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();
};
})();
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Reference - What does this symbol mean in JavaScript?user1106925– user11069252012-03-07 16:04:12 +00:00Commented Mar 7, 2012 at 16:04
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Logical operator || in javascript, 0 stands for Boolean false?Evan Mulawski– Evan Mulawski2012-03-07 16:17:53 +00:00Commented Mar 7, 2012 at 16:17
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possible duplicate of What does this construct (x = x || y) mean?Felix Kling– Felix Kling2013-06-09 18:47:14 +00:00Commented Jun 9, 2013 at 18:47
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5 Answers
window.myapp = window.myapp || {};
It means: create window.myapp as an empty object if it does not already exist.
Comments
The a = a || b; syntax is equivalent to
if (!a)
a = b;
or
a = a ? a : b;
Comments
window.myapp = window.myapp || {};
is equivalent to this code
if(!(window.myapp)) {
window.myapp = {};
}
2 Comments
Kit Sunde
Why the additional parens in
!(window.myapp) instead of !window.myapp?
Sergio Tulentsev
@KitSunde just to be sure that negation applies to the whole expression. Probably we don't need it here.
|| 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.
|| 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 = {};
}
