The native array sort in Javascript is old and mutates the array instead of creating a new one. If you’re not using ES6, I would suggest using concat on a new array, to avoid mutating the old one: var newUsers =. Here we create a new array, add all the items of users to it, plus the newUser. I have an array of users, and I want to change the age John to 23. Almost all of the array related logic can be written using these three methods. When you’re writing logic dealing with arrays, these 3 are you’re friends. You’re always sure that you are working with the array or object you expect, without having to worry about it being mutated somewhere else. Not mutating data makes your code less error prone. In functional programming mutating data is considered evil. The Mutators mutate the Array and the Accessors and the Iterators do not, they return a new array every time you call them. You can check out this MDN article about array methods. We could seperate the methods that work with arrays into two categories: mutating and not mutating. But what’s a good, functional way to deal with them? To mutate or not to mutate Big lists, small lists, nested lists, lists of objects. It can be DOM elements you’re traversing through, or a list of users your retrieve from the server. Let’s see how we can leverage Javascript’s three most powerful built in functional array methods.

We prefer a functional programming style in our Javascript applications.

There is a clear trend going on in the Javascript world.

Javascript array map reduce