Oh, I thought this came only with what was related to EMScripten before WASM (I forgot what the fast optimization standard is/was called). This took some years to propagate to all browsers.

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.)