What does var x = x

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What does “var FOO = FOO || {}” (assign a variable or an empty object to that variable) mean in Javascript? 7 answers

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|| is the logical OR .

The expression

var x = x OR {};

should become more obvious then.

If x has a falsy value (like null , undefined , 0 , "" ), we assign x an empty object {} , otherwise just keep the current value. The long version of this would look like

var x = x ? x : {};

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如果x未定义（或null或任何其他false值），则它变为空对象。

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One should never write "var x = x || {};" per se.

The only circumstance where this is not identical to "var x = {};" is when x was previously initialized in the same scope. That is immoral. Note:

function() {
  x = {foo:"bar"};
  var x = x || {};
}

Is the same as, merely more confusing than,

function() {
  var x = {foo:"bar"};
  x = x || {};
}

In neither case is there any reference to the value of the symbol "x" in the global scope.

This expression is a confused variant of the legitimate lazy property initialization idiom:

function( foo ) { 
  foo.x = foo.x || {};
  foo.x.y = "something";
}

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