What does the Javascript expression 'a = a || function() {…}' mean?

Question

I\'m not sure what this construct means but I\'ve seen it a few times. The example below is from another Stack Overflow question. I\'m not sure how to interpret the initial

Answer 1

This looks incomplete to me, but it seems as if it is a shim for Object.keys. Basically, if the property doesn't exist (in non standards compliant browsers, for example), we implement it ourselves.

The or operator will evaluate the second operand only if the first one is falsy. As such

alert(false || "Hello, world");

Will alert "Hello, world". In this case, Object.keys would be undefined, which evaluates to false.