JavaScript: How to check if variable exists by variable name?

Question

I am having this issue. I have a script that checks if variable exists, because some scripts load asyncronously, like FB for Facebook or twttr for Twitter.

function whenAvailable(name, callback, interval) {
    interval || (interval = 100); // ms
    window.setTimeout(function() {
        if ((window.hasOwnProperty && window.hasOwnProperty(name)) || window[name] || !!eval(name)) {
            return callback();
        } else {
            window.setTimeout(arguments.callee, interval);
        }
    }, interval);
}

Looks like this

if ((window.hasOwnProperty && window.hasOwnProperty(name)) || window[name] || !!eval(name))

does not work. IE throws error for eval(name) -- e.g. if name = 'FB', it says it can't eval 'FB' which is undefined.

window.hasOwnProperty(name) does not work if name == 'twttr.widgets'.

Is there a universal and cross browser check for existence of var by var name?

quick question, do those variables always exist in the scripts loaded? — Joseph
– Joseph, Commented Mar 29, 2012 at 15:51
There can't be a variable named twttr.widgets. There can be a variable named twttr which may have a property widgets, but hopefully you can see it's not as simple as "does this property exist" — Gareth
– Gareth, Commented Mar 29, 2012 at 15:52
Joseph - no, they may never exist. Gareth - technically you're right but what I wanted needs to work with properties as well. — mvbl fst
– mvbl fst, Commented Mar 29, 2012 at 16:36
@Gareth: There can certainly be a property of window called twttr.widgets. It would be unusual, but it's perfectly legal. window['twittr.widgets'] = "foo";. — T.J. Crowder
– T.J. Crowder, Commented Mar 29, 2012 at 21:10

T.J. Crowder · Accepted Answer · 2012-03-29 15:51:21Z

7

First off, you don't need that eval (you almost never do), you can index into a JavaScript object using a string and bracketed notation, e.g.:

window['FB'];

or

name = 'FB';
foo = window[name];

So to check:

if (typeof window[name] === "undefined")

...except that doesn't differentiate between the property not existing at all, or existing but having the value undefined.

or

if (name in window)

...but that checks the prototype as well as the object. It's fine for window, though.

window.hasOwnProperty(name) does not work if name == 'twttr.widgets'.

Right, you have to break it up:

var index = 0,
    parts = name.split('.'), // doesn't handle [] notation
    result;
result = window;
index = 0;
try {
    while (typeof result !== "undefined" && result !== null && index < parts.length) {
        result = result[parts[index++]];
    }
}
catch (e) {
}
if (index < parts.length) {
    // Didn't find all of it
}

...or something to that effect.

answered Mar 29, 2012 at 15:51

T.J. Crowder

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2 Comments

mvbl fst Over a year ago

Thanks T.J. I only changed if (index < parts.length) to if (index < parts.length || (typeof result == 'undefined' && result === undefined)) return false;

T.J. Crowder Over a year ago

@SODA: Good deal. FWIW, if typeof result == "undefined" the odds are very high (though granted not 100% if you're dealing with cross-window stuff) that result === undefined, you can probably drop the second bit and just keep the typeof check. Also note that I included result !== null for a reason (typeof null is not "undefined" but if you try to dereference null with brackets, you'll get an exception).

kiswa · Accepted Answer · 2012-03-29 17:44:39Z

3

I've always just used things like:

if (window[name]) {
    return 'it exists';
} else {
    return 'nope';
}

EDIT: This works because if window[name] isn't there, the if determines undefined to be false.

edited Mar 29, 2012 at 17:44

answered Mar 29, 2012 at 15:52

kiswa

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1 Comment

mvbl fst Over a year ago

No, in your case if I do var name = 'FB', window.name will return 'FB' but what I need is to return window['FB'] or window.FB

JaredMcAteer · Accepted Answer · 2012-03-29 15:51:23Z

0

What you're looking for is:

if ('key' in object)

so in your example

if (name in window)

answered Mar 29, 2012 at 15:51

JaredMcAteer

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Comments

user578895 · Accepted Answer · 2012-03-29 15:52:49Z

0

To resolve twttr.widgets as well as twttr, something like:

var parts = name.split( '.' )
  ,  aVar = window;

while( parts.length ){
    aVar = aVar[ aVar.shift() ];
    return setTimeout( ... );
}

// aVar will now be your resolved variable
return callback();

answered Mar 29, 2012 at 15:52

user578895

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