React, Flux, GraphQL, Hack, HHVM...? All of this and more!
WebAssembly allows writing browser applications in any language that is strongly typed languages. These include ones like C, C++, Java and more recently Rust. Web developers often use dynamically typed languages like JavaScript, PHP and Python.
Building communications and computing technology is a painstakingly long task. The route from initial idea will typically take years first to spec, implementation and finally mainstream adoption. WebAssembly is now on this route, it is already usable - but the official specification work continues.
Since the 1990s the only standard language available to developers targeting the web developers has been JavaScript. This is a great thing and has lead to the thriving JavaScript ecosystem we have today. This has been a given for over two decades, but with WebAssembly it could change.