for (let i = 0; i < a.length; i++) {
    a[i] = a[i]*2
  }

Has to be converted into something that can be run onto a machine that won't cause a fatal error if the array `a` doesn't have an item at index `i` (e.g. negative index). It can be converted to the following by a naive compiler:

  for (let i = 0; i < a.length; i++) {
    if (i > 0 && a < a.length) { // safe to access
      a[i] = a[i] * 2
    } else {
      throw('in a safe way')
    }
  }

Or, to avoid repetitive checks to a.length:

  const temp = a.length
  for (let i = 0; i < temp; i++) {
    if (i > 0 && a < temp) { // safe to access
      a[i] = a[i] * 2
    }
  }

A more advanced compiler might realise that the if statement is irrelevant, we've defined it in the loop that we'll never have a negative `i` or one above the length of the array, so we can just avoid those checks entirely and actually revert back to the original code to compile. This is not always the case:

  const temp = a.length
  for (let i = 0; i < temp; i++) {
    a[i] = a[i]*2
  }

This comes up in interesting places, like why it is faster to go through an array backwards than forwards[0].

[0] https://stackoverflow.com/questions/8689573/why-is-iterating...

I think it is that since the length is never mutated in the body the check can be simplified. From a generic dictionary lookup to comparison to a constant int.

If the array structure itself were being modified, the "i < a.length" check would have to be done every iteration, but in this case there's no mutation of the array itself, so it's OK to move "a.length" to a constant outside the loop... and from there it's trivial to move bounds checks outside the loop (and from there: eliminate them entirely)