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How to check if a variable is set in Javascript?

Answer 1

+2 A:

Recommended:

if (typeof states["AL"] == 'undefined')
    alert("Invalid state");

Litteral undefined is possible and will work most of the time, but won't in some browser (IE mac, maybe others).

streetpc 2010-02-15 15:18:38

undefined should be in quotes?

Click Upvote 2010-02-15 15:19:23

Yes, because `typeof` returns a string. However, I'm not sure why using `typeof` is better in the first place.

Matt Ball 2010-02-15 15:20:53

@streetpc What if states["AL"] is null? Then `typeof states["AL"]` will return `"object"` and you won't handle an invalid value correctly.

Jonathon 2010-02-15 15:26:58

@Jonathon: What was asked was whether `states["AL"]` has been set. It being set it to `null` still counts as being set. @Bears: `typeof` is better for two reasons: one, in some older browsers, the global object doesn't have an `undefined` property, and two, because `undefined` is a property of the global object rather than an immutable literal like `null`, it can be inadvertently redefined. For example, a typo in an equality check could do it: `if (undefined = someVar) {...}`.

Tim Down 2010-02-15 15:59:30

Answer 2

+2 A:

You don't have to do a literal comparison at all; the following will work just fine...

if(states["AL"]) 
  alert("State exists"); 
else
  alert("State does not exist");

Josh Stodola 2010-02-15 15:20:04

really? will that work in all browsers?

Click Upvote 2010-02-15 15:21:14

@Click Yes, if states["AL"] has not been set it will return undefined which will evaluate to false in a boolean expression.

Jonathon 2010-02-15 15:22:16

@Click Yes, Jonathon is right.

Josh Stodola 2010-02-15 15:22:43

I would not recommend that for this particular test. What you have above checks if `states["AL"]` is a truthy value or not - for instance `states["AL"]` might be set to `false` but that's different from being undefined.

Matt Ball 2010-02-15 15:23:14

so if i did `if (! states["AL"])` would that work as well?

Click Upvote 2010-02-15 15:23:44

@Click - http://11heavens.com/falsy-and-truthy-in-javascript

Matt Ball 2010-02-15 15:25:13

@Bears This seems ok to me. If states["AL"] is any sort of valid value (non-empty string that doesn't evaluate to false), this will handle it fine.

Jonathon 2010-02-15 15:28:21

This checks for true/false. So if `states["AL"]` is zero, it will return false, which is technically not what you want.

James Wiseman 2010-02-15 15:31:59

Well, it depends on what the intent of the check is. It sounded like the OP wanted to see whether or not an entry is defined at all - not check to see if it is a falsy value.

Matt Ball 2010-02-15 15:33:02

Answer 3

A:

function State(ab, name){
    if(states && states[ab])return states[ab];

    this.ab=ab;
    this.name=name;
    //whatever else, no return;
}

kennebec 2010-02-15 15:54:18

Answer 4

+1 A:

Alternately, you could say if ("AL" in states) { ... }.

Sean 2010-02-15 18:19:17

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How to check if a variable is set in Javascript?

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