Multi threading will not be done in EcmaScript but can be exposed in host environments.
The classic examples are WebWorkers which allows you to spin up a background worker to do work in and abusing <iframe>'s as a way to spawn new processes.
It should be noted that multi threading in JavaScript is not needed (there are exceptions, mainly graphics related programs). You don't need multiple threads, you already have an event loop for your GUI and your graphics rendering (canvas) is hardware accelerated (meaning the GPU renders your graphics in parallel for you).
Although projects like webcl are pretty exciting.