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Array.prototype.filter()

The filter() method creates a new array with all elements that pass the test implemented by the provided function.

Syntax

arr.filter(callback[, thisArg])

Parameters

callback
Function to test each element of the array. Invoked with arguments (element, index, array). Return true to keep the element, false otherwise.
thisArg
Optional. Value to use as this when executing callback.

Description

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 true value or 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:

  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 passed to callback when invoked, for use as its this value.  Otherwise, the value undefined will be passed for use 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.

Examples

Example: 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;
}
var filtered = [12, 5, 8, 130, 44].filter(isBigEnough);
// filtered is [12, 130, 44]

Example: Filtering invalid entries from JSON

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

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

var invalidEntries = 0;

function filterByID(obj) {
  if ('id' in obj && typeof(obj.id) === 'number' && !isNaN(obj.id)) {
    return true;
  } else {
    invalidEntries++;
    return false;
  }
}

var arrByID = arr.filter(filterByID);

console.log('Filtered Array\n', arrByID); 
// [{ id: 15 }, { id: -1 }, { id: 0 }, { id: 3 }, { id: 12.2 }]

console.log('Number of Invalid Entries = ', invalidEntries); 
// 4

Polyfill

filter() was added to the ECMA-262 standard in the 5th edition; as such 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 the one specified in ECMA-262, 5th edition, assuming that fn.call evaluates to the original value of Function.prototype.call(), and that Array.prototype.push() has its original value.

if (!Array.prototype.filter) {
  Array.prototype.filter = function(fun/*, thisArg*/) {
    'use strict';

    if (this === void 0 || this === null) {
      throw new TypeError();
    }

    var t = Object(this);
    var len = t.length >>> 0;
    if (typeof fun !== 'function') {
      throw new TypeError();
    }

    var res = [];
    var thisArg = arguments.length >= 2 ? arguments[1] : void 0;
    for (var i = 0; i < len; i++) {
      if (i in t) {
        var val = t[i];

        // NOTE: Technically this should Object.defineProperty at
        //       the next index, as push can be affected by
        //       properties on Object.prototype and Array.prototype.
        //       But that method's new, and collisions should be
        //       rare, so use the more-compatible alternative.
        if (fun.call(thisArg, val, i, t)) {
          res.push(val);
        }
      }
    }

    return res;
  };
}

Specifications

Specification Status Comment
ECMAScript 5.1 (ECMA-262)
The definition of 'Array.prototype.filter' in that specification.
Standard Initial definition. Implemented in JavaScript 1.6.
ECMAScript 2015 (6th Edition, ECMA-262)
The definition of 'Array.prototype.filter' in that specification.
Standard  

Browser compatibility

Feature Chrome Firefox (Gecko) Internet Explorer Opera Safari
Basic support (Yes) 1.5 (1.8) 9 (Yes) (Yes)
Feature Android Chrome for Android Firefox Mobile (Gecko) IE Mobile Opera Mobile Safari Mobile
Basic support (Yes) (Yes) 1.0 (1.8) (Yes) (Yes) (Yes)

See also

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