JavaScript Labels

WAT? JavaScript doesn’t have labels.. Well sure it does! Well, to be fair, they’re only for controlling loop flow. Let’s take a look!

Imagine the case where we have two loops and you want to break from the inner-most loop out of the outer loop. This is often achieved with a flag:

var stop = false;
for (var i = 0; i < 10; i++) {
  for (var j = 0; j < 10; j++) {
    stop = true;
    break;
  }
  if (stop) {
    break;
  }
}

With the labels, you can rewrite the above as:

outer:
for (var i = 0; i < 10; i++) {
  for (var j = 0; j < 10; j++) {
    break outer;
  }
}

You can do the same with a continue inside of a nested loop. In that case, you’ll break into the next iteration of the loop you continue to:

outer:
for (var i = 0; i < 10; i++) {
  for (var j = 0; j < 10; j++) {
    continue outer;
  }
  console.log('this will never print');
}

Whether you think this type of mild-goto usage is a good plan (I think it is when used correctly), its good to know they’re around for when you see them in use.