In my opinion this pattern is better if you write it like this:

  _ = isDefined(user) || throw new Error("user must be defined")

This reads way more natural for me. "A user is defined OR throw an error"...

I've also seen this in Perl (`do_something() || die()`) and shell scripts (`grep -q || die "not found"`).

In perl you would want to use `or` instead of `||` to take advantage of the low precedence, so you can type things like `dostuff $foo or die` which is logically 'do stuff, and if it fails, die' as opposed to 'die if $foo is false', which you'd get with ||

It's a stage 2 proposal, which technically can be used today via babel[0] (though personally, I don't recommend using stuff below stage 4)

[0] https://babeljs.io/docs/en/babel-plugin-proposal-throw-expre...

My preference for this is to use an assertion function, eg in nodejs:

assert(isDefined(user),”User must be defined”) // throws if Falsy

This pattern is quite common in React to do conditional rendering:

  {isLoading && <Loading />}