@dcodeIO wdyt should it also optionally add for BigInt64Array and BigUint64Array?

Hmm, good question. There is no good i64 story yet, and portable doesn't yet try to polyfill any of that, including the basic type. I suggest leaving the PR as-is for now, and think about better portable i64 support in general later, for example we'd also have to alias BigInt64Array to Int64Array first.

Perhaps on a second note, one way to polyfill i64 in general might be to hijack the i64.xy set of instructions, so one can write i64.add(x, y) etc. in both JS and Wasm to make it work. That would also fit well into the compiler I guess, with an i64 being a Long.js object in JS. However, there are no inline-assembler variants for very basics ops yet, like the mentioned i64.add, but that shouldn't be too hard to add (just a lot of work to add all of them for i32, f32, f64 etc.).