Why does Javascript's OR return a value other than true/false?

Question

I saw this construction in order to get the browser viewport width:

function () { return window.innerWidth || document.documentElement.clientWidth || documen         


        

Answer 1

The OR function is a short-circuit OR evaluation - it returns the first element that is not false, or the last false element otherwise.

This is actually quite useful, so you can write expressions like

a = a || someValue;

Which is the same as

if (a==null)
  a = someValue;