filter

Summary

Returns the elements of an array that meet the condition specified in a callback function.

Syntax

filter( callbackfn [, thisArg ])

callbackfn

Required. A function that accepts up to three arguments. The filter method calls the callbackfn function one time for each element in the array.

thisArg

Optional. An object to which the this keyword can refer in the callbackfn function. If thisArg is omitted, undefined is used as the this value.

Return Value

A new array that contains all the values for which the callback function returns true. If the callback function returns false for all elements of array1 , the length of the new array is 0.

Examples

The following example shows how to use the filter method.

// Define a callback function.
 function CheckIfPrime(value, index, ar) {
     high = Math.floor(Math.sqrt(value)) + 1;

     for (var div = 2; div <= high; div++) {
         if (value % div == 0) {
             return false;
         }
     }
     return true;
 }

 // Create the original array.
 var numbers = [31, 33, 35, 37, 39, 41, 43, 45, 47, 49, 51, 53];

 // Get the prime numbers that are in the original array.
 var primes = numbers.filter(CheckIfPrime);

 document.write(primes);
 // Output: 31,37,41,43,47,53

In the following example, the callbackfn argument includes the code of the callback function.

// Create the original array.
 var arr = [5, "element", 10, "the", true];

 // Create an array that contains the string
 // values that are in the original array.
 var result = arr.filter(
     function (value) {
         return (typeof value === 'string');
     }
 );

 document.write(result);
 // Output: element, the

The following example displays the names of properties that start with the letter “css” in the window DOM object.

var filteredNames = Object.getOwnPropertyNames(window).filter(IsC);

     for (i in filteredNames)
         document.write(filteredNames[i] + "<br/>");

 // Check whether the string starts with "css".
 function IsC(value) {
     var firstChar = value.substr(0, 3);
     if (firstChar.toLowerCase() == "css")
         return true;
     else
         return false;
     }

 // Output:
 // CSSRule
 // CSSFontFaceRule
 // CSSImportRule
 // CSSMediaRule
 // CSSNamespaceRule
 // CSSPageRule
 // CSSRuleList
 // CSSStyleDeclaration
 // CSSStyleRule
 // CSSStyleSheet

The following example illustrates the use of the thisArg argument, which specifies an object to which the this keyword can refer.

var checkNumericRange = function(value) {
     if (typeof value !== 'number')
         return false;
     else
         return value >= this.minimum && value <= this.maximum;
 }

 var numbers = [6, 12, "15", 16, "the", -12];

 // The obj argument enables use of the this value
 // within the callback function.
 var obj = { minimum: 10, maximum: 20 }

 var result = numbers.filter(checkNumericRange, obj);

 document.write(result);
 // Output: 12,16

The filter method can be applied to a string instead of an array. The following example shows how to do this.

// Define a callback function that returns true
 // if the current array element follows a space
 // or is the first character.
 function CheckValue(value, index, ar) {
     if (index == 0)
         return true;
     else
         return ar[index - 1] === " ";
 }

 // Create a string.
 var sentence = "The quick brown fox jumps over the lazy dog.";

 // Create an array that contains all characters that follow a space.
 var subset = [].filter.call(sentence, CheckValue);

 // You can use this alternative syntax.
 //var subset = Array.prototype.filter.call(sentence, CheckValue);

 document.write(subset);
 // Output: T,q,b,f,j,o,t,l,d

Remarks

The filter method calls the callbackfn function one time for each element in the array, in ascending index order. The callback function is not called for missing elements of the array.

In addition to array objects, the filter method can be used by any object that has a length property and that has numerically indexed property names.

The syntax of the callback function is as follows:

function callbackfn(value, index, array1)

You can declare the callback function by using up to three parameters.

The following table lists the callback function parameters.

The filter method does not directly modify the original array, but the callback function might modify it. The following table describes the results of modifying the array object after the filter method starts.

Exceptions

If the callbackfn argument is not a function object, a TypeError exception is thrown.

Requirements

Supported in the following document modes: Internet Explorer 9 standards, Internet Explorer 10 standards, and Internet Explorer 11 standards. Also supported in Windows Store apps. Not supported in the following document modes: Quirks, Internet Explorer 6 standards, Internet Explorer 7 standards, Internet Explorer 8 standards.

See also

Specification

[15.4.4.20 Array.prototype.filter ( callbackfn , thisArg )]

ECMAScript® Language Specification Standard ECMA-262 5.1 Edition / June 2011

Array.prototype.filter ( callbackfn [ , thisArg ] )

Attributions

• Microsoft Developer Network: Article