javascript - Why is it common practice in JS libraries to duplicate variable name and adding 'or object' -

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i trying better grip on js syntax , wondering why common practice in many libraries repeat object name after variable same name has been declared. please see below:

var backbone = backbone || {};    // <= why duplication?

or

var listeners = this._listeners || (this._listeners = {});

update:

after further research have found written article evaluates many idiomatic expressions.

the article explains use behind invoked function expressions (iife), i.e wrapping of function within function, such:

(function() {   console.log('hello!'); })();

which 1 of js brainteasers newbies me can't quite understand.

var backbone = backbone || {};

means if backbone undefined or null or false, set {}

longer explanation:

the assignment operator evaluates right left , logical operators(even though javascript doesn't have real logical operators work on non-boolean operands well) evaluate left right.

an expression a || b returns b if a undefined, null, or false.

so a = || b either keeps a value if has 1 or assigns b a.


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