How can you sort an array without mutating the original array?

Question

Let's suppose I wanted a sort function that returns a sorted copy of the inputted array. I naively tried this

function sort(arr) {
  return arr.sort();
}

and I tested it with this, which shows that my sort method is mutating the array.

var a = [2,3,7,5,3,7,1,3,4];
sort(a);
alert(a);  //alerts "1,2,3,3,3,4,5,7,7"

I also tried this approach

function sort(arr) {
  return Array.prototype.sort(arr);
}

but it doesn't work at all.

Is there a straightforward way around this, preferably a way that doesn't require hand-rolling my own sorting algorithm or copying every element of the array into a new one?

Answer 1

You need to copy the array before you sort it. One way with es6:

const sorted = [...arr].sort();

The spread-syntax as array literal (copied from mdn):

var arr = [1, 2, 3];
var arr2 = [...arr]; // like arr.slice()

https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Operators/Spread_operator

Answer 2

Just copy the array. There are many ways to do that:

function sort(arr) {
  return arr.concat().sort();
}

// Or:
return Array.prototype.slice.call(arr).sort(); // For array-like objects

Answer 3

Try the following

function sortCopy(arr) { 
  return arr.slice(0).sort();
}

The slice(0) expression creates a copy of the array starting at element 0.

Answer 4

You can use slice with no arguments to copy an array:

var foo,
    bar;
foo = [3,1,2];
bar = foo.slice().sort();

Answer 5

You can also do this

d = [20, 30, 10]
e = Array.from(d)
e.sort()

This way d will not get mutated.

function sorted(arr) {
  temp = Array.from(arr)
  return temp.sort()
}

//Use it like this
x = [20, 10, 100]
console.log(sorted(x))

Answer 6

Update - Array.prototype.toSorted() proposal

The Array.prototype.toSorted(compareFn) -> Array is a new method that was proposed to be added to the Array.prototype and is currently in stage 3 (Soon to be available).

This method will keep the target Array untouched and return a copy with the change performed instead.

Answer 7

ES2023 Array Method toSorted():

The toSorted() method of Array instances is the copying version of the sort() method. It returns a new array with the elements sorted in ascending order.

const arr = [2, 1, 3];
const arrSorted = arr.toSorted();
console.log(arr); //[2, 1, 3]
console.log(arrSorted); //[1, 2, 3]

PS: toSorted() method is supported nearly by all browsers and on Node.js version 20+.
see browser compatibility

Answer 8

Anyone who wants to do a deep copy (e.g. if your array contains objects) can use:

let arrCopy = JSON.parse(JSON.stringify(arr))

Then you can sort arrCopy without changing arr.

arrCopy.sort((obj1, obj2) => obj1.id > obj2.id)

Please note: this can be slow for very large arrays.

Answer 9

Try this to sort the numbers. This does not mutate the original array.

function sort(arr) {
  return arr.slice(0).sort((a,b) => a-b);
}

Answer 10

There's a new tc39 proposal, which adds a toSorted method to Array that returns a copy of the array and doesn't modify the original.

For example:

const sequence = [3, 2, 1];
sequence.toSorted(); // => [1, 2, 3]
sequence; // => [3, 2, 1]

As it's currently in stage 3, it will likely be implemented in browser engines soon, but in the meantime a polyfill is available here or in core-js.

Answer 11

You can also extend the existing Array functionality. This allows chaining different array functions together.

Array.prototype.sorted = function (compareFn) {
    const shallowCopy = this.slice();
    shallowCopy.sort(compareFn);

    return shallowCopy;
}

[1, 2, 3, 4, 5, 6]
    .filter(x => x % 2 == 0)
    .sorted((l, r) => r - l)
    .map(x => x * 2)

// -> [12, 8, 4]

Same in typescript:

// extensions.ts
Array.prototype.sorted = function (compareFn?: ((a: any, b: any) => number) | undefined) {
    const shallowCopy = this.slice();
    shallowCopy.sort(compareFn);

    return shallowCopy;
}

declare global {
    interface Array<T> {
        sorted(compareFn?: (a: T, b: T) => number): Array<T>;
    }
}

export {}

// index.ts
import 'extensions.ts';


[1, 2, 3, 4, 5, 6]
    .filter(x => x % 2 == 0)
    .sorted((l, r) => r - l)
    .map(x => x * 2)

// -> [12, 8, 4]

Answer 12

To sort a function without mutating the original, simply use .map() before sort to create a copy of the original array:

const originalArr = [1, 45, 3, 21, 6];
const sortedArr = originalArr.map(value => JSON.parse(JSON.stringify(value))).sort((a, b) => a - b);

console.log(sortedArr); // the logged output will be 1,3,6,21,45

The original array has not been modified, but you have a sorted version of it available to you. JSON.parse(JSON.stringify()) make sure it is a deep copy, not a shallow copy.