No, because changing browsers to interpret `arr[-1]` as `arr[arr.length - 1]` breaks existing sites that expect `arr[-1]` to be interpreted as `arr['-1']`: That

No, because changing browsers to interpret `arr[-1]` as `arr[arr.length - 1]` breaks existing sites that expect `arr[-1]` to be interpreted as `arr['-1']`: That is, the value stored on object `arr` at key name '-1'. Changing browsers to interpret `arr.get(-1)` as `arr[arr.length - 1]` doesn't affect any old code using `arr[-1]`. It's not about supporting old browsers. It's about supporting old code.