Array.prototype.sort()
The sort()
method sorts the elements of an array in place 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((a, b) => { /* ... */ } )
// Compare function
sort(compareFn)
// Inline compare function
sort(function compareFn(a, b) { /* ... */ })
Parameters
compareFn
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.
a
-
The first element for comparison.
b
-
The second element for comparison.
Return value
The sorted array. Note that the array is sorted in place, 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 \uD855\uDE51
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
).
compareFunction(a, b) return value |
sort order |
---|---|
> 0 | sort b before a |
< 0 | sort a before b |
=== 0 | keep original order of a and b |
Note: compareFunction(a, b)
must always return the same value when given a specific pair of
elements a
and b
as its two arguments.
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;
}
Note: Applications should not rely on order of arguments or calls, as those are implementation dependent.
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:
const 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.
const 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.
const 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) {
const nameA = a.name.toUpperCase(); // ignore upper and lowercase
const 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 numberArray = [40, 1, 5, 200];
let numericStringArray = ['80', '9', '700'];
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.
const 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 data = ['delta', 'alpha', 'charlie', 'bravo'];
// temporary array holds objects with position and sort-value
const mapped = data.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 => data[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 stability 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
];
Specifications
Specification |
---|
ECMAScript Language Specification # sec-array.prototype.sort |
Browser compatibility
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