filter Method (Array) (JavaScript)

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

Syntax


array1.filter(callbackfn[, thisArg])  

Parameters

Parameter Definition
array1 Required. An array object.
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.

Exceptions

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

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.

Callback Function Syntax

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.

Callback argument Definition
value The value of the array element.
index The numeric index of the array element.
array1 The array object that contains the element.

Modifying the Array Object

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.

Condition after the filter method starts Element passed to callback function?
Element is added beyond the original length of the array. No.
Element is added to fill in a missing element of the array. Yes, if that index has not yet been passed to the callback function.
Element is changed. Yes, if that element has not yet been passed to the callback function.
Element is deleted from the array. No, unless that element has already been passed to the callback function.

Example

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  

Example

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  

Example

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  

Example

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  

Example

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  

Requirements

Supported in the following document modes: Internet Explorer 9 standards, Internet Explorer 10 standards, and Internet Explorer 11 standards. Also supported in Store apps (Windows 8 and Windows Phone 8.1). See Version Information.
Not supported in the following document modes: Quirks, Internet Explorer 6 standards, Internet Explorer 7 standards, Internet Explorer 8 standards.

See Also

Array Object
Using Arrays
map Method (Array)
forEach Method (Array)