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What does this construct (x = x || y) mean?

a lot of times i can see that variables set like that

var d = d || {}

or

var = var || []

a few question came up to mind

1. what it’s mainly purpose ?
2. how can it be to use OR in variable define?

Answer 1

The || operator returns the left hand side if it is true and the right hand side if the left hand side isn’t true.

So:

var d = d || {}

Is a quick way of saying:

var d;
if (!d) {
    d = {};
}