How does the "||=" operator differ from "?=" in CoffeeScript?

Question

What is the difference between these two? From Ruby, I am used to apple ||= walrus setting apple equal to walrus only if apple is null. That seems to be the same in JS/CS, though ?= seems to do the same thing. What confuses me is this:

apple = 0
walrus = 9
apple ?= walrus // outputs 0
apple ||= walrus // outputs 9

wat

Answer 1

The best thing to do would be to look at the resulting JS.

Coffeescript:

apple = 0
walrus = 9
apple ?= walrus // outputs 0
apple ||= walrus // outputs 9

JavaScript:

var apple, walrus;
apple = 0;
walrus = 9;
if (apple == null) apple = walrus;
apple || (apple = walrus);

As you can see, the ?= explicitly checks is something is null or undefined in JS. This is very different from ||= which just relies on whether apple is a falsy value.

To expand a bit more apple ||= walrus is equivalent in meaning to apple = apple || walrus, so any value of apple that is truthy will short-circuit the logic and not change the value of apple.