Does .sort function change original array?

Question

I have that code:

arr = arr.sort(function (a, b) {
    return a.time>b.time
})

Do I need to redefine arr or it is possible just to call sort function? like this:

arr.sort(function (a, b) {
    return a.time>b.time
})

Will the sort and filter functions change the original array?

Answer 1

Use slice() to sort a copy of the original array.

var arr =[{time:4},{time:3},{time:6}];

arr.sort(function (a, b) {
  return a.time-b.time;
});

will mutate the original array and returns :

[ { time: 3 }, { time: 4 }, { time: 6 } ]

and console.log(arr) returns

[ { time: 3 }, { time: 4 }, { time: 6 } ]

but

var arr =[{time:4},{time:3},{time:6}];
arr.slice().sort(function (a, b) {
  return a.time-b.time;
});

returns

[ { time: 3 }, { time: 4 }, { time: 6 } ]

but will not affect the original array.

console.log(arr) returns

[ { time: 4 }, { time: 3 }, { time: 6 } ]

Answer 2

It sorts the array in place (modifying the array). From MDN:

The sort() method sorts the elements of an array in place and returns the array. The sort is not necessarily stable. The default sort order is according to string Unicode code points.

Answer 3

It's a decent question, and let's answer it properly:

const a = [1, 2, 3];
const b = a.sort();
console.log(a === b); // true

there is your answer. The === operator for objects will compare memory locations, so it's the same object in memory.

Which is a shame because it would be better if sort created a new array (immutability etc), but in many languages it does not return a new array, but the same array (reordered).

So if you want it to be immutable, you can do:

const a = [1, 2, 3];
const b = a.slice(0).sort();