Among other things, wasm has access to float32, int64 and vector types that JS does not. It also has operations like reinterpret_cast that you can't do in JS without a store/load pair using a scratch buffer.
Parent referred to int64. BigInt and int64 are not the same thing.
JS doesn't have integers; all numbers are represented as doubles. One of the reasons having actual integers is important is performance, as integer operations are often faster than floating point operations. BigInt doesn't help with that.
