What does the || operator do?

Question

Attacklab.wmd_env.buttons=Attacklab.wmd_env.buttons||_4;

what does the || do in this case?
Adds _4 to the array which is Attacklab.wmd_env.buttons?

Answer 1

The || operator in JavaScript returns the value on the left if that value does not evaluate to false, otherwise it returns the value on the right.

From Mozilla's Core JavaScript 1.5 Reference:

expr1 || expr2
Returns expr1 if it can be converted to true; otherwise, returns expr2. Thus, when used with Boolean values, || returns true if either operand is true; if both are false, returns false.

So, in this case, if Attacklab.wmd_env.buttons doesn't have a value, it sets the value to _4.

Answer 2

It's a fancy way of writing

if(!Attacklab.wmd_env.buttons)
    Attacklab.wmd_env.buttons = _4;

It's nice for providing default values. Keep in mind that not only null and undefined will trigger the conditional, but also 0, false and '', ie everything which is considered false in boolean contexts.