|| in javascript

Question

Can somebody please tell me what || this doing here

(function()
{
window.myapp = window.myapp || {};
window.myapp.lang = window.myapp.lang || {};
myapp.lang.Extend = function(subClass, superClass)
{
subClass.prototype = new superClass();
};
})();

[Reference - What does this symbol mean in JavaScript?](http://stackoverflow.com/questions/9549780/reference-what-does-this-symbol-mean-in-javascript) — , Mar 07 '12 at 16:04
[Logical operator || in javascript, 0 stands for Boolean false?](http://stackoverflow.com/questions/9579262/logical-operator-in-javascript-0-stands-for-boolean-false) — Evan Mulawski, Mar 07 '12 at 16:17
possible duplicate of [What does this construct (x = x || y) mean?](http://stackoverflow.com/questions/2802055/what-does-this-construct-x-x-y-mean) — Felix Kling, Jun 09 '13 at 18:47

score 4 · Accepted Answer · answered Mar 07 '12 at 16:02

4

window.myapp = window.myapp || {};

It means: create window.myapp as an empty object if it does not already exist.

answered Mar 07 '12 at 16:02

David Frank

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score 2 · Answer 2 · answered Mar 07 '12 at 16:02

2

The a = a || b; syntax is equivalent to

if (!a)
  a = b;

or

a = a ? a : b;

answered Mar 07 '12 at 16:02

Alexander Pavlov

29,538
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score 2 · Answer 3 · answered Mar 07 '12 at 16:02

2

window.myapp = window.myapp || {};

is equivalent to this code

if(!(window.myapp)) {
  window.myapp = {};
}

answered Mar 07 '12 at 16:02

Sergio Tulentsev

210,238
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Why the additional parens in `!(window.myapp)` instead of `!window.myapp`? – Kit Sunde Mar 07 '12 at 16:57
@KitSunde just to be sure that negation applies to the whole expression. Probably we don't need it here. – Sergio Tulentsev Mar 07 '12 at 17:00

score 2 · Answer 4 · answered Mar 07 '12 at 16:02

2

|| is the logical OR operator in Javascript. In this case

window.myapp = window.myapp || {};

assigns window.myapp to itself if it is not null or false, otherwise it assigns an empty object {} to window.myapp.

answered Mar 07 '12 at 16:02

1

*"if it is not null or false"* ...or one of four other falsey values... `NaN`, `undefined`, `0`, `""` – Mar 07 '12 at 16:09

score 0 · Answer 5 · answered Mar 07 '12 at 16:16

|| is a short-circuiting operator

while evaluating x || y , x is evaluated first, if it is true - then there is no need to evaluate y because the operation is anyway going to be true.

in javascript the following values result in 'false' when used as if condition -

0 -0 null "" false undefined NaN

so in your case if window.myapp is 'undefined', that will be evaluated to 'false', and the or operator has to evaluate the next operand {} to complete the operation, which is assigned to window.myapp

so all it is doing is -

if(!(window.myapp)) {
  window.myapp = {};
}

|| in javascript

5 Answers5

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