Webpack is weird in that it's lumped with task runners/"Make"-like programs but it actually it's a configurable and hackable version of the browser asset pipeline.

Nit: not sure why you mention const. It does not provide any kind of immutability assurance.

I just use const instead of var/let all the time and it prevented me from overriding the variable later, which forced me to write my code a bit more functional then before I used it.

I know that I can still change indirect references or override valueOf() etc.

You probably know this, but for anyone else reading, the problem is that if you assign an object or array using const, the object/array members are still mutable. If you have a tree structure, you have to recursively Object.freeze the entire tree to get immutability, which AFAIK is basically what immutable.js does for you. I also use const by default, but I find that more often than not my variables are trees and lists.