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

From what I can tell, that code attempts to define the function Object.keys if it isn't already defined (or if it's false). The function to the left of || will become the function Object.keys.

The reason I said "from what I can tell" is that you haven't posted the entire code snippet. Notice that the code after || reads (function(){ instead of just function(){. It's possible that the author has set up the function to be self invoking.

If, after the function definition, you see })(), then the return value of the function is stored in Object.keys. If not, then the function itself is stored there.