supporting foreach style for-loops

One pattern I use a lot is a for-loop that looks like this:

for (let i = 0, token; token = tokens[i]; i++) {
  // do something with token
}

This errors out, saying:

Expected a conditional expression and instead saw an assignment

I think this is totally appropriate. Unfortunately there’s no way to get around this error without changing the loop completely. Here’s a syntax that I think should work:

for (let i = 0, token; (token = tokens[i]); i++) {
  // do something with token
}

Since it’s explicit that there’s an assignment, but it’ll give the same error as previously.

[timoxley]

Still kind unreadable

@dcousens whoops somehow I read this as your comment, my bad.

[matthewmueller]

@jieverson

This approach saves a function allocation and allows you to do this more easily:

for (let i = 0, url; (url = urls[i]); i++) {
  await fetch(url)
}

@dcousens yep:

Just have to be a bit more careful about falsey values. In practice I’ve only run into that issue like 3 times over the last couple years though.

also, it’s perfectly safe for other types when you have control over what you’re looping over, which happens quite a bit internally.