JS has a lot of limitations and the developers around the world have been continuously working on overcoming them by building JS transcompilers (which convert code from one language into JS code).
WebAssembly allows the programmers to compile their code for browsers in one language (it is C/C++ as of now) which is then executed inside the JS engine of the browsers. WebAssembly can decode the code a lot faster as it doesn’t have to parse the full code at one go.
When implemented in all web browsers, WebAssembly will provide the developers with a single compilation target for the web, and hence will become a new standard of the web.
JS files are text files which are downloaded from the web server along with the other elements of a web page. These files are then parsed and compiled the JS engine of the web browsers, which takes a lot of time.
WebAssembly on the other hand is in binary format which can not only be compressed more than the JS text files, but can also be decoded much faster. For instance, WebAssembly’s current prototype is 23x faster than parsing Mozilla asm.js code.
WebAssembly not only meets the target set by Mozilla’s asm.js for a super-fast web, but also offers the same functionality. Further, the developers would also be able to use the same tools they use for compiling asm.js code and WebAssembly code.
So whatever might be the case, WebAssembly has to have a lot of potential.
"Google, Microsoft, and Mozilla Announce ‘WebAssembly’- A Faster Web Standard",