In recent days I have seen multiple posts regarding JS performance, claiming while loops are faster than for-loops. Given that there is no reason for this to be the case, these claims suprised me so I did a very simple test: compare the code generated for a for-loop vs. a while-loop in JSC. They are identical:
function forLoop(imgs) {
    for (var i=0; i < imgs.length; i++) {
         var e = imgs[i];
         // ...
    }
}
function whileLoop(imgs) {
    var i = 0;
    while (i < imgs.length) {
         var e = imgs[i];
         // ...
         i++;
    }
}
// instruction description


[   6] e = imgs[i]
[  10] ++i
[  12] r2 = imgs.length
[  20] if (i < r2) goto 6

[   0] enter
[   1] mov       r0, 0(@k0)
[   4] jmp       8(->12)
[   6] get_by_val    r1, r-9, r0
[  10] pre_inc       r0
[  12] get_by_id     r2, r-9, length(@id0)
[  20] loop_if_less  r0, r2, -14(->6)
[  24] ret       undefined(@k1)

Identifiers:
  id0 = length

Constants:
   k2 = 0
   k3 = undefined
[   0] enter
[   1] mov       r0, 0(@k0)
[   4] jmp       8(->12)
[   6] get_by_val    r1, r-9, r0
[  10] pre_inc       r0
[  12] get_by_id     r2, r-9, length(@id0)
[  20] loop_if_less  r0, r2, -14(->6)
[  24] ret       undefined(@k1)

Identifiers:
  id0 = length

Constants:
   k2 = 0
   k3 = undefined