Well, I haven't actually tried the asm.js approach for this, so maybe I'm missing something.

But since asm.js is just (a subset of) javascript, I assumed I could just pass ArrayBuffers around.

With wasm, I could pass a Uint8Array out of it. If I wanted to pass it in, I had to call malloc from the javascript side to allocate in the wasm heap. But since I already had an arraybuffer (from a file upload), that meant an extra copy.