Are there legitimate uses for JavaScript's “with” statement?

Question

Alan Storm\'s comments in response to my answer regarding the with statement got me thinking. I\'ve seldom found a reason to use this particular language feature, and had ne

Answer 1

As Andy E pointed out in the comments of Shog9's answer, this potentially-unexpected behavior occurs when using with with an object literal:

for (var i = 0; i < 3; i++) {
  function toString() {
    return 'a';
  }
  with ({num: i}) {
    setTimeout(function() { console.log(num); }, 10);
    console.log(toString()); // prints "[object Object]"
  }
}

Not that unexpected behavior wasn't already a hallmark of with.

If you really still want to use this technique, at least use an object with a null prototype.

function scope(o) {
  var ret = Object.create(null);
  if (typeof o !== 'object') return ret;
  Object.keys(o).forEach(function (key) {
    ret[key] = o[key];
  });
  return ret;
}

for (var i = 0; i < 3; i++) {
  function toString() {
    return 'a';
  }
  with (scope({num: i})) {
    setTimeout(function() { console.log(num); }, 10);
    console.log(toString()); // prints "a"
  }
}

But this will only work in ES5+. Also don't use with.