How to execute statement in javascript if variable defined

Question

Possible Duplicate:
How to check a not defined variable in javascript
Determining if a javascript object has a given property

In my beforeSend function i have this

$.myloader.show();

But some scripts dont have that element so my this line gives me error

$.myloader.show(); gives me error that $.myloader does not exists

How can i do something like

if($.myloader exists)
$.myloader.show();

Or http://stackoverflow.com/questions/858181/how-to-check-a-not-defined-variable-in-javascript?rq=1 (and other links visible at the right...) — Denys Séguret, Oct 11 '12 at 07:17
try { some_var.xxx.xxx.xxx x = 't'; }catch(e){ x = 'f'; } console.debug(x); — Ruthe, Sep 11 '15 at 07:56

Denys Séguret · Accepted Answer · 2012-10-11T07:31:44.723

5

The most generic and solid solution is :

if (typeof $.myloader != 'undefined') {

If you're sure your variable can't hold anything else than a function or undefined, you might use

if ($.myloader) {

But do this only when you're sure of the possible values because this test also match false, 0 and ''.

edited Oct 11 '12 at 07:31

answered Oct 11 '12 at 07:06

Denys Séguret

372,613
87
782
758

It does not match `'0'`, `[]`, etc, and the test is reversed. – pimvdb Oct 11 '12 at 07:13
That's something different. `if('0')` *will* execute. – pimvdb Oct 11 '12 at 07:14
@pimvdb You're right, I got a little carried over... – Denys Séguret Oct 11 '12 at 07:15
1

No problem :). I still think the test is reversed, though. – pimvdb Oct 11 '12 at 07:26
@pimvdb I need a second coffee I think... I should not come on SO before coding... – Denys Séguret Oct 11 '12 at 07:32

score 0 · Answer 2 · answered Oct 11 '12 at 07:09

0

You can do it as folllow...

if(typeof $.myloader != 'undefined')
{    // your code here.  };

answered Oct 11 '12 at 07:09

Amol Kolekar

2,307
5
30
45

You most certainly want `!==`, not `!=` – nfechner Oct 11 '12 at 07:12
@nfechner that doesn't seem needed : [typeof](https://developer.mozilla.org/en-US/docs/JavaScript/Reference/Operators/typeof) always return a string. – Denys Séguret Oct 11 '12 at 07:18

score 0 · Answer 3 · answered Oct 11 '12 at 07:16

0

Undefined values are 'falsey', and evaluate to false in an if statement, so all you need is:

if( $.myloader )
    $.myloader.show();

answered Oct 11 '12 at 07:16

nbrooks

18,126
5
54
66

score 0 · Answer 4 · answered Oct 11 '12 at 07:19

0

Given that jQuery ($) is an object:

if ($.hasOwnProperty('myloader'))
{
    //should work
    //try: $.hasOwnProperty('fn'), it'll return true, too
}

if the myloader object isn't directly bound to the jQuery object, but via one of its prototypes:

if ('myloader' in $)
{
    //works, like:
    if ('hasOwnProperty' in $)
    {//is true, but it's not a property/method of the object itself:
        console.log($.hasOwnProperty('hasOwnProperty'));//logs false
    }
}

You can check for the methods you want to use in the same way:

if ('myloader' in $ && 'show' in $.myloader)
{
    $.myloader.show();
}

answered Oct 11 '12 at 07:19

Elias Van Ootegem

74,482
9
111
149

`$` is actually a function, but of course also inherits from `Object`... in the end, everything does (apart from `Object.create(null)`), so it should not really matter what `$` is. – Felix Kling Oct 11 '12 at 07:20
@FelixKling, yes, it's a function, but functions are first class objects. Tomato - tomato, really – Elias Van Ootegem Oct 11 '12 at 07:23

score 0 · Answer 5 · answered Oct 11 '12 at 07:21

0

You can do this even more shortly:

$.myloader.show() ? console.log('yep') : console.log('nope');

Or:

$.myloader.show() || console.log('fail');

answered Oct 11 '12 at 07:21

happyCoda

418
2
2

score 0 · Answer 6 · answered Oct 11 '12 at 08:00

If you want it short, you can also do it like this:

$.myloader && $.myloader.show();

The first operand (before the &&) is called a guard. However some people argue that this is not readable and safe enough, so you might want to use the if (typeof ...) solution.

How to execute statement in javascript if variable defined

6 Answers6