As for interpreted JS, a binary operator returns a 32-bit integer value, but it would be still stored as a Number (float). (Meaning, a | 0 => int, b = a | 0 => float stored in b, there is no other primitive numeric type. – Instead of optimizing for speed, you would be adding implicit type conversions.)
Nowadays the semantics if what happens are still the same, but things happen a bit more optimized by usually avoiding the round-trip to double when not necessary.
(I think, this had been originally introduced by Mozilla and propagated to other browsers to varying extent. This optimization allowed a significant speed up for things like EMScripten before WASM.)
