WebAssembly's ultimate goal is not to replace JavaScript, but rather to complement it. JavaScript's rich ecosystem and flexibility still makes it the ideal language for the web. WebAssembly's JavaScript API makes interoperability between the two technologies relatively simple. So will you be able to build a web application using just WebAssembly? One of the explicit goals of WebAssembly is portability, and replicating all of JavaScript's functionality could inhibit that goal. However, the official site includes a goal to execute and integrate well with the existing web platform, so only time will tell. It may not be practical to write the entire code base in a language that compiles down to WebAssembly, but moving some of the application logic to Wasm modules could be beneficial in terms of performance and load times.