You’re thinking of ASM.js, a method of writing and optimizing JavaScript as a compile target for C. It involved a lot of |0 to force the VMs of the time to assume every number was a int32 instead of a float64. Firefox’s engine briefly used a special optimizer for asm.js but it’s just standard compliant JS.

WebAssembly is a new virtual machine that isn’t related to JavaScript. There’s no garbage collector or automatic memory management. It uses a binary compiled format for machine code instead of plain text. It’s kinda like Java if you replace the Java memory model with the C memory model, and the Java language with something that looks more like LLVM mashed up with Lisp.