Core JavaScript 1.5 Reference:Global Objects:Array:filter

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Contents 1 Summary 2 Syntax 3 Parameters 4 Description 5 Compatibility 6 Examples 6.1 Example: Filtering out all small values

[edit] Summary

Creates a new array with all elements that pass the test implemented by the provided function.

Method of Array
Implemented in:	JavaScript 1.6 (Gecko 1.8b2 and later)
ECMAScript Edition:	none

[edit] Syntax

var filteredArray = array.filter(callback[, thisObject]);

[edit] Parameters

callback 

Function to test each element of the array.

thisObject 

Object to use as this when executing callback.

[edit] 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. 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: the value of the element, the index of the element, and the Array object being traversed.

If a thisObject parameter is provided to filter, it will be used as the this for each invocation of the callback. If it is not provided, or is null, the global object associated with callback is used instead.

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.

[edit] Compatibility

filter is a JavaScript extension to the ECMA-262 standard; as such it may not be present in other 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 used in Firefox and SpiderMonkey.

if (!Array.prototype.filter)
{
  Array.prototype.filter = function(fun /*, thisp*/)
  {
    var len = this.length;
    if (typeof fun != "function")
      throw new TypeError();

    var res = new Array();
    var thisp = arguments[1];
    for (var i = 0; i < len; i++)
    {
      if (i in this)
      {
        var val = this[i]; // in case fun mutates this
        if (fun.call(thisp, val, i, this))
          res.push(val);
      }
    }

    return res;
  };
}

[edit] Examples

[edit] 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(element, index, array) {
  return (element >= 10);
}
var filtered = [12, 5, 8, 130, 44].filter(isBigEnough);