What does “var FOO = FOO || {}” (assign a variable or an empty object to that variable) mean in Javascript?

Question

Looking at an online source code I came across this at the top of several source files.

var FOO = FOO || {};
FOO.Bar = …;

But I have no ide

Answer 1

The || operator takes two values:

a || b

If a is truthy, it will return a. Otherwise, it will return b.

The falsy values are null, undefined, 0, "", NaN and false. The truthy values are everything else.

So if a has not been set (is it undefined) it will return b.