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    <p>The difference is subtle.</p> <p>In JavaScript an <code>undefined</code> variable is a variable that as never been declared, or never assigned a value. Let's say you declare <code>var a;</code> for instance, then <code>a</code> will be <code>undefined</code>, because it was never assigned any value.</p> <p>But if you then assign <code>a = null;</code> then <code>a</code> will now be <code>null</code>. In JavaScript <code>null</code> is an object (try <code>typeof null</code> in a JavaScript console if you don't believe me), which means that null is a value (in fact even <code>undefined</code> is a value).</p> <p>Example:</p> <pre><code>var a; typeof a; # =&gt; "undefined" a = null; typeof null; # =&gt; "object" </code></pre> <p>This can prove useful in function arguments. You may want to have a default value, but consider null to be acceptable. In which case you may do:</p> <pre><code>function doSomething(first, second, optional) { if (typeof optional === "undefined") { optional = "three"; } // do something } </code></pre> <p>If you omit the <code>optional</code> parameter <code>doSomething(1, 2) then</code>optional will be the <code>"three"</code> string but if you pass <code>doSomething(1, 2, null)</code> then optional will be <code>null</code>.</p> <p>As for the equal <code>==</code> and strictly equal <code>===</code> comparators, the first one is weakly type, while strictly equal also checks for the type of values. That means that <code>0 == "0"</code> will return true; while <code>0 === "0"</code> will return false, because a number is not a string.</p> <p>You may use those operators to check between <code>undefined</code> an <code>null</code>. For example:</p> <pre><code>null === null # =&gt; true undefined === undefined # =&gt; true undefined === null # =&gt; false undefined == null # =&gt; true </code></pre> <p>The last case is interesting, because it allows you to check if a variable is either undefined or null and nothing else:</p> <pre><code>function test(val) { return val == null; } test(null); # =&gt; true test(undefined); # =&gt; true </code></pre>
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    1. USJulien Portalier
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