Javascript Variable Declaration with ||

Question

I'm investigating Three.js at the moment and have come across this variable declaration at the top of the main source file:

var THREE = THREE || { REVISION: '52' };

I'm just wondering what the OR (||) is doing in there - what is the function of it?

score 4 · Accepted Answer · answered Nov 07 '12 at 11:30

4

The above means:

If the value of THREE evaluates to true, assign the value of THREE to the THREE variable, otherwise initialize it to the object { REVISION: '52' }.

answered Nov 07 '12 at 11:30

Oded

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1

It tests if `THREE` evaluates as true, not if it is defined. – Quentin Nov 07 '12 at 11:32
@Quentin - Answer updated - better? – Oded Nov 07 '12 at 11:33

Mario S · Answer 2 · 2012-11-07T11:49:10.047

1

In code, it's like saying:

var THREE;
if (THREE) {
    THREE = { REVISION: '52' };
}
else {
    THREE = THREE;
}

Or:

var THREE = (THREE) ? { REVISION: '52' } : THREE;

edited Nov 07 '12 at 11:49

answered Nov 07 '12 at 11:33

Mario S

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What if `THREE` was `0`? – Quentin Nov 07 '12 at 11:36
@Quentin Good point! Hope I got it right now(?) =) – Mario S Nov 07 '12 at 11:49

score 0 · Answer 3 · answered Nov 07 '12 at 11:32

0

lazy instantiation. If the variable is declared already, then assign a value to it.

answered Nov 07 '12 at 11:32

Ahmad

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Javascript Variable Declaration with ||

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