JavaScript engines do optimize integers. They usually represent integers up to +-2^30 as integers and apply integer operations to them. But of course that's not observable.
You are half correct about 2^53-1 being used (around 9 quadrillion). It is the largest integer representable with 64-bit float. JS even includes a `Number.MAX_SAFE_INTEGER`.
That said, these only get used in the rare cases where your number exceeds around 1 billion which is fairly rare.
JS engines use floats only when they cannot prove/speculate that a number can be an i32. They only use 31 of the 32 bits for the number itself with the last bit used for tagging. i32 takes fewer cycles to do calculations with (even with the need to deal with the tag bit) compared to f64. You fit twice as many i32 in a cache line (affects prefetching). i32 uses half the RAM (and using half the cache increases the hit rate). Finally, it takes way more energy to load two numbers into the ALU/FPU than it does to perform the calculation, so cutting the size in half also reduces power consumption. The max allowable size of a JS array is also 2^32.
JS also has BigInt available for arbitrary precision integers and these are probably what someone should be using if they expect to go over that 2^31-1 limit because hitting a number that big generally means you have something unbounded and might go over that 2^53-1 limit.
