sort() in an array

JavaScriptJunior~2 min read

The sort() method in JavaScript is used to sort the elements of an array. It mutates the original array (unlike most other methods) and can work with both numbers and strings if you provide a compare function.

Syntax

javascript

array.sort(compareFunction);

Where `compareFunction` is a callback of the form:

javascript

(a, b) => {
  // return a negative value if a should come before b
  // return a positive value if a should come after b
  // return 0 if the order doesn't matter
}

Example 1. Simple string sorting

javascript

const fruits = ['banana', 'apple', 'cherry'];

fruits.sort();

console.log(fruits); // ['apple', 'banana', 'cherry']

By default sort() sorts elements as Unicode strings, even if they are numbers! That's why numbers without a callback are sorted not by magnitude, but by character.

Example 2. A mistake when sorting numbers without a compare function

javascript

const numbers = [1, 10, 2, 5];
numbers.sort();
console.log(numbers); // [1, 10, 2, 5] - sorted as strings!

Fixed:

javascript

numbers.sort((a, b) => a - b);
console.log(numbers); // [1, 2, 5, 10]

The function (a, b) => a - b sorts in ascending order, (a, b) => b - a - in descending order.

Example 3. Sorting by an object property

javascript

const users = [
  { name: 'Tim', age: 25 },
  { name: 'Alex', age: 30 },
  { name: 'John', age: 20 }
];

users.sort((a, b) => a.age - b.age);

console.log(users);
// [
//   { name: 'John', age: 20 },
//   { name: 'Tim', age: 25 },
//   { name: 'Alex', age: 30 }
// ]

A common scenario is sorting an array of objects by a numeric field.

Example 4. Sorting strings alphabetically (case-insensitive)

javascript

const names = ['tim', 'Alex', 'john'];

names.sort((a, b) => a.localeCompare(b, 'en', { sensitivity: 'base' }));

console.log(names); // ['Alex', 'john', 'tim']

localeCompare is the correct way to sort strings taking language and case into account.

Example 5. Sorting in descending order

javascript

const prices = [100, 500, 200, 50];

prices.sort((a, b) => b - a);

console.log(prices); // [500, 200, 100, 50]

Important: `sort()` mutates the original array

javascript

const arr = [3, 1, 2];
const sorted = arr.sort();

console.log(sorted); // [1, 2, 3]
console.log(arr);    // [1, 2, 3] - the same array, mutated!

If you need to keep the original array, copy it before sorting:

javascript

const sortedCopy = [...arr].sort((a, b) => a - b);

Example 6. Sorting by multiple criteria

javascript

const users = [
  { name: 'Tim', age: 25 },
  { name: 'Alex', age: 25 },
  { name: 'John', age: 20 }
];

users.sort((a, b) => {
  if (a.age !== b.age) return a.age - b.age;
  return a.name.localeCompare(b.name);
});

console.log(users);
// [
//   { name: 'John', age: 20 },
//   { name: 'Alex', age: 25 },
//   { name: 'Tim', age: 25 }
// ]

If the age is the same, sorting falls back to the name.

Mistakes to watch for

Without a callback, strings are sorted instead of numbers

javascript

[10, 2, 1].sort(); // ['1', '10', '2']

Solution:

javascript

[10, 2, 1].sort((a, b) => a - b); // [1, 2, 10]

Mutating the original array

javascript

const a = [3, 1, 2];
const b = a.sort();
console.log(a === b); // true - it's the same object

Lack of stability in older JS versions

Sorting could be unstable in older engines (before ES2019). In modern ECMAScript versions sort() is stable - elements with equal order keep their original relative position.

When to use `sort()`

Goal	Is `sort()` suitable?
Sort numbers or strings	Yes
Sort objects by a field	Yes
Get a new sorted array without changing the original	Yes, but only with a copy (`[...arr]`)
Check a condition	No
Transform data	No

In short:

sort() sorts the elements of an array in place, by default as strings. To sort numbers, provide a compare function.

Memorization formula: arr.sort((a, b) => a - b) -> ascending arr.sort((a, b) => b - a) -> descending

Comparison with other methods

Method	Returns a new array	Mutates the original	Purpose
map()	yes	no	Transforms elements
filter()	yes	no	Selects elements
find()	no (an element)	no	Finds the first matching element
reduce()	no (a single value)	no	Reduces to a single result
sort()	no (the same array)	yes	Sorts elements

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