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In this lesson, I explain what an Array is, properties of Array instances, and methods of Array.prototype that were added in EcmaScript 5.

What is an Array[edit]

Array instances also have a special definition for the internal method called [[DefineOwnProperty]] ( P, Desc, Throw ) used for property assignment and a special length property that affects and is affected by the properties in the array. The [[Class]] internal property value is "Array" (§ 15.4.5).

Arrays are Objects. Array instances inherit properties from the Array prototype (§15.4.4) object and Array.prototype inherits properties from Object.prototype.

Length and [[DefineOwnProperty]][edit]

<source lang="javascript"> // length. var a = ["a", "b" , "c", undefined]; "3" in a; // true; a[3]; // undefined; a.length = 2; "2" in a; // false.

// DefineOwnProperty a[4] = "CC"; a.length; // 5; "3" in a; // true. a[3]; // undefined.

// Sparse array DO NOT DO THIS. delete a[2]; // true "2" in a; // false </source>

NodeLists Are Not Arrays[edit]

There are many collections in the DOM that have indexed properties but are not Arrays. NodeList is one such example. For example, <source lang="javascript"> document.getElementsByTagName("div"); </source> — returns a live collection.

In modern browsers Array methods can be used generically on these Array-like host objects. For example:

<source lang="javascript"> var slice = Array.prototype.slice;

var divs = document.getElementsByTagName("div"); divArray =; // create a new Array

function filterOutEmptyDivs(el) { // Using ES5 String.prototype.trim; assuming textContent is supported.

 return el.getElementsByTagName("*").length > 0 && !el.textContent.trim();


divArray.filter(filterOutEmptyDivs); </source>

However, IE8 and below will throw an error when slice is called on a Host object. See also:

EcmaScript 5 Array Methods[edit]

The normative reference for Array is the EcmaScript 5.1 specification, a mature, official standard. See §15.4 Array.

EcmaScript 5 formally standardized the "Array Extras" that were previously introduced in Firefox 1.5 (Mozilla JavaScript™ 1.6) in 2006.

These added methods are present in common modern browsers, but can have a noticeable performance impact for lengthy arrays, particularly on limited devices, and especially with large data sets.

<source lang="javascript"> Array.prototype.indexOf Array.prototype.lastIndexOf Array.prototype.every Array.prototype.some Array.prototype.forEach Array.prototype.filter Array.prototype.reduce Array.prototype.reduceRight </source>

Each method description has an overview of what it does, followed by the algorithm. The complete list of methods is to be found in the EcmaScript specification. See Array.prototype.

MDC Array

It is interesting that these methods can also be used generically on something that is Array-like. <source lang="javascript"> var filter = Array.prototype.filter;"foo", function (ch) { return ch == "o"; }).join(""); </source>

Array-like was proposed as a standard interface (I championed this idea for a while), but was not included. Perhaps it will be, some day.

But for now, it is useful to know that these methods are natively supported in modern browsers.

Assignment 1 Write a function that removes an element from an array.[edit]

Write a function that that takes an array and a specified element, and returns the array, minus the first occurence of the element from the array.

Given: <source lang="javascript"> var thing = {}, f = function(){}; var input = [f, [], /foo/, thing, []];

removeElement(input, thing); // [f, [], /foo/, []]; </source>

Assignment 2 Write a function that removes duplicate numbers from an array.[edit]

<source lang="javascript"> var input = [1, 2, 3, 1, 0, 1]; var result = arrayUniqueNumbers( input ); // [1, 2, 3, 0]; </source>

Assignment 3 Write a function that sorts an array of strings case-insensitively.[edit]

Write a function that sorts an array of strings case-insensitively. <source lang="javascript"> var input = ["penguin", "Beard", "pelican", "bear", "ostrich"]; sortStringsCI(input); // ["Bear", "pelican", "penguin", "ostrich"]; </source>