The team behind this new project has stated pretty clearly that this project is not meant to replace JavaScript, but be used for specific cases, mostly working alongside JS. For now, the project is mostly has “bridge the gap” intentions as it is still in very early development. None of the project’s specifications or design have been finalized yet. However, with such major developers supporting the project, it’s hard to think it will be long before it’s ready.
“While WebAssembly will, over time, allow many languages to be compiled to the Web, JS has an incredible amount of momentum and will remain the single, privileged dynamic language of the Web,” reads the official WebAssembly statement.