Follow ES5 implementation for alternatives to map, forEach, filter,...

[jdalton]

Underscore uses native array methods when possible, but doesn't follow ES5 spec in the alternative code forks. This can be seen in how sparse arrays are handled.

Something like

for (var i = 0, l = obj.length; i < l; i++) {
  if (iterator.call(context, obj[i], i, obj) === breaker) return;
}

should be changed to

for (var i = 0, l = obj.length; i < l; i++) {
  if (i in obj && iterator.call(context, obj[i], i, obj) === breaker) return;
}

[jashkenas]

Sure thing -- there's the patch that adds the i in obj to the emulated forEach function.

[jdalton]

Rock!