Immutable Javascript means that we're never going to change a value once it's been set on an object. It might sound complicated, but it's really quite simple. We simply always create new objects and copy their values over.

If you want to learn Redux for React.js applications, you have to know how to code immutable JS objects, since it's built upon that. Instead of changing values on a store, we write reducers to modify those values and return a new object instead.