W3cubDocs

/JavaScript

Array.prototype.filter()

The filter() method creates a shallow copy of a portion of a given array, filtered down to just the elements from the given array that pass the test implemented by the provided function.

Try it

Syntax

// Arrow function
filter((element) => { /* … */ } )
filter((element, index) => { /* … */ } )
filter((element, index, array) => { /* … */ } )

// Callback function
filter(callbackFn)
filter(callbackFn, thisArg)

// Inline callback function
filter(function(element) { /* … */ })
filter(function(element, index) { /* … */ })
filter(function(element, index, array){ /* … */ })
filter(function(element, index, array) { /* … */ }, thisArg)

Parameters

callbackFn

Function is a predicate, to test each element of the array. Return a value that coerces to true to keep the element, or to false otherwise.

The function is called with the following arguments:

element

The current element being processed in the array.

index

The index of the current element being processed in the array.

array

The array on which filter() was called.

thisArg Optional

Value to use as this when executing callbackFn.

Return value

A shallow copy of a portion of the given array, filtered down to just the elements from the given array that pass the test implemented by the provided function. If no elements pass the test, an empty array will be returned.

Description

filter() calls a provided callbackFn function once for each element in an array, and constructs a new array of all the values for which callbackFn returns a value that coerces to true. callbackFn is invoked only for indexes of the array which have assigned values; it is not invoked for indexes which have been deleted or which have never been assigned values. Array elements which do not pass the callbackFn test are skipped, and are not included in the new array.

callbackFn is invoked with three arguments:

  1. the value of the element
  2. the index of the element
  3. the Array object being traversed

If a thisArg parameter is provided to filter, it will be used as the callback's this value. Otherwise, the value undefined will be used as its this value. The this value ultimately observable by callbackFn is determined according to the usual rules for determining the this seen by a function.

filter() does not mutate the array on which it is called.

The range of elements processed by filter() is set before the first invocation of callbackFn. Elements which are assigned to indexes already visited, or to indexes outside the range, will not be visited by callbackFn. If existing elements of the array are deleted in the same way they will not be visited.

Warning: Concurrent modification of the kind described in the previous paragraph frequently leads to hard-to-understand code and is generally to be avoided (except in special cases).

Examples

Filtering out all small values

The following example uses filter() to create a filtered array that has all elements with values less than 10 removed.

function isBigEnough(value) {
  return value >= 10;
}

const filtered = [12, 5, 8, 130, 44].filter(isBigEnough);
// filtered is [12, 130, 44]

Find all prime numbers in an array

The following example returns all prime numbers in the array:

const array = [-3, -2, -1, 0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13];

function isPrime(num) {
  for (let i = 2; num > i; i++) {
    if (num % i === 0) {
      return false;
    }
  }
  return num > 1;
}

console.log(array.filter(isPrime)); // [2, 3, 5, 7, 11, 13]

Filtering invalid entries from JSON

The following example uses filter() to create a filtered JSON of all elements with non-zero, numeric id.

const arr = [
  { id: 15 },
  { id: -1 },
  { id: 0 },
  { id: 3 },
  { id: 12.2 },
  {},
  { id: null },
  { id: NaN },
  { id: 'undefined' },
];

let invalidEntries = 0;

function filterByID(item) {
  if (Number.isFinite(item.id) && item.id !== 0) {
    return true;
  }
  invalidEntries++;
  return false;
}

const arrByID = arr.filter(filterByID);

console.log('Filtered Array\n', arrByID);
// Filtered Array
// [{ id: 15 }, { id: -1 }, { id: 3 }, { id: 12.2 }]

console.log('Number of Invalid Entries = ', invalidEntries);
// Number of Invalid Entries = 5

Searching in array

Following example uses filter() to filter array content based on search criteria.

const fruits = ['apple', 'banana', 'grapes', 'mango', 'orange'];

/**
 * Filter array items based on search criteria (query)
 */
function filterItems(arr, query) {
  return arr.filter((el) => el.toLowerCase().includes(query.toLowerCase()));
}

console.log(filterItems(fruits, 'ap')); // ['apple', 'grapes']
console.log(filterItems(fruits, 'an')); // ['banana', 'mango', 'orange']

Affecting Initial Array (modifying, appending and deleting)

The following example tests the behavior of the filter method when the array is modified.

// Modifying each word
let words = ['spray', 'limit', 'exuberant', 'destruction', 'elite', 'present'];

const modifiedWords = words.filter((word, index, arr) => {
  arr[index + 1] += ' extra';
  return word.length < 6;
});

console.log(modifiedWords);
// Notice there are three words below length 6, but since they've been modified one is returned
// ["spray"]

// Appending new words
words = ['spray', 'limit', 'exuberant', 'destruction', 'elite', 'present'];
const appendedWords = words.filter((word, index, arr) => {
  arr.push('new');
  return word.length < 6;
})

console.log(appendedWords);
// Only three fits the condition even though the `words` itself now has a lot more words with character length less than 6
// ["spray" ,"limit" ,"elite"]

// Deleting words
words = ['spray', 'limit', 'exuberant', 'destruction', 'elite', 'present'];
const deleteWords = words.filter((word, index, arr) => {
  arr.pop();
  return word.length < 6;
})

console.log(deleteWords);
// Notice 'elite' is not even obtained as it's been popped off 'words' before filter can even get there
// ["spray" ,"limit"]

Specifications

Browser compatibility

Desktop Mobile Server
Chrome Edge Firefox Internet Explorer Opera Safari WebView Android Chrome Android Firefox for Android Opera Android Safari on IOS Samsung Internet Deno Node.js
filter
1
12
1.5
9
9.5
3
≤37
18
4
10.1
1
1.0
1.0
0.10.0

See also

© 2005–2022 MDN contributors.
Licensed under the Creative Commons Attribution-ShareAlike License v2.5 or later.
https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Array/filter