Fighting Array Functions with ES6

Apr 07 2019

Yesterday, I came across an interesting bug regarding JavaScript Arrays, and I wanted to share my approach to fixing it. At a basic level, I wanted to pass part of an array to a function, but wanted to use the original array later on.

let arr = [1, 2, 3, 4, 5]
let something = arr.splice(0, 3)
do(something) // []

Thinking that Array.prototype functions don’t mutate the array directly, I moved on with my day. This lead to a bunch of problems down the line. Some array methods in the EcmaScript specification are designed to mutate arrays, while others do not.

Non-mutating functions

  • Array.prototype.slice()
  • Array.prototype.join()

These functions do not mutate the array they are called on. For example:

let arr = [1, 2, 3, 4, 5];
let partOfArr = arr.slice(1, 2);
console.log(partOfArr); // [2, 3]
console.log(arr); // [1, 2, 3, 4, 5]

Mutating functions

  • Array.prototype.sort()
  • Array.prototype.splice()
  • Array.prototype.reverse()

These methods mutate the array directly. This can lead to unreadable code, as the value can be manipulated from anywhere. For example:

let arr = [5, 2, 4];
console.log(arr); // [2, 4, 5]

To me, it is very unclear, which functions do, and which don’t mutate arrays directly. But, there’s a simple trick you can use to stop letting the functions mutate arrays directly, ultimately leading to more readable and reliable code.

Enter: The ES6 Spread Operator!

Spread Operator

Take a look at this snippet:

let arr = [3, 5, 1, 2, 4];
let sorted = [...arr].sort();
console.log(arr); // [3, 5, 1, 2, 4]
console.log(sorted); // [1, 2, 3, 4, 5]

Voilà! We have a sorted array, and the original one is also around. The spread operator([...arr]) is used to create a new array with every value of arr . You can use this for arrays, as well as objects:

let obj = {
  field: "example",
let extendedObj = {
  anotherField: 42,
console.log(extendedObj.field); // "example"


ES6 brought us awesome features like let and const assignments, as well as arrow functions. A more unknown feature however is the spread operator. I hope you now know how to use the spread operator, and that you can adopt it for cleaner and simpler code.

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Garrit Franke

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