Why does `(state == 1 && 3)` make sense? [duplicate]

Question

I came across this code in Mithril.js:

finish(state == 1 && 3)

To my (Java/C programmer's) eyes it looks like it should always invoke finish(true) if state is 1 and finish(false) if state is not 1. But it actually seems to do finish(3) for the former and finish(false) for the latter.

What is the logic behind this?

Is this idiomatic in JavaScript, or is it a bad idea? To me it's horribly obscure.

Answer 1

You can interpret the operators || and && like this:

    A || B
→   A ? A : B

    A && B
→   A ? B : A

But without evaluating A twice.

Answer 2

It is a characteristic of JavaScript, && and || operators always return the last value it evaluated.