Array.prototype.sort()

The sort() method sorts the elements of an array [in place](https://en.wikipedia.org/wiki/In-placealgorithm)_ and returns the sorted array. The default sort order is ascending, built upon converting the elements into strings, then comparing their sequences of UTF-16 code units values.

The time and space complexity of the sort cannot be guaranteed as it depends on the implementation.

Syntax

// Functionless
sort()

// Arrow function
sort((firstEl, secondEl) => { ... } )

// Compare function
sort(compareFn)

// Inline compare function
sort(function compareFn(firstEl, secondEl) { ... })

Parameters

compareFunction Optional Specifies a function that defines the sort order. If omitted, the array elements are converted to strings, then sorted according to each character's Unicode code point value.

firstEl The first element for comparison.

secondEl The second element for comparison.

Return value

The sorted array. Note that the array is sorted [in place](https://en.wikipedia.org/wiki/In-placealgorithm)_, and no copy is made.

Description

If compareFunction is not supplied, all non-undefined array elements are sorted by converting them to strings and comparing strings in UTF-16 code units order. For example, "banana" comes before "cherry". In a numeric sort, 9 comes before 80, but because numbers are converted to strings, "80" comes before "9" in the Unicode order. All undefined elements are sorted to the end of the array.

Note: In UTF-16, Unicode characters above \uFFFF are encoded as two surrogate code units, of the range \uD800-\uDFFF. The value of each code unit is taken separately into account for the comparison. Thus the character formed by the surrogate pair \uD655\uDE55 will be sorted before the character \uFF3A.

If compareFunction is supplied, all non-undefined array elements are sorted according to the return value of the compare function (all undefined elements are sorted to the end of the array, with no call to compareFunction). If a and b are two elements being compared, then:

  • If compareFunction(a, b) returns less than 0, leave a and b unchanged.
  • If compareFunction(a, b) returns 0, leave a and b unchanged with respect to each other, but sorted with respect to all different elements. Note: the ECMAScript standard only started guaranteeing this behavior in 2019, thus, older browsers may not respect this.
  • If compareFunction(a, b) returns greater than 0, sort b before a.
  • compareFunction(a, b) must always return the same value when given a specific pair of elements a and b as its two arguments. If inconsistent results are returned, then the sort order is undefined.

So, the compare function has the following form:

function compare(a, b) {
  if (a is less than b by some ordering criterion) {
    return -1;
  }
  if (a is greater than b by the ordering criterion) {
    return 1;
  }
  // a must be equal to b
  return 0;
}

To compare numbers instead of strings, the compare function can subtract b from a. The following function will sort the array in ascending order (if it doesn't contain Infinity and NaN):

function compareNumbers(a, b) {
  return a - b;
}

The sort method can be conveniently used with function expressions:

var numbers = [4, 2, 5, 1, 3];
numbers.sort(function(a, b) {
  return a - b;
});
console.log(numbers);

// [1, 2, 3, 4, 5]

ES2015 provides arrow function expressions with even shorter syntax.

let numbers = [4, 2, 5, 1, 3];
numbers.sort((a, b) => a - b);
console.log(numbers);

// [1, 2, 3, 4, 5]

Arrays of objects can be sorted by comparing the value of one of their properties.

var items = [
  { name: 'Edward', value: 21 },
  { name: 'Sharpe', value: 37 },
  { name: 'And', value: 45 },
  { name: 'The', value: -12 },
  { name: 'Magnetic', value: 13 },
  { name: 'Zeros', value: 37 }
];

// sort by value
items.sort(function (a, b) {
  return a.value - b.value;
});

// sort by name
items.sort(function(a, b) {
  var nameA = a.name.toUpperCase(); // ignore upper and lowercase
  var nameB = b.name.toUpperCase(); // ignore upper and lowercase
  if (nameA < nameB) {
    return -1;
  }
  if (nameA > nameB) {
    return 1;
  }

  // names must be equal
  return 0;
});

Examples

Creating, displaying, and sorting an array

The following example creates four arrays and displays the original array, then the sorted arrays. The numeric arrays are sorted without a compare function, then sorted using one.

let stringArray = ['Blue', 'Humpback', 'Beluga'];
let numericStringArray = ['80', '9', '700'];
let numberArray = [40, 1, 5, 200];
let mixedNumericArray = ['80', '9', '700', 40, 1, 5, 200];

function compareNumbers(a, b) {
  return a - b;
}

stringArray.join(); // 'Blue,Humpback,Beluga'
stringArray.sort(); // ['Beluga', 'Blue', 'Humpback']

numberArray.join(); // '40,1,5,200'
numberArray.sort(); // [1, 200, 40, 5]
numberArray.sort(compareNumbers); // [1, 5, 40, 200]

numericStringArray.join(); // '80,9,700'
numericStringArray.sort(); // [700, 80, 9]
numericStringArray.sort(compareNumbers); // [9, 80, 700]

mixedNumericArray.join(); // '80,9,700,40,1,5,200'
mixedNumericArray.sort(); // [1, 200, 40, 5, 700, 80, 9]
mixedNumericArray.sort(compareNumbers); // [1, 5, 9, 40, 80, 200, 700]

Sorting non-ASCII characters

For sorting strings with non-ASCII characters, i.e. strings with accented characters (e, é, è, a, ä, etc.), strings from languages other than English, use String.localeCompare. This function can compare those characters so they appear in the right order.

var items = ['réservé', 'premier', 'communiqué', 'café', 'adieu', 'éclair'];
items.sort(function (a, b) {
  return a.localeCompare(b);
});

// items is ['adieu', 'café', 'communiqué', 'éclair', 'premier', 'réservé']

Sorting with map

The compareFunction can be invoked multiple times per element within the array. Depending on the compareFunction's nature, this may yield a high overhead. The more work a compareFunction does and the more elements there are to sort, it may be more efficient to use map for sorting. The idea is to traverse the array once to extract the actual values used for sorting into a temporary array, sort the temporary array, and then traverse the temporary array to achieve the right order.

// the array to be sorted
const in = ['delta', 'alpha', 'charlie', 'bravo'];

// temporary array holds objects with position and sort-value
const mapped = in.map((v, i) => {
  return { i, value: someSlowOperation(v) };
})

// sorting the mapped array containing the reduced values
mapped.sort((a, b) => {
  if (a.value > b.value) {
    return 1;
  }
  if (a.value < b.value) {
    return -1;
  }
  return 0;
});

const result = mapped.map(v => in[v.i]);

There is an open source library available called mapsort which applies this approach.

Sort stability

Since version 10 (or EcmaScript 2019), the specification dictates that Array.prototype.sort is stable.

For example, say you had a list of students alongside their grades. Note that the list of students is already pre-sorted by name in alphabetical order:

const students = [
  { name: "Alex",   grade: 15 },
  { name: "Devlin", grade: 15 },
  { name: "Eagle",  grade: 13 },
  { name: "Sam",    grade: 14 },
];

After sorting this array by grade in ascending order:

students.sort((firstItem, secondItem) => firstItem.grade - secondItem.grade);

The students variable will then have the following value:

[
  { name: "Eagle",  grade: 13 },
  { name: "Sam",    grade: 14 },
  { name: "Alex",   grade: 15 }, // original maintained for similar grade (stable sorting)
  { name: "Devlin", grade: 15 }, // original maintained for similar grade (stable sorting)
];

It's important to note that students that have the same grade (for example, Alex and Devlin), will remain in the same order as before calling the sort. This is what a stable sorting algorithm guarantees.

Before version 10 (or EcmaScript 2019), sort stabiliy was not guaranteed, meaning that you could end up with the following:

[
  { name: "Eagle",  grade: 13 },
  { name: "Sam",    grade: 14 },
  { name: "Devlin", grade: 15 }, // original order not maintained
  { name: "Alex",   grade: 15 }, // original order not maintained
];