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

Notice that in some version of IE this code won't work as expected. Because the var, the variable is redefined and assigned so – if I recall correctly the issue – you'll end up to have always a new object. That should fix the issue:

var AEROTWIST;
AEROTWIST = AEROTWIST || {};