Javascript conditional order evaluation

Question

Can I count on Javascript failing immediately when one condition in an expression results to false?

f = {'a':'b'};
if (f.a !== undefined || f.a === 'b') {
  // Is this OK to use, because the second condition will never be evaluated?
}

Answer 1

Yes, this is known as short circuit evaluation .

With an AND logical operator, if the first evaluates to false , then the second is never evaluated, because the condition knows enough already to be met.

With the OR logical operator, if the first one is false , it will evaluate the second one. Otherwise if the first is true it won't evaluate the second (no need to).

This is also why you see...

var a = function(b) {
   b = b || 7;
}