TypeScript does something similar to what less or sass does for CSS. They are super sets of it, which means that every JS code you write is valid TypeScript code. Plus you can use the other goodies that it adds to the language, and the transpiled code will be valid js. You can even set the JS version that you want your resulting code on.
Currently TypeScript is a super set of ES2015, so might be a good choice to start learning the new js features and transpile to the needed standard for your project.