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

var AEROTWIST = AEROTWIST || {};

Basically this line is saying set the AEROTWIST variable to the value of the AEROTWIST variable, or set it to an empty object.

The double pipe || is an OR statement, and the second part of the OR is only executed if the first part returns false.

Therefore, if AEROTWIST already has a value, it will be kept as that value, but if it hasn't been set before, then it will be set as an empty object.

it's basically the same as saying this:

if(!AEROTWIST) {var AEROTWIST={};}

Hope that helps.