strong>Examples</strong></p> <pre><code>void 0 void (0) void "hello" void (new Date()) //all will return undefined </code></pre> <h2>What's the point of that?</h2> <p>It seems pretty useless, doesn't it? If it always returns <code>undefined</code>, what's wrong with just using <code>undefined</code> itself?</p> <p>In a perfect world we would be able to safely just use <code>undefined</code>: it's much simpler and easier to understand than <code>void 0</code>. But in case you've never noticed before, <em>this isn't a perfect world</em>, especially when it comes to Javascript. </p> <p>The problem with using <code>undefined</code> was that <code>undefined</code> is not a reserved word (<a href="https://web.archive.org/web/20160311231335/http://wtfjs.com/2010/02/15/undefined-is-mutable" rel="noreferrer">it is actually a property of the global object <sup>[wtfjs]</sup></a>). That is, <code>undefined</code> is a permissible variable name, so you could assign a new value to it at your own caprice.</p> <pre><code>alert(undefined); //alerts "undefined" var undefined = "new value"; alert(undefined) // alerts "new value" </code></pre> <p>Note: This is no longer a problem in any environment that supports ECMAScript 5 or newer (i.e. in practice everywhere but IE 8), which defines the <code>undefined</code> property of the global object as read-only (so it is only possible to shadow the variable in your own local scope). However, this information is still useful for backwards-compatibility purposes.</p> <pre><code>alert(window.hasOwnProperty('undefined')); // alerts "true" alert(window.undefined); // alerts "undefined" alert(undefined === window.undefined); // alerts "true" var undefined = "new value"; alert(undefined); // alerts "new value" alert(undefined === window.undefined); // alerts "false" </code></pre> <p><code>void</code>, on the other hand, cannot be overidden. <code>void 0</code> will <em>always</em> return <code>undefined</code>. <code>undefined</code>, on the other hand, can be whatever Mr. Javascript decides he wants it to be.</p> <h2>Why <code>void 0</code>, specifically?</h2> <p>Why should we use <code>void 0</code>? What's so special about <code>0</code>? Couldn't we just as easily use <code>1</code>, or <code>42</code>, or <code>1000000</code> or <code>"Hello, world!"</code>?</p> <p>And the answer is, yes, we could, and it would work just as well. The only benefit of passing in <code>0</code> instead of some other argument is that <code>0</code> is short and idiomatic.</p> <h2>Why is this still relevant?</h2> <p>Although <code>undefined</code> can generally be trusted in modern JavaScript environments, there is one trivial advantage of <code>void 0</code>: it's shorter. The difference is not enough to worry about when writing code but it can add up enough over large code bases that most code minifiers replace <code>undefined</code> with <code>void 0</code> to reduce the number of bytes sent to the browser.</p>
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    1. POWhat does `void 0` mean?
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    1. USPeter Olson
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    1. CO@Randomblue I edited to answer your comment
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      1. USPeter Olson
    2. CO@PeterOlson the main benefit of `void 0` is that the `0` is the de-facto standard for using `void` to generate an `undefined` value.
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    3. COFirst, `undefined` is not a keyword, it's just a property of the global object. Second, in ECMAScript5, it is write protected, so the `void 0` trick is only neccessary if someone uses `undefined` as a local variable within a function.
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