What does this Javascript code mean? [duplicate]

This question already has answers here: Closed 11 years ago.

Possible Duplicate:

What does “options = options || {}” mean in Javascript?

Looking at the YouTube source...

var yt = yt || 开发者_C百科{};

Does that mean.. set yt to yt if yt exists, else create a new object?

If that's the case, I didn't think you can put a condition when declaring a variable.

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Assign the value of yt back to yt unless it is any of 0, NaN, false, null, "", or undefined (i.e. it's falsy), in which case assign {} to yt.

This works because each of the values in the list above evaluate to false in a boolean expression.

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It means exactly that: If the content does not evaluate to false, assign it to itself (which is a neutral operation), otherwise create a new object and assign it to yt. It's typically used to instantiate objects to use as namespaces, first checking if the namespace already exists.

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Evaluate yt, if it evaluates falsey, then instantiate it as an object.

The first time I saw somthing like this was :

function handleEvent(e){
    e=e||window.event;
}

pretty nifty~ anyone know of other languages that support this syntax? (Not PHP =(

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Yes, the whole right side of the expression is evaluated first before the assignment. So if yt==false the value of the expression on the RHS will be {} and get passed to the var yt