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    <p>In JavaScript, <code>null</code> is an object. There's another value for things that don't exist, <code>undefined</code>. The DOM returns <code>null</code> for almost all cases where it fails to find some structure in the document, but in JavaScript itself <code>undefined</code> is the value used.</p> <p>Second, no, there is not a direct equivalent. If you really want to check for specifically for <code>null</code>, do:</p> <pre><code>if (yourvar === null) // Does not execute if yourvar is `undefined` </code></pre> <p>If you want to check if a variable exists, that can only be done with <code>try</code>/<code>catch</code>, since <code>typeof</code> will treat an undeclared variable and a variable declared with the value of <code>undefined</code> as equivalent.</p> <p>But, to check if a variable is declared <em>and</em> is not <code>undefined</code>:</p> <pre><code>if (typeof yourvar !== 'undefined') // Any scope </code></pre> <p>If you know the variable exists, and want to check whether there's any value stored in it:</p> <pre><code>if (yourvar !== undefined) </code></pre> <p>If you want to know if a member exists independent but don't care what its value is:</p> <pre><code>if ('membername' in object) // With inheritance if (object.hasOwnProperty('membername')) // Without inheritance </code></pre> <p>If you want to to know whether a variable is <a href="https://developer.mozilla.org/en-US/docs/Glossary/Truthy" rel="noreferrer">truthy</a>:</p> <pre><code>if (yourvar) </code></pre> <p><a href="http://lists.evolt.org/archive/Week-of-Mon-20050214/099714.html" rel="noreferrer">Source</a></p>
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    1. POHow to check a not-defined variable in JavaScript
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    1. COundefined is not a reserved word; you (or someone else's code) can do "undefined = 3" and that will break two of your tests.
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      1. USJason S
    2. CO"If you know the variable exists but don't know if there's any value stored in it" -- huh?!
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    3. COI think he is referring to a variable declared that has not been assigned to. eg: var foo; // foo exists but does not have a value
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      1. USWally Lawless
 

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