[kevingadd]

WebAssembly adds new functionality to the browser runtime environment that isn't available to regular JavaScript. It was originally 1:1 with asm.js/JS but that stopped being true relatively quick.

[notriddle]

It's faster. That's literally it. There aren't any new I/O facilities.

[kevingadd]

A few examples of things it adds that are not widely available in JS (if at all, or requiring awkward hacks/hints in JS), off the top of my head:

int64

uint32

popcnt

copysign

reinterpret_cast equivalent (the js hack for this is abysmal)

pthreads

SIMD

AOT compilation

To be fair, it's been a while since I was last one of the authors of the webassembly spec, so I could be missing some things.

[Bombthecat]

Could ads be served with web assembly?

[kevingadd]

Yes, though I don't know if it makes sense to do it for any reason other than hiding from ad blockers.

[Bombthecat]

Well, for some it is the only reason needed...