Javascript Variable Declaration with || [duplicate]

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?

Oded · Accepted Answer · 2012-11-07 11:30:19Z

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 7, 2012 at 11:30

Oded

501k102 gold badges900 silver badges1k bronze badges

Sign up to request clarification or add additional context in comments.

2 Comments

Quentin Over a year ago

It tests if THREE evaluates as true, not if it is defined.

Oded Over a year ago

@Quentin - Answer updated - better?

Mario S · Accepted Answer · 2012-11-07 11:49:10Z

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 7, 2012 at 11:49

answered Nov 7, 2012 at 11:33

Mario S

12k24 gold badges42 silver badges47 bronze badges

2 Comments

Quentin Over a year ago

What if THREE was 0?

Mario S Over a year ago

@Quentin Good point! Hope I got it right now(?) =)

Ahmad · Accepted Answer · 2012-11-07 11:32:58Z

0

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

answered Nov 7, 2012 at 11:32

Ahmad

12.8k7 gold badges53 silver badges98 bronze badges

Collectives™ on Stack Overflow

Javascript Variable Declaration with || [duplicate]

3 Answers 3

2 Comments

2 Comments

Comments

Linked

Hot Network Questions

Collectives™ on Stack Overflow

3 Answers 3

2 Comments

2 Comments

Comments

Linked

Related