Editing JavaScript/Notes/Array

In this lesson, I explain what an <code>Array</code> is, properties of Array instances, and methods of <code>Array.prototype</code> that were added in EcmaScript 5.
== Array Instances ==
Array instances also have a special definition for the internal method called <code><nowiki>[[DefineOwnProperty]] ( P, Desc, Throw )</nowiki></code> used for property assignment and a special <code>length</code> property that affects and is affected by the properties in the array. The <code><nowiki>[[Class]]</nowiki></code> internal property value is "Array" ([http://www.ecma-international.org/ecma-262/5.1/#sec-15.4.5 &sect; 15.4.5]).

Array instances inherit properties from the <code>Array prototype</code> ([http://www.ecma-international.org/ecma-262/5.1/#sec-15.4.4 &sect;15.4.4]) object. === Length and <nowiki>[[DefineOwnProperty]]</nowiki> ===
<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 == 
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>
&mdash; 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 = slice.call(divs); // 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:
* [http://www.ecma-international.org/ecma-262/5.1/#sec-15.5.4.13 String.prototype.slice (start, end)]
* [http://lists.w3.org/Archives/Public/public-webapps/2008JulSep/0480.html Re: [whatwg] WebIDL and HTML5]
* [https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Array/slice Slice:MDN]
* [http://www.jibbering.com/faq/#hostObject What is a Host Object?]

== EcmaScript 5 Array Methods == 
The normative reference for Array is the EcmaScript 5.1 specification, a mature, official standard. See [http://www.ecma-international.org/ecma-262/5.1/#sec-15.4.4 &sect;15.4.4 Array].

EcmaScript 5 formally standardized the "Array Extras" that were previously introduced in Firefox 1.5 (Mozilla JavaScript&trade; 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.map
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 [http://www.ecma-international.org/ecma-262/5.1/#sec-15.4.4 Array.prototype].

[https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Array 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;

filter.call("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. == 
Write a function that removes an element from an 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. ==
<source lang="javascript">
var input = [1, 2, 3, 1, 0, 1];
var result = arrayUnique( input ); // [1, 2, 3, 0];
</source>

== Assignment 3 Write a function that sorts an array of strings case-insensitively. == 
Write a function that sorts an array of strings case-insensitively.