What does this Javascript line do? [duplicate]

This question already has answers here: Closed 11 ye开发者_运维问答ars ago.

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What is “var _gaq = _gaq || []; ” for ?

var _gaq = _gaq || [];

I'm not sure what this line is doing? Can someone explain this to me?

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This is similar to doing

var _gaq = _gaq ? : _gaq : [];

It means that if _gaq is set, it'll set it to _gaq, otherwise it will default to a new empty array.

• var means it's local scope
• _gaq is the name of the variable
• || means or

It's saying that if _gaq doesn't already exist, set it to a new array which is what [] means.

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It declares a variable named _gaq. If that variable was already defined, and is a truthy value, then the line is equivalent to writing

var _gaq = _gaq;

If _gaq is a falsy value, then the newly declare variable is an empty array.

Some reference on truthiness and falsiness in JavaScript:

• http://11heavens.com/falsy-and-truthy-in-javascript
• http://javascript.crockford.com/style2.html

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It checks if _gaq is defined if not assigns an array object to _gaq.

its equivalent to

if(!_gaq){
 var _gaq = [];
}