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

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];
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, prefereably a way that doesn't require hand-rolling my own sorting algorithm or copying every element of the array into a new one?



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

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

// Or:
return; // For array-like objects

Another way with es6 (non-deep copy):

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

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

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


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.


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

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

You can also do this

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

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]

I use Object.assign() for most of my copies:

var copyArray = Object.assign([], originalArray).sort();

However, after looking through the OP comments, I researched a bit of deep copying and turns out Object.assign not only performs a shallow copy, but also only selects enumerable and own properties (as answered in this post).


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) => >

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


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