WebAssembly doesn't appear to be a way for threading to come to JS, unless you count adding a JS API that lets you invoke multithreaded code in a totally different format and memory/execution model as "threading coming to JS". I don't think anyone is itching to move V8/SpiderMonkey/etc. to concurrent garbage collection, so concurrent memory access in multithreaded JS code will likely be limited to SharedArrayBuffer/WebAssembly for the foreseeable future.