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POWhen to check for undefined and when to check for null
 

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    <p><strong>[Bounty Edit]</strong></p> <p>I'm looking for a good explanation when you should set/use <code>null</code> or <code>undefined</code> and where you need to check for it. Basically what are common practices for these two and is really possible to treat them separately in generic maintainable codee?</p> <p>When can I safely check for <code>=== null</code>, safely check for <code>=== undefined</code> and when do I need to check for both with <code>== null</code></p> <p>When should you use the keyword <code>undefined</code> and when should one use the keyword <code>null</code> </p> <p>I have various checks in the format of </p> <p><code>if (someObj == null)</code> or <code>if (someObj != null)</code> which check for both null and undefined. I would like to change all these to either <code>=== undefined</code> or <code>=== null</code> but I'm not sure how to guarantee that it will only ever be one of the two but not both.</p> <p>Where should you use checks for <code>null</code> and where should you use checks for <code>undefined</code></p> <p>A concrete example:</p> <pre><code>var List = []; // ordered list contains data at odd indexes. var getObject = function(id) { for (var i = 0; i &lt; List.length; i++) { if (List[i] == null) continue; if (id === List[i].getId()) { return List[i]; } } return null; } var deleteObject = function(id) { var index = getIndex(id) // pretty obvouis function // List[index] = null; // should I set it to null? delete List[index]; // should I set it to undefined? } </code></pre> <p>This is just one example of where I can use both <code>null</code> or <code>undefined</code> and I don't know which is correct.</p> <p>Are there any cases where you must check for both <code>null</code> and <code>undefined</code> because you have no choice?</p>
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    1. COFor my own edification, why do you care? I mean, why not just test with `== undefined` unless you **know** you don't want to match `null`?
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    2. COFirstly JSLint complains. It's neater to use `===`. I would like to be able to treat `undefined` and `null` seperately. I think there are clear cases where you should use one or the other but I don't understand them. Almost everyone doesn't bother understanding the minor differences between the two and just uses `== null` or `== undefined`. Javascript does seem to have both without any distinction.
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    3. COUse the *bang operator* (`!`): `if (!someObj)`
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