Treating it as a dealbreaker seems a bit strong, but immutability has become very popular among JS devs recently. JS has pretty respectable functional-programming features, libraries like Immutable.js have cropped up to add memory-saving immutable data structures, and major frameworks like React and Redux play much more naturally with immutable data than mutable. The overall goal is scalability through reduction of side-effects. Whether the trade off is worth it or not, it's absolutely the fashion right now.