explorers' club

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JavaScript null vs undefined & default values

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[note to self]

During my recent journey into learning more about Javascript, I ran into an interesting little issue.  It seems that the default value (or lack thereof) in JavaScript is undefined, not null as it is in ActionScript.

I fired up Firebug’s console and did a little test (which by the way is a great means of learning JavaScript by means of experimentation)

Let’s start with the basic equality:

>>> foo
>>> foo == null
>>> foo == undefined
>>> null == undefined

And now let’s use strict equality:

>>> foo === null
>>> foo === undefined
>>> null === undefined

Nothing revolutionary however it does illustrate that while you can evaluate a value to null in most checks, there is that caveat that if you haven’t explicitly set a value to null (which is a value/type itself in laymen’s speak) then it is not null but rather undefined.  Doing more research I found this post by a guy who explains it a bit better than I have – http://saladwithsteve.com/2008/02/javascript-undefined-vs-null.html


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