In this JavaScript tutorial we’re going to learn about the filter JavaScript Array Method and how it can be used to create a new array which will contain all the values for which the callback function returns a truthy value.
The filter()
method creates a new array with all elements that pass the test implemented by the provided function.
const words = ['spray', 'limit', 'elite', 'exuberant', 'destruction', 'present'];
const result = words.filter(word => word.length > 6);
console.log(result);
// expected output: Array ["exuberant", "destruction", "present"]
let newArray = arr.filter(callback(element[, index, [array]])[, thisArg])
callback
Function is a predicate, to test each element of the array. Return true
to keep the element, false
otherwise.
It accepts three arguments:
element
The current element being processed in the array.
index
| Optional
The index of the current element being processed in the array.
array
| Optional
The array filter
was called upon.
thisArg
| Optional
Value to use as this
when executing callback
.
A new array with the elements that pass the test. If no elements pass the test, an empty array will be returned.
filter()
calls a provided callback
function once for each element in an array, and constructs a new array of all the values for which callback
returns a value that coerces to true
. callback
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 callback
test are simply skipped, and are not included in the new array.
callback
is invoked with three arguments:
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 callback
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 callback
. Elements which are appended to the array after the call to filter()
begins will not be visited by callback
. If existing elements of the array are changed, or deleted, their value as passed to callback
will be the value at the time filter()
visits them; elements that are deleted are not visited.
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
}
let filtered = [12, 5, 8, 130, 44].filter(isBigEnough)
// filtered is [12, 130, 44]
The following example returns all prime numbers in the array:
let array = [4, 6, 8, 9, 12, 53, -17, 2, 5, 7, 31, 97, -1, 17];
function isPrime(num) {
if (num <= 1)
return false;
else if (num === 2)
return true;
else {
for (let i = 2; i < num; i++)
if (num % i === 0)
return false;
return true;
}
}
console.log(array.filter(isPrime)); // [53, 2, 5, 7, 31, 97, 17]
The following example uses filter()
to create a filtered json of all elements with non-zero, numeric id
.
let arr = [
{ id: 15 },
{ id: -1 },
{ id: 0 },
{ id: 3 },
{ id: 12.2 },
{ },
{ id: null },
{ id: NaN },
{ id: 'undefined' }
]
let invalidEntries = 0
function isNumber(obj) {
return obj !== undefined && typeof(obj) === 'number' && !isNaN(obj)
}
function filterByID(item) {
if (isNumber(item.id) && item.id !== 0) {
return true
}
invalidEntries++
return false;
}
let 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
Following example uses filter()
to filter array content based on search criteria.
let fruits = ['apple', 'banana', 'grapes', 'mango', 'orange']
/**
* Filter array items based on search criteria (query)
*/
function filterItems(arr, query) {
return arr.filter(function(el) {
return el.toLowerCase().indexOf(query.toLowerCase()) !== -1
})
}
console.log(filterItems(fruits, 'ap')) // ['apple', 'grapes']
console.log(filterItems(fruits, 'an')) // ['banana', 'mango', 'orange']
const fruits = ['apple', 'banana', 'grapes', 'mango', 'orange']
/**
* Filter array items based on search criteria (query)
*/
const filterItems = (arr, query) => {
return arr.filter(el => el.toLowerCase().indexOf(query.toLowerCase()) !== -1)
}
console.log(filterItems(fruits, 'ap')) // ['apple', 'grapes']
console.log(filterItems(fruits, 'an')) // ['banana', 'mango', 'orange']
The following examples tests the behavior of the filter
method when the array is modified.
// Modifying each words
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 its been popped off `words` before filter can even get there
// ["spray" ,"limit"]
filter()
was added to the ECMA-262 standard in the 5th edition. Therefore, it may not be present in all implementations of the standard.
You can work around this by inserting the following code at the beginning of your scripts, allowing use of filter()
in ECMA-262 implementations which do not natively support it. This algorithm is exactly equivalent to the one specified in ECMA-262, 5th edition, assuming that fn.call
evaluates to the original value of Function.prototype.bind()
, and that Array.prototype.push()
has its original value.
if (!Array.prototype.filter){
Array.prototype.filter = function(func, thisArg) {
'use strict';
if ( ! ((typeof func === 'Function' || typeof func === 'function') && this) )
throw new TypeError();
var len = this.length >>> 0,
res = new Array(len), // preallocate array
t = this, c = 0, i = -1;
var kValue;
if (thisArg === undefined){
while (++i !== len){
// checks to see if the key was set
if (i in this){
kValue = t[i]; // in case t is changed in callback
if (func(t[i], i, t)){
res[c++] = kValue;
}
}
}
}
else{
while (++i !== len){
// checks to see if the key was set
if (i in this){
kValue = t[i];
if (func.call(thisArg, t[i], i, t)){
res[c++] = kValue;
}
}
}
}
res.length = c; // shrink down array to proper size
return res;
};
}
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