{}" construct in javascript

Possible Duplicate:
What does “var FOO = FOO || {}” mean in Javascript?
Javascript - Can you add condition to variable declaration

I believe this has to do with scoping, and not redefining scope. I see a lot of this in popular javascript frameworks:

var something = something || {};

Is that to do with not accidentally redefining a top-level variable?

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You are correct. If the variable already exists (ours or not), don't change it. If it doesn't exist, let's create a new one.

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var something = something || {};

can be used inside of functions with parameters that may not be set

    function doStuff( arg1, arg2, arg3 ) {
        arg2 = arg2 || {};
        arg3 = arg3 || arg1 + 2;
    };

this makes the second and third arguments optional when you call the function doStuff

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This creates a new object with local scope. If the outer something is null, false, or undefined (or otherwise falsy), the new variable will be an empty object {} .

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