Question

I have seen two different “styles” for setting a default value default to the variable argument in JavaScript:

argument = argument || default     // Version 1
argument || (argument = default)   // Version 2

Are these completely equivalent? Is one faster than the other, or better in some other way?

Answer 1

The second one avoids any assignment whatsoever if argument is already truthy, so it may theoretically be a smidge faster.

Really though I doubt you’d ever see a difference.

---

Just to break it down a bit more

1. Assign argument to itself if argument is already truthy, otherwise assign argument to default

2. Evaluate the boolean condition argument || (argument = default). If argument is already truthy, the evaluation will short circuit, and nothing else will happen. If argument is not truthy, the second half of the boolean condition will hit, and argument will be assigned default