Difference between revisions of "JavaScript"

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JavaScript is the most widely used scripting language for implementing client-side functionality in Web applications.  Unlike PHP, JavaScript code is interpreted and executed in a client's web browser (not on the web server).
+
JavaScript is the most widely used scripting language for implementing client-side functionality in Web applications, and, in recent years, it's become very popular for server-side application, too.  Unlike PHP, JavaScript code can be interpreted and executed in a client's web browser, instead of on the web server, depending on how it's being used.
  
 
== JavaScript?  Did you mean Java? ==
 
== JavaScript?  Did you mean Java? ==
  
'''JavaScript''' is a prototype-based programming language focussed on web interactivity.  It was first introduced in Netscape Navigator, and it in turn gave rise to what is now '''[http://www.ecmascript.org/ ECMAScript]'''.  Originally, JavaScript was called "LiveScript".  Sun, the developer of Java at the time, wanted a language that would complement its compiled Java language, and so LiveScript was renamed JavaScript.
+
'''JavaScript''' is a prototype-based programming language focused on web interactivity.  It was first introduced in Netscape Navigator, and it in turn gave rise to what is now '''[http://www.ecmascript.org/ ECMAScript]'''.  Originally, JavaScript was called "LiveScript".  Sun, the developer of Java at the time, wanted a language that would complement its compiled Java language, and so LiveScript was renamed JavaScript.
  
 
Beyond that, '''Java and JavaScript are as similar as Ham and Hamster.'''
 
Beyond that, '''Java and JavaScript are as similar as Ham and Hamster.'''
  
Java and JavaScript have similar syntax, but the under workings are quite different.  For example, Java requires strict typing, but JavaScript allows for dynamic typing.  Java is object-oriented, but JavaScript is prototype-oriented.  Java is compiled, but JavaScript is interpreted.
+
Java and JavaScript have similar syntax, but the under workings are quite different.  For example, Java requires strict typing, but JavaScript allows for dynamic typing.  Java is object-oriented, but JavaScript is prototype-oriented.  Java is compiled, but JavaScript is interpreted.  Java supports event listeners, but JavaScript makes them a fundamental part of the language.  Java variables are scoped to the innermost method or loop, but JavaScript variables are scoped only to the innermost function.
  
 
== JavaScript Language Components ==
 
== JavaScript Language Components ==
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This section highlights some features of the JavaScript language that differentiate it from other languages.  For a more comprehensive JavaScript tutorial, the following tutorials are both quality:
 
This section highlights some features of the JavaScript language that differentiate it from other languages.  For a more comprehensive JavaScript tutorial, the following tutorials are both quality:
  
* [http://docs.webplatform.org/wiki/javascript/tutorials WebPlatform]: Great for learning JavaScript for the first time.  Advocates best practices for modern JavaScript development.
+
* [https://developer.mozilla.org/en-US/docs/Web/JavaScript Mozilla Developer Network]: Great for learning JavaScript for the first time.  Advocates best practices for modern JavaScript development.
* [http://www.quirksmode.org/js/contents.html QuirksMode]: More advanced than the WebPlatform tutorial, but it thoroughly covers the most powerful aspects of pure JavaScript and gives unparalleled advice on browser compatibility.
+
* [http://www.quirksmode.org/js/contents.html QuirksMode]: More advanced than the Mozilla tutorial, but it thoroughly covers the most powerful aspects of pure JavaScript and gives unparalleled advice on browser compatibility.
  
 
Advice: Do '''NOT''' Google for "JavaScript Tutorial".  The first few hits advocate bad or outdated practices.  Stick with the ones listed here.
 
Advice: Do '''NOT''' Google for "JavaScript Tutorial".  The first few hits advocate bad or outdated practices.  Stick with the ones listed here.
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=== Variables and Scope ===
 
=== Variables and Scope ===
  
To define a variable in JavaScript, use the '''var''' keyword.
+
To define a variable in JavaScript, use the '''let''' keyword.
  
 
<source lang="javascript">
 
<source lang="javascript">
var foo = "bar";
+
let foo = "bar";
 
</source>
 
</source>
 +
 +
To define a constant in JavaScript, use the '''const''' keyword.
 +
 +
<source lang="javascript">
 +
const foo = "bar";
 +
</source>
 +
 +
'''Note: We should NOT see any variables initialized with var in your code. Some might appear in the linked examples, since those are from older versions of the language so please replace those with let and const in your mind'''
 +
Var creates ambiguity to anyone reading your code and is becoming deprecated in JavaScript.
  
 
All variables are objects.  In the case above, the variable '''foo''' is an object of the class '''String'''.
 
All variables are objects.  In the case above, the variable '''foo''' is an object of the class '''String'''.
Line 35: Line 44:
  
 
<source lang="javascript">
 
<source lang="javascript">
var a = "hello";
+
const a = "hello";
var b = "world";
+
const b = "world";
  
 
function sayGoodByeWorld(){
 
function sayGoodByeWorld(){
var a = "goodbye";
+
const a = "goodbye";
  
 
alert(a); // goodbye
 
alert(a); // goodbye
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<source lang="javascript">
 
<source lang="javascript">
var apple = {
+
const apple = {
 
color: "red",
 
color: "red",
 
flavor: "sweet",
 
flavor: "sweet",
Line 76: Line 85:
 
</source>
 
</source>
  
Pass a parameter to a function like this:
+
Pass an argument to a function like this:
  
 
<source lang="javascript">
 
<source lang="javascript">
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</source>
 
</source>
  
In JavaScript, functions are objectsThis means that they can be assigned to variables, manipulated on the fly, and even passed as arguments to other functions!  As we will see in a couple sections, this is an extremely important feature of JavaScript that makes it such a robust language for web development.
+
====Arrow Functions====
 +
The latest versions of JavaScript have introduced arrow functions.  They provide a cleaner syntax for writing functions, and maintain scope when using classes.
 +
 
 +
The general syntax is:
 +
<source lang="javascript">
 +
(param1, param2, ... , paramN) => { statements }
 +
 
 +
//removing the brackets around { statements } is equivalent to
 +
{ return statements; }
 +
 
 +
//No parameters should be 2 empty parentheses as such:
 +
() => { statements }
 +
</source>
 +
 
 +
The example above could be written as follows:
  
If you want to define a scope-specific function, you can assign it to a variable like this:
+
<source lang="javascript">
 +
const sayHello = (name) => alert("Hello, "+name);
 +
sayHello("Todd");
 +
</source>
  
 +
Arrow functions can be particularly useful when you're looking to contain function logic within cleanly and compactly. For example, iterating over an array and making updates to each item of the array.
 
<source lang="javascript">
 
<source lang="javascript">
var sayHello = function(){
+
let arr = [1,2,3,4,5];
 +
let newArr = arr.map(number => number*5); //newArr is an array containing [5,10,15,20,25]
 +
</source>
 +
 
 +
==== Functions are Objects ====
 +
 
 +
In JavaScript, functions are objects.  This means that they can be assigned to variables, manipulated on the fly, and even passed as arguments to other functions!  '''''This is an extremely important feature of JavaScript''''' that makes it such a robust language for web development.
 +
 
 +
You create a function variable like this:
 +
 
 +
<source lang="javascript">
 +
const sayHello = function(){
 
alert("Hello World");
 
alert("Hello World");
 
}
 
}
Line 97: Line 135:
 
sayHello(); // call the function
 
sayHello(); // call the function
 
</source>
 
</source>
 +
 +
Observe that you can make [[#Variables and Scope|scope-dependent]] functions in this way.
  
 
=== Closures ===
 
=== Closures ===
 +
 +
{{DoctypeTV
 +
|youtube_id=HaPlj796ngQ
 +
|time=5m45s
 +
|desc=JavaScript Closures
 +
}}
  
 
A '''closure''' is a way to separate a certain snippet of JavaScript code from the global scope.  This is useful if you do not want to "muck up" global variables.  Closures are nothing more than anonymous functions that are called as soon as they are created:
 
A '''closure''' is a way to separate a certain snippet of JavaScript code from the global scope.  This is useful if you do not want to "muck up" global variables.  Closures are nothing more than anonymous functions that are called as soon as they are created:
  
 
<source lang="javascript">
 
<source lang="javascript">
​var a = "hello";
+
const a = "hello";
  
 
(​​​​function(){
 
(​​​​function(){
var b = "world";
+
const b = "world";
 
 
 
alert(a); // hello
 
alert(a); // hello
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alert(b);​ // ReferenceError: b is not defined​​​​​​​​​​​​​​​
 
alert(b);​ // ReferenceError: b is not defined​​​​​​​​​​​​​​​
 
</source>
 
</source>
 +
 +
{{JSFiddleExample
 +
|title=Passing Variables to a Closure
 +
|desc=This example demonstrates how you can "save" previous versions of a variable using closures.  This is possible because of scoping and the fact that function variables can be returned as objects.
 +
|fiddle_id=YURqa
 +
}}
  
 
Most JavaScript libraries (like jQuery) write all of their code in a closure.  What this means is that except for one or two variable names (jQuery or $), everything associated with the library is completely self-contained.
 
Most JavaScript libraries (like jQuery) write all of their code in a closure.  What this means is that except for one or two variable names (jQuery or $), everything associated with the library is completely self-contained.
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</source>
 
</source>
  
To "extend" an object, just copy its prototype to a new object.  However, since classical inheritance and prototypal inheritance are different concepts, copying prototypes may have results that you don't expect.  [http://beej.us/blog/data/javascript-prototypes-inheritance/ This article] seems to have a pretty good explanation with example implementations.
+
To "extend" an object, just copy its prototype to a new object.  However, since classical inheritance and prototypal inheritance are different concepts, copying prototypes may have results that you don't expect.  [http://beej.us/blog/data/javascript-prototypes-inheritance/ This article] seems to have a pretty good explanation of how to "inherit" prototypes with example implementations.
 +
 
 +
==== Extending Built-In Prototypes ====
 +
 
 +
You should be aware that there is an active debate in the web development community about whether it is good programming style to extend the prototypes of built-in objects like String and Array, such as in the ''String.prototype.bang'' example above.  There are legitimate arguments on both sides.  [http://stackoverflow.com/questions/8859828/javascript-what-dangers-are-in-extending-array-prototype This Stack Overflow post] has good links to some articles discussing this issue.
  
 
=== Context ===
 
=== Context ===
 +
 +
{{DoctypeTV
 +
|youtube_id=0a4r4bm1nQA
 +
|time=4m32s
 +
|desc=The ''this'' Variable in JavaScript
 +
}}
  
 
Every function is called on a certain object, which is not necessarily the same object in which it was defined.  This is called '''context'''.
 
Every function is called on a certain object, which is not necessarily the same object in which it was defined.  This is called '''context'''.
Line 145: Line 207:
  
 
<source lang="javascript">
 
<source lang="javascript">
var sayThisColor = function(){
+
const sayThisColor = function(){
 
alert(this.color);
 
alert(this.color);
 
}
 
}
  
var apple = { color: "red" }
+
const apple = { color: "red" }
  
 
sayThisColor();  // undefined
 
sayThisColor();  // undefined
Line 164: Line 226:
  
 
<source lang="javascript">
 
<source lang="javascript">
var sayHello = function(){
+
const sayHello = function(){
 
alert("Hello World");
 
alert("Hello World");
 
}
 
}
Line 173: Line 235:
 
{{JSFiddleExample
 
{{JSFiddleExample
 
|title=Binding Callbacks to a Button Click
 
|title=Binding Callbacks to a Button Click
|fiddle_id=Ah7uK
+
|fiddle_id=L8c4Z
 +
|cse330=true
 
|desc=In this example, notice how we bind the callback function when the ''DOMContentLoaded'' event fires.  This is because JavaScript requires that the button has been parsed by the browser before it can add event listeners to it.
 
|desc=In this example, notice how we bind the callback function when the ''DOMContentLoaded'' event fires.  This is because JavaScript requires that the button has been parsed by the browser before it can add event listeners to it.
 
}}
 
}}
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*: The callback function is passed one parameter: the event object.
 
*: The callback function is passed one parameter: the event object.
 
* A boolean representing bubble (false) or capture (true) phase
 
* A boolean representing bubble (false) or capture (true) phase
*: You usually want Bubble phase.  For more information on event phase, see [[#Special Topic: Event Phase]]
+
*: You usually want Bubble phase.  For more information on event phase, see [[#Event Phase]]
  
 
Note that versions of Internet Explorer prior to version 9 required that you use a different, albeit similar, function for assigning event listeners called '''attachEvent'''.  attachEvent takes the same arguments as addEventListener, except you need to put '''on''' before the event type (e.g. "onclick"), and the third argument (phase) is not necessary.
 
Note that versions of Internet Explorer prior to version 9 required that you use a different, albeit similar, function for assigning event listeners called '''attachEvent'''.  attachEvent takes the same arguments as addEventListener, except you need to put '''on''' before the event type (e.g. "onclick"), and the third argument (phase) is not necessary.
Line 190: Line 253:
 
<source lang="javascript">
 
<source lang="javascript">
 
document.getElementById("hello").addEventListener("click", function(event){
 
document.getElementById("hello").addEventListener("click", function(event){
alert("Hello, World!  Event Type: "+event.type);
+
alert(`Hello, World!  Event Type: ${event.type}`);
 
}, false);
 
}, false);
 
</source>
 
</source>
Line 202: Line 265:
 
<source lang="javascript">
 
<source lang="javascript">
 
document.getElementById("my_btn").addEventListener("click", function(event){
 
document.getElementById("my_btn").addEventListener("click", function(event){
var strout = "";
+
let strout = "";
 
for(var i in event){
 
for(var i in event){
strout += i + ": " + event[i] + "\n";
+
strout += `${i}: ${event[i]}\n`;
 
}
 
}
 
console.log(strout);
 
console.log(strout);
Line 221: Line 284:
 
'''Note:''' If your callback function does not take any parameters, it will still run; you just won't have as easy of access to the event object.
 
'''Note:''' If your callback function does not take any parameters, it will still run; you just won't have as easy of access to the event object.
  
=== Special Topic: Event Phase ===
+
=== Event Handlers and Memory Management ===
 +
 
 +
JavaScript is an automatically garbage-collected language, meaning that memory management is mostly taken care of for you.  However, there are some instances when you need to be careful about creating "zombie" objects.  In JavaScript, this mostly revolves around the idea of event listeners.
 +
 
 +
I could give an explanatory example, but instead I'll tell you straightaway that '''the moral of the story is that for every "addEventListener", there should probably be a "removeEventListener".'''  Here are some good articles to satisfy your hunger for more details on event listeners and JavaScript memory management:
 +
 
 +
* http://www.vladalexandruionescu.com/2012/08/javascript-memory-leaks.html
 +
* http://lostechies.com/derickbailey/2012/03/19/backbone-js-and-javascript-garbage-collection/
 +
* http://stackoverflow.com/questions/5326300/garbage-collection-with-node-js
 +
 
 +
When you start working with Node.JS in the next module, it's more important than ever to keep good accounting of your event listeners.
 +
 
 +
=== Event Phase ===
  
 
In JavaScript, events occur in two phases: '''Capture Phase''' and '''Bubble Phase'''.  Consider the following HTML document:
 
In JavaScript, events occur in two phases: '''Capture Phase''' and '''Bubble Phase'''.  Consider the following HTML document:
Line 244: Line 319:
 
# Capture Phase CLICK on the Body
 
# Capture Phase CLICK on the Body
 
# Capture Phase CLICK on the Div#box
 
# Capture Phase CLICK on the Div#box
# Capture Phase CLICK on the Button#btn
+
# Target Phase CLICK on the Button#btn
# Bubble Phase CLICK on the Button#btn
 
 
# Bubble Phase CLICK on the Div#box
 
# Bubble Phase CLICK on the Div#box
 
# Bubble Phase CLICK on the Body
 
# Bubble Phase CLICK on the Body
Line 263: Line 337:
  
 
<source lang="javascript">
 
<source lang="javascript">
var disableToolbarFunc = function(e){
+
const disableToolbarFunc = function(e){
 
e.stopPropogation();
 
e.stopPropogation();
 
}
 
}
Line 275: Line 349:
  
 
'''Word of caution:''' Like many great things in web development, capture phase does not work in versions of Internet Explorer prior to version 9.  There is no elegant substitution for older versions of Internet Explorer.
 
'''Word of caution:''' Like many great things in web development, capture phase does not work in versions of Internet Explorer prior to version 9.  There is no elegant substitution for older versions of Internet Explorer.
 +
 +
'''Note:''' When the event reaches the innermost element, there is no distinction between capture phase and bubble phase.  The order in which event handlers were added to the element determines the order in which those callbacks are evaluated.
  
 
== Document Object Model ==
 
== Document Object Model ==
Line 339: Line 415:
  
 
<source lang="javascript">
 
<source lang="javascript">
var newLi = document.createElement("li");  // creates a node with the tag name li
+
const newLi = document.createElement("li");  // creates a node with the tag name li
 
newLi.appendChild(document.createTextNode("Broccoli"));
 
newLi.appendChild(document.createTextNode("Broccoli"));
 
newLi.setAttribute("class", "veggies");
 
newLi.setAttribute("class", "veggies");
Line 348: Line 424:
  
 
<source lang="javascript">
 
<source lang="javascript">
var apple = document.getElementById("my-list").getElementsByClassName("fruits")[0];
+
const apple = document.getElementById("my-list").getElementsByClassName("fruits")[0];
 
document.getElementById("my-list").removeChild(apple);
 
document.getElementById("my-list").removeChild(apple);
 
</source>
 
</source>
Line 355: Line 431:
  
 
<source lang="javascript">
 
<source lang="javascript">
var apple = document.getElementById("my-list").getElementsByClassName("fruits")[0];
+
const apple = document.getElementById("my-list").getElementsByClassName("fruits")[0];
 
document.getElementById("my-list").appendChild(apple);
 
document.getElementById("my-list").appendChild(apple);
 
</source>
 
</source>
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</source>
 
</source>
  
'''Important:''' You need to be careful to prevent XSS-type attacks if you display information innerHTML.  You should be sure, for example, that your JSON data is sanitized on the server.
+
'''Important:''' You need to be careful to prevent XSS-type attacks if you display information using innerHTML.  For example, be sure to escape a user-supplied string before appending it to innerHTML.
  
==== DOM Manipulation using jQuery ====
+
=== JavaScript Inspectors ===
  
jQuery's approach is a cross between the W3C Node model and a more traditional .innerHTML approach.  Here is how you would add a paragraph saying "Hello World" to the element with ID "holder" in jQuery:
+
{{DoctypeTV
 
+
|youtube_id=PKzrmbN-pMg
<source lang="javascript">
+
|time=4m13s
var code_to_insert = $("<p>Hello World</p>");
+
|desc=Using Firebug, WebKit Inspector, and IE Developer Tools
$("#holder").append(code_to_insert);
+
}}
</source>
 
  
=== Inspecting the DOM ===
+
Various browsers and browser plugins give you, the developer, tools to debug your code, inspect the DOM, and much moreUsing these tools is essential for an efficient workflow.
 
 
Various browsers and browser plugins allow you, the developer, to look at the DOM of your web page and find what properties and methods are associated with each nodeSome options include:
 
  
 
* '''Firebug'''.  Firebug is a plugin for Firefox that is a toolkit full of web developer tools, one of which is a DOM inspector.  To inspect the DOM of a web page, simply open the Firebug window while viewing the web page.
 
* '''Firebug'''.  Firebug is a plugin for Firefox that is a toolkit full of web developer tools, one of which is a DOM inspector.  To inspect the DOM of a web page, simply open the Firebug window while viewing the web page.
 
* '''WebKit Inspector'''.  The WebKit Inspector, which is available in both Chrome and Safari, enables you to see the DOM tree and relationships between elements.  However, its DOM inspection tools are not as robust as those of Firebug.
 
* '''WebKit Inspector'''.  The WebKit Inspector, which is available in both Chrome and Safari, enables you to see the DOM tree and relationships between elements.  However, its DOM inspection tools are not as robust as those of Firebug.
* '''Opera Dragonfly'''.  The web browser Opera comes with a web developer toolkit similar to the WebKit Inspector called Dragonfly.  It is important that you test your sites in Opera, because although Opera has only a small fraction of the desktop browser market, it is second only to WebKit on the mobile market.
+
* '''Opera Dragonfly'''.  The web browser Opera comes with a web developer toolkit similar to the WebKit Inspector called Dragonfly.  It is important that you test your sites in Opera, because although Opera has only a small fraction of the desktop browser market, it is second only to WebKit on the mobile market.  Opera also has extremely strong CSS support, so it is good for testing for bugs in your CSS.
 
 
== Asynchronous Javascript And XML ==
 
 
 
Asynchronous Javascript And XML, or '''''AJAX''''', is a group of related web development techniques, standards, and technologies used on the client (web browser) side of the standard web server-client communication model.
 
 
 
Here's the general concept: JavaScript is used to send requests to the web server in the background and retrieve data, without the need to refresh the pageThe result: dynamic, fast, user-friendly web pages.
 
 
 
The requests are performed ''asynchronously'': this means that the end user can continue interacting with your web page while the server data is being loaded.
 
 
 
The key idea is that a large fraction of the work happens at the web browser, and larger requests happen behind the scenes, keeping waiting times for data to come back over the network to a minimum.  AJAX is generally also associated with higher-level APIs and libraries that aid in producing cleaner user interfaces for web applications.
 
 
 
=== The XMLHttpRequest Object ===
 
 
 
The JavaScript specification defines the '''XMLHttpRequest''' object, which is used to exchange data with the web servers without needing to reload the entire web page in the browser.
 
 
 
'''Note:''' All modern browsers support the XMLHttpRequest object as specified by the W3C.  However, if you wish to support older browsers (e.g., Netscape Navigator or Internet Explorer prior to version 9), you need to do some additional work on the back end.  ''In this class, your JavaScript needs to work in only Firefox and Chrome.''
 
 
 
==== Initializing an XMLHttpRequest Object ====
 
 
 
As stated above, the XMLHttpRequest object handles communication with the web servers.  To use it, you must first initialize a variable to be an instance of the '''XMLHttpRequest''' "class".
 
 
 
<source lang="javascript">
 
var xmlHttp = new XMLHttpRequest();
 
</source>
 
 
 
Next, use your XMLHttpRequest instance to open a connection to your server.
 
 
 
<source lang="javascript">
 
xmlHttp.open("METHOD", "URL_OF_SERVER_SCRIPT", ASYNCRONOUS_FLAG);
 
// METHOD: either GET or POST.
 
// URL_OF_SERVER_SCRIPT: the path to your server-side script relative to the current page.  For example, process_ajax.php
 
// ASYNCRONOUS_FLAG: Should be true in order to perform the request asynchronously.  (Having a value of false will freeze the browser until the request is completed.)
 
</source>
 
 
 
'''Note:''' For security reasons, you cannot perform AJAX requests to other domains, unless that domain has a special security policy set up to enable such requests.
 
 
 
==== Sending Data ====
 
 
 
===== GET Data =====
 
 
 
To send GET data, append the information to the end of the URL_OF_SERVER_SCRIPT, as you would normally do, and then call xmlHttp.send(), passing ''null'' as the parameter:
 
 
 
<source lang="javascript">
 
var xmlHttp = new XMLHttpRequest();
 
xmlHttp.open("GET", "process_ajax.php?var1=val1&var2=val2", true);
 
xmlHttp.send(null);
 
</source>
 
 
 
===== POST Data =====
 
 
 
To send POST data, pass a URL-encoded string to xmlHttp.send().  You also need to change the MIME Type of the outgoing request.
 
 
 
<source lang="javascript">
 
var xmlHttp = new XMLHttpRequest();
 
xmlHttp.open("POST", "process_ajax.php", true);
 
xmlHttp.setRequestHeader("Content-Type", "application/x-www-form-urlencoded");
 
xmlHttp.send("var1=val1&var2=val2");
 
</source>
 
 
 
==== Listening for a Response ====
 
 
 
XMLHttpRequest will fire an [[#Events|event]] when the request has been completed.  We therefore need to use a '''[[#Events|callback function]]''' in order to listen for the '''load''' event on the XMLHttpRequest object.
 
 
 
The server's plain text response is available as the '''responseText''' property of your XMLHttpRequest instance, which is available in the '''target''' property of the event.  For example:
 
 
 
<source lang="javascript">
 
var xmlHttp = new XMLHttpRequest();
 
xmlHttp.open("GET", "hello.txt", true);
 
xmlHttp.addEventListener("load", ajaxCallback, false);
 
xmlHttp.send(null);
 
 
 
// ...
 
 
 
function ajaxCallback(event){
 
alert( "Your file contains the text: " + event.target.responseText );
 
}
 
</source>
 
 
 
===== Browser Support =====
 
 
 
Many examples that you find when Googling for "AJAX" use the '''onreadystatechange''' property of the XMLHttpRequest instance to react on the completion of the request, rather than using an event listener.  This approach works, and it is required for comprehensive browser support, but it is no longer recommended for future use in modern web applications.  The implementation of XMLHttpRequest documented here (so-called "XHR level 2") works in the following browsers:
 
* Firefox 3.5+ (Jun 2009)
 
* Safari 5+ (Jun 2010)
 
* Chrome 7+ (Sep 2010)
 
* Opera 12+ (May 2012)
 
* Internet Explorer 10+ (unreleased)
 
Together, browsers supporting XHR level 2 account for more than half of the market as of October 2012.
 
 
 
==== Response Formats ====
 
 
 
More often than not, a plaintext response form the server is not very helpful: you want to transmit more information than simply a single string of text.  There are two good options to get more information from a single AJAX request.
 
 
 
===== XML =====
 
 
 
Your server can respond with an XML document (whence, ''XMLHttpRequest'').  An '''XML document''' uses syntax that is extremely close to HTML  For example:
 
 
 
<source lang="xml">
 
<?xml version="1.0" standalone="yes" ?>
 
<note>
 
  <date year="2013" month="02" day="14" />
 
  <to>Sally</to>
 
  <message>Happy Valentine's Day!</message>
 
  <from>Your Secret Admirer</from>
 
</note>
 
</source>
 
 
 
Like HTML, XML has a DOM that can be traversed in JavaScript.
 
 
 
If you make an AJAX request to a script generating an XML document, the root XML node is accessible in the '''responseXML''' property.
 
 
 
<source lang="javascript">
 
function ajaxCallback(event){
 
var xmlDocument = event.target.responseXML;
 
 
alert(xmlDocument.getElementsByTagName("to")[0].textContent);  // Sally (in the case of the example XML above)
 
}
 
</source>
 
 
 
===== JSON =====
 
 
 
JavaScript Object Notation, or '''JSON''', is a rising standard interface format because of its lightweight syntax that makes it especially useful for AJAX requests.
 
 
 
The JSON syntax closely follows the syntax for [[#Object Literals|object literals]].  For example:
 
 
 
<source lang="javascript">
 
{
 
  "apple": {
 
      "color": "red",
 
      "flavor": "sweet"
 
  },
 
  "lemon": {
 
      "color": "yellow",
 
      "flavor": "sour"
 
  }
 
}
 
</source>
 
 
 
Usually when JSON is transferred over the internet, it is condensed down to one line, like this:
 
 
 
<source lang="javascript">
 
{"apple":{"color":"red","flavor":"sweet"},"lemon":{"color":"yellow","flavor":"sour"}}
 
</source>
 
 
 
In JavaScript, you can convert a JSON string to an object using '''JSON.parse()'''.  For example:
 
 
 
<source lang="javascript">
 
var jsonData = JSON.parse('{"apple":{"color":"red","flavor":"sweet"},"lemon":{"color":"yellow","flavor":"sour"}}');
 
 
 
alert(jsonData.apple.color);  // red
 
</source>
 
 
 
Here's how to use it in conjunction with your AJAX response callback:
 
 
 
<source lang="javascript">
 
function ajaxCallback(event){
 
var jsonData = JSON.parse(event.target.responseText);
 
 
alert(jsonData.lemon.flavor);  // sour (in the case of the example JSON above)
 
}
 
</source>
 
 
 
Here is a graphic to illustrate the JSON process.
 
 
 
[[File:JSON-Flowchart.png‎]]
 
 
 
=== Examples ===
 
 
 
==== Logging In a User ====
 
 
 
Suppose you had the following PHP script:
 
 
 
<source lang="php">
 
<?php
 
// login_ajax.php
 
 
 
header("Content-Type: application/json"); // Since we are sending a JSON response here (not an HTML document), set the MIME Type to application/json
 
 
 
$username = $_POST['username'];
 
$password = $_POST['password'];
 
 
 
// Check to see if the username and password are valid.  (You learned how to do this in Module 3.)
 
 
 
if( /* valid username and password */ ){
 
session_start();
 
$_SESSION['username'] = $username;
 
$_SESSION['token'] = substr(md5(rand()), 0, 10);
 
 
 
echo json_encode(array(
 
"success" => true
 
));
 
exit;
 
}else{
 
echo json_encode(array(
 
"success" => false,
 
"message" => "Incorrect Username or Password"
 
));
 
exit;
 
}
 
?>
 
</source>
 
 
 
'''Note:''' This example uses PHP's very handy '''json_encode()''' function to translate an associative array into a JSON object.  This means that you can access in JavaScript the same structure that you made in PHP!
 
 
 
And suppose you had the following HTML content:
 
 
 
<source lang="html4strict">
 
<input type="text" id="username" placeholder="Username" />
 
<input type="password" id="password" placeholder="Password" />
 
<button id="login_btn">Log In</button>
 
 
 
<script type="text/javascript" src="ajax.js"></script> <!-- load the JavaScript file -->
 
</source>
 
 
 
You could use the following JavaScript code to prepare, send, and receive a request from the server to see if the user is logged in, without ever having to refresh the page.
 
 
 
<source lang="javascript">
 
// ajax.js
 
 
 
function loginAjax(event){
 
var username = document.getElementById("username").value; // Get the username from the form
 
var password = document.getElementById("password").value; // Get the password from the form
 
 
 
// Make a URL-encoded string for passing POST data:
 
var dataString = "username=" + encodeURIComponent(username) + "&password=" + encodeURIComponent(password);
 
 
var xmlHttp = new XMLHttpRequest(); // Initialize our XMLHttpRequest instance
 
xmlHttp.open("POST", "login_ajax.php", true); // Starting a POST request (NEVER send passwords as GET variables!!!)
 
xmlHttp.setRequestHeader("Content-Type", "application/x-www-form-urlencoded"); // It's easy to forget this line for POST requests
 
xmlHttp.addEventListener("load", function(event){
 
var jsonData = JSON.parse(event.target.responseText); // parse the JSON into a JavaScript object
 
if(jsonData.success){  // in PHP, this was the "success" key in the associative array; in JavaScript, it's the .success property of jsonData
 
alert("You've been Logged In!");
 
}else{
 
alert("You were not logged in.  "+jsonData.message);
 
}
 
}, false); // Bind the callback to the load event
 
xmlHttp.send(dataString); // Send the data
 
}
 
 
 
document.getElementById("login_btn").addEventListener("click", loginAjax, false); // Bind the AJAX call to button click
 
</source>
 
 
 
'''Note:''' The above example uses an '''anonymous function''' (a function without a name that is declared in an attribute) as the callback.  If you are using a certain callback function only once in your program, this is a perfectly valid approach to reduce the amount of code you need to write.
 
 
 
==== Loading Data from an XML File for Display in HTML ====
 
 
 
Suppose you have a file '''note.xml''' that contains the following information:
 
 
 
<source lang="xml">
 
<?xml version="1.0" standalone="yes" ?>
 
<note>
 
  <date year="2013" month="02" day="14" />
 
  <to>Sally</to>
 
  <message>Happy Valentine's Day!</message>
 
  <from>Your Secret Admirer</from>
 
</note>
 
</source>
 
 
 
Further, suppose you had an HTML element with ID '''note''':
 
 
 
<source lang="html4strict">
 
<div id="note"></div>
 
</source>
 
 
 
You could load this XML file and then display it inside of your HTML document like this:
 
 
 
<source lang="javascript">
 
function getNoteAjax(event){
 
// The XMLHttpRequest is simple this time:
 
var xmlHttp = new XMLHttpRequest();
 
xmlHttp.open("GET", "note.xml", true);
 
xmlHttp.addEventListener("load", getNoteCallback, false);
 
xmlHttp.send(null);
 
}
 
 
 
function getNoteCallback(event){
 
var htmlParent = document.getElementById("note"); // Get the HTML element into which we want to write the note
 
var xmlDocument = event.target.responseXML; // Get the XML root node from the response
 
 
// Find the <date/> element from the XML response.  We expect only 1 such element, so we can call
 
// xmlDocument.getElementsByTagName() and then get the first (and only) element in the array all in
 
// one step.  This is called chaining.
 
var xmlDateElement = xmlDocument.getElementsByTagName("date")[0];
 
 
// Put together a YYYY-MM-DD type date into a string based on the attributes of the <date/> element.
 
var dateString = xmlDateElement.getAttribute("year") + "-" + xmlDateElement.getAttribute("month")
 
+ "-" + xmlDateElement.getAttribute("day");
 
 
// Get the content of the <to>, <message>, and <from> tags.  Again, you can use a chain to do this in one line for each.
 
var to = xmlDocument.getElementsByTagName("to")[0].textContent;
 
var message = xmlDocument.getElementsByTagName("message")[0].textContent;
 
var from = xmlDocument.getElementsByTagName("from")[0].textContent;
 
 
// Make a <strong> tag, append the date string as its content, and then append it to the HTML element.
 
var htmlDateObj = document.createElement("strong");
 
htmlDateObj.appendChild(document.createTextNode(dateString));
 
htmlParent.appendChild(htmlDateObj);
 
 
// Make a <p> tag to contain the note.
 
var htmlParagraphObj = document.createElement("p");
 
htmlParagraphObj.appendChild(document.createTextNode("Dear "+to+",")); // First append the greeting
 
htmlParagraphObj.appendChild(document.createElement("br")); // Append a couple line feeds
 
htmlParagraphObj.appendChild(document.createElement("br"));
 
htmlParagraphObj.appendChild(document.createTextNode(message)); // Write the message itself
 
htmlParagraphObj.appendChild(document.createElement("br")); // Append a couple more line feeds
 
htmlParagraphObj.appendChild(document.createElement("br"));
 
htmlParagraphObj.appendChild(document.createTextNode("Love, "+from)); // Finally, append the salutation
 
htmlParent.appendChild(htmlParagraphObj); // Append the newly engineered <p> to the HTML element.
 
}
 
 
 
document.addEventListener("DOMContentLoaded", getNoteAjax, false); // Bind the AJAX call to page load
 
</source>
 
 
 
This example uses a lot of techniques documented in the [[#Document Object Model]] section above.  The resulting DOM will be something like this:
 
 
 
<source lang="html4strict">
 
<div id="note">
 
  <strong>2013-02-14</strong>
 
  <p>Dear Sally,<br /><br />Happy Valentine's Day!<br /><br />Love, Your Secret Admirer</p>
 
</div>
 
</source>
 
  
If you think that this is a lot of code to produce a relatively simple output, you are not alone.  You will learn how to do this job in much fewer lines when you learn about [[jQuery]].
+
All of the above-mentioned tools enable you to look at the DOM of your web page and find what properties and methods are associated with each node.  This is helpful because if you have JavaScript that manipulates HTML elements, you cannot tell which properties are associated with which elements by looking at the source code alone.
  
 
[[Category:Module 6]]
 
[[Category:Module 6]]

Latest revision as of 11:00, 2 March 2020

JavaScript is the most widely used scripting language for implementing client-side functionality in Web applications, and, in recent years, it's become very popular for server-side application, too. Unlike PHP, JavaScript code can be interpreted and executed in a client's web browser, instead of on the web server, depending on how it's being used.

JavaScript? Did you mean Java?

JavaScript is a prototype-based programming language focused on web interactivity. It was first introduced in Netscape Navigator, and it in turn gave rise to what is now ECMAScript. Originally, JavaScript was called "LiveScript". Sun, the developer of Java at the time, wanted a language that would complement its compiled Java language, and so LiveScript was renamed JavaScript.

Beyond that, Java and JavaScript are as similar as Ham and Hamster.

Java and JavaScript have similar syntax, but the under workings are quite different. For example, Java requires strict typing, but JavaScript allows for dynamic typing. Java is object-oriented, but JavaScript is prototype-oriented. Java is compiled, but JavaScript is interpreted. Java supports event listeners, but JavaScript makes them a fundamental part of the language. Java variables are scoped to the innermost method or loop, but JavaScript variables are scoped only to the innermost function.

JavaScript Language Components

This section highlights some features of the JavaScript language that differentiate it from other languages. For a more comprehensive JavaScript tutorial, the following tutorials are both quality:

  • Mozilla Developer Network: Great for learning JavaScript for the first time. Advocates best practices for modern JavaScript development.
  • QuirksMode: More advanced than the Mozilla tutorial, but it thoroughly covers the most powerful aspects of pure JavaScript and gives unparalleled advice on browser compatibility.

Advice: Do NOT Google for "JavaScript Tutorial". The first few hits advocate bad or outdated practices. Stick with the ones listed here.

Variables and Scope

To define a variable in JavaScript, use the let keyword.

let foo = "bar";

To define a constant in JavaScript, use the const keyword.

const foo = "bar";

Note: We should NOT see any variables initialized with var in your code. Some might appear in the linked examples, since those are from older versions of the language so please replace those with let and const in your mind Var creates ambiguity to anyone reading your code and is becoming deprecated in JavaScript.

All variables are objects. In the case above, the variable foo is an object of the class String.

Notice that like PHP, you do not have to explicitly define a type for a variable like int or String; this is because JavaScript and PHP are dynamically typed languages.

JavaScript code can access any variable higher in the “function tree”, but not vice-versa. This is called scope. All functions create their own scope.

When you reference a variable, JavaScript will look up through the "function tree" until it finds the variable you asked for. This means that you could define variables of the same name as global variables inside of functions. For demonstration of concept:

const a = "hello";
const b = "world";

function sayGoodByeWorld(){
	const a = "goodbye";

	alert(a); // goodbye
	alert(b); // world
}

sayGoodByeWorld();

alert(a); // hello

Object Literals

A very common way of storing and transporting data in JavaScript is by using objects. You can define an object with certain properties using an object literal:

const apple = {
	color: "red",
	flavor: "sweet",
	season: "autumn"
}

alert(apple.color);  // red

Functions

To define a function in JavaScript, use the function keyword.

function sayHello(){
	alert("Hello World");
}

sayHello(); // call the function

Pass an argument to a function like this:

function sayHello(name){
	alert("Hello, "+name);
}

sayHello("Todd"); // call the function and pass an argument

Arrow Functions

The latest versions of JavaScript have introduced arrow functions. They provide a cleaner syntax for writing functions, and maintain scope when using classes.

The general syntax is:

(param1, param2, ... , paramN) => { statements }

//removing the brackets around { statements } is equivalent to
{ return statements; }

//No parameters should be 2 empty parentheses as such: 
() => { statements }

The example above could be written as follows:

const sayHello = (name) => alert("Hello, "+name);
sayHello("Todd");

Arrow functions can be particularly useful when you're looking to contain function logic within cleanly and compactly. For example, iterating over an array and making updates to each item of the array.

let arr = [1,2,3,4,5];
let newArr = arr.map(number => number*5); //newArr is an array containing [5,10,15,20,25]

Functions are Objects

In JavaScript, functions are objects. This means that they can be assigned to variables, manipulated on the fly, and even passed as arguments to other functions! This is an extremely important feature of JavaScript that makes it such a robust language for web development.

You create a function variable like this:

const sayHello = function(){
	alert("Hello World");
}

sayHello(); // call the function

Observe that you can make scope-dependent functions in this way.

Closures

Watch on DOCTYPE

JavaScript Closures

A closure is a way to separate a certain snippet of JavaScript code from the global scope. This is useful if you do not want to "muck up" global variables. Closures are nothing more than anonymous functions that are called as soon as they are created:

const a = "hello";

(​​​​function(){
	const b = "world";
	
	alert(a); // hello
	alert(b); // world
})();

alert(a); // hello
alert(b); // ReferenceError: b is not defined​​​​​​​​​​​​​​​

Example: Passing Variables to a Closure

View in JSFiddle

This example demonstrates how you can "save" previous versions of a variable using closures. This is possible because of scoping and the fact that function variables can be returned as objects.

Most JavaScript libraries (like jQuery) write all of their code in a closure. What this means is that except for one or two variable names (jQuery or $), everything associated with the library is completely self-contained.

Prototypal Inheritance

JavaScript implements prototypal inheritance, as opposed to a language like Java or C++, which implements classical inheritance. JavaScript is one of only a handful of languages implementing prototype-based inheritance.

Here's how it works. Every callable object (function) has a prototype. When instances of that object are created, their properties and methods are copied (inherited) from the prototype. Additionally, changes made to the prototype later in the program are "copied" to every instance.

For example, if you wanted to make a method on all strings that added a “!”, you could modify String’s prototype:

String.prototype.bang = function(){
	return this+"!"; // Notice: the context is the instance itself
}

alert("Hello".bang()); // alerts Hello!

To "extend" an object, just copy its prototype to a new object. However, since classical inheritance and prototypal inheritance are different concepts, copying prototypes may have results that you don't expect. This article seems to have a pretty good explanation of how to "inherit" prototypes with example implementations.

Extending Built-In Prototypes

You should be aware that there is an active debate in the web development community about whether it is good programming style to extend the prototypes of built-in objects like String and Array, such as in the String.prototype.bang example above. There are legitimate arguments on both sides. This Stack Overflow post has good links to some articles discussing this issue.

Context

Watch on DOCTYPE

The this Variable in JavaScript

Every function is called on a certain object, which is not necessarily the same object in which it was defined. This is called context.

To call a function on an arbitrary context, use either call or apply. call takes a list of arguments, while apply takes them all in an array.

To access the current context, use the this keyword. Demonstration of concept:

const sayThisColor = function(){
	alert(this.color);
}

const apple = { color: "red" }

sayThisColor();   // undefined
sayThisColor.call(apple); // red

Events

Events are really what make JavaScript such a powerful language for web development.

Rather than being thread based, JavaScript is event based. Since all JavaScript code runs in a single thread, there are no concurrency issues. When an event occurs, JavaScript automatically calls the event callback as soon as all previous events' callbacks have finished executing.

Code in the form of a callback function is associated with events via a listener. For example, the following JavaScript causes sayHello() (the callback function) to be run whenever the button with ID "hello" is clicked (the event):

const sayHello = function(){
	alert("Hello World");
}

document.getElementById("hello").addEventListener("click", sayHello, false);

Example: Binding Callbacks to a Button Click

View in JSFiddle

In this example, notice how we bind the callback function when the DOMContentLoaded event fires. This is because JavaScript requires that the button has been parsed by the browser before it can add event listeners to it.

The addEventListener method takes three parameters:

  • The event type
  • The callback function
    The callback function is passed one parameter: the event object.
  • A boolean representing bubble (false) or capture (true) phase
    You usually want Bubble phase. For more information on event phase, see #Event Phase

Note that versions of Internet Explorer prior to version 9 required that you use a different, albeit similar, function for assigning event listeners called attachEvent. attachEvent takes the same arguments as addEventListener, except you need to put on before the event type (e.g. "onclick"), and the third argument (phase) is not necessary.

It is perfectly valid to use an anonymous function as your callback function:

document.getElementById("hello").addEventListener("click", function(event){
	alert(`Hello, World!  Event Type: ${event.type}`);
}, false);

QuirksMode has perhaps the best series of articles about the ins and outs of JavaScript events on the internet: http://www.quirksmode.org/js/contents.html#events

The Event Object

Callback functions are passed one argument: an event object. You see above that one property of the event object is event.type. To see the full list of properties of the event object, you could run the code:

document.getElementById("my_btn").addEventListener("click", function(event){
	let strout = "";
	for(var i in event){
		strout += `${i}: ${event[i]}\n`;
	}
	console.log(strout);
}, false);

Some of the most useful properties and methods include:

  • event.target references the object on which the event occurred.
  • event.currentTarget references the object whose listener is currently being processed.
    Note: event.target and event.currentTarget might be the same or might be different, depending on whether you are listening for bubbled events.
  • event.preventDefault() stops the normal result of the event from being executed. For example, if you listen for a form's submit event, you can call event.preventDefault() to stop the form from being submitted.
  • event.stopPropagation() prevents any other event listeners from being executed. For more detail, read the section on Event Phase below.

For more detail on the event object, see the W3C specification.

Note: If your callback function does not take any parameters, it will still run; you just won't have as easy of access to the event object.

Event Handlers and Memory Management

JavaScript is an automatically garbage-collected language, meaning that memory management is mostly taken care of for you. However, there are some instances when you need to be careful about creating "zombie" objects. In JavaScript, this mostly revolves around the idea of event listeners.

I could give an explanatory example, but instead I'll tell you straightaway that the moral of the story is that for every "addEventListener", there should probably be a "removeEventListener". Here are some good articles to satisfy your hunger for more details on event listeners and JavaScript memory management:

When you start working with Node.JS in the next module, it's more important than ever to keep good accounting of your event listeners.

Event Phase

In JavaScript, events occur in two phases: Capture Phase and Bubble Phase. Consider the following HTML document:

<!DOCTYPE html>
<html>
<head><title>Event Test</title></head>
<body>
  <div id="box">
    <button id="btn">
      Hello
    </button>
  </div>
</body>
</html>

Suppose the user clicks on the "Hello" button. Events will be fired in the following order:

  1. Capture Phase CLICK on the Document
  2. Capture Phase CLICK on the Body
  3. Capture Phase CLICK on the Div#box
  4. Target Phase CLICK on the Button#btn
  5. Bubble Phase CLICK on the Div#box
  6. Bubble Phase CLICK on the Body
  7. Bubble Phase CLICK on the Document

Example: Using Capture Phase to Enable or Disable a Toolbar

View in JSFiddle

We bind an event in the capture phase to the div with ID toolbar. When e.stopPropagation() is called, the event does not reach the buttons in the toolbar.

Graphically, here is the order in which an event occurs in three generic nested elements:

EventPhase.gif

Why is this useful? Suppose you have a control panel (toolbar, etc) that you want to temporarily deactivate. (For example, Bold/Italic/Underline buttons on a graphic text editor that currently has no content.) You could keep bubble phase event listeners on your buttons, but prevent those events from occurring by stopping the event in the capture phase on the parent element before the event reaches your buttons! Example:

const disableToolbarFunc = function(e){
	e.stopPropogation();
}

// To disable toolbar:
document.getElementById("box").addEventListener("click", disableToolbarFunc, true);

// To re-enable toolbar:
document.getElementById("box").removeEventListener("click", disableToolbarFunc, true);

Word of caution: Like many great things in web development, capture phase does not work in versions of Internet Explorer prior to version 9. There is no elegant substitution for older versions of Internet Explorer.

Note: When the event reaches the innermost element, there is no distinction between capture phase and bubble phase. The order in which event handlers were added to the element determines the order in which those callbacks are evaluated.

Document Object Model

The Document Object Model (DOM) is what enables JavaScript to manipulate the content of your web page.

Nodes and Traversing the DOM

Each element on your page, whether it is a <p>, <img/>, a string of text, or even an attribute, is called a node.

Every node, except for the document itself, has exactly one parent. A single node may have multiple children.

Consider the following HTML code:

<ul id="my-list">
	<li class="fruits">Apples</li>
	<li class="citrus">Oranges and <strong>Lemons</strong></li>
	<li class="berries">Cherries</li>
</ul>

Here, the ul with ID "my-list" has three child element nodes (the three li's) as well as four child text nodes (the whitespace counts as a text node) and a child attribute node. The second LI has one child text node as well as one child element node and one child attribute node.

The DOM can be traversed in JavaScript. First, let's look at an example.

e = document.getElementById("my-list").getElementsByClassName("citrus")[0].lastChild.nodeName;
alert(e);

Example: Currently Selected Radio Button

View in JSFiddle

In pure JavaScript, in order to determine the currently selected radio button, it is necessary to iterate over an array of pointers to all possible radio buttons, and stop when you find one that is "checked".

The above snippet gets the element with class name citrus that is a child of the element with ID my-list (the second LI in the example above), and alerts the type of its second child node (which in this case is STRONG).

Some common methods for traversing the DOM include:

  • Node.parentNode
  • Node.childNodes - returns an array of nodes
  • Node.firstChild - same as Node.childNodes[0]
  • Node.lastChild - same as Node.childNodes[Node.childNodes.length-1]
  • Node.previousSibling
  • Node.nextSibling

You can also search for child (including all descendants) using one of the following methods:

  • Node.getElementsByTagName("tag-name") - returns an array of element nodes having the tag name tag-name that are descendants of Node
  • Node.getElementById("id") - returns a single node having the ID id that is a descendant of Node
  • Node.getElementsByName("what-name") - returns an array of element nodes having their name attribute set to what-name. Useful for radio buttons and other form elements.
  • Node.getElementsByClassName("class") - returns an array of nodes having the class class
    Note: As of October 2012, 82% of the browser market supports Node.getElementsByClassName (more info). In order to support the other 18%, you need to either add your own getElementsByClassName to Node's prototype or use a JavaScript library like jQuery or MooTools.

It is common in JavaScript to have long statements like the one in the example above, where you call methods on several objects in sequence.

Creating, Moving, and Removing Nodes

You will frequently find yourself wanting to modify the DOM. JavaScript provides methods that help you do this.

Consider the same HTML as above. If you wanted to add another element to the list, you could use this code:

const newLi = document.createElement("li");  // creates a node with the tag name li
newLi.appendChild(document.createTextNode("Broccoli"));
newLi.setAttribute("class", "veggies");
document.getElementById("my-list").appendChild(newLi);

If you wanted to remove the apples from the list, you could do:

const apple = document.getElementById("my-list").getElementsByClassName("fruits")[0];
document.getElementById("my-list").removeChild(apple);

If you wanted to move the apples to the end of the list, you could do this:

const apple = document.getElementById("my-list").getElementsByClassName("fruits")[0];
document.getElementById("my-list").appendChild(apple);

Notice that Node.append() both removes a node from its previous location and appends it to its new location, which is always going to be as the last child of the parent node.

In summary, here are the methods to know from this section:

  • Node.append(otherNode) - removes otherNode from its current location in the DOM (if applicable) and then adds it as the last child of Node
  • Node.removeChild(otherNode) - removes otherNode, a child of Node, from the DOM
  • document.createElement("tag-name") - create a new node with the tag name "tag-name"
  • document.createTextNode("text") - create a node containing only the string text
  • Node.setAttribute("attribute", "value") - sets the attribute node of name "attribute" to "value"

DOM Shortcuts

In the above sections, we have been creating, reading, modifying, and deleting Nodes. This is the most "correct" way to change the content of a web page from inside JavaScript.

This section documents a few shortcuts that are not as "pure" as using Nodes but can be less confusing and cumbersome.

Example: Displaying Information

View in JSFiddle

This example shows how to use some of the shortcut methods for using JavaScript to display information on the page.

textContent

Using textContent to display information is probably the easiest method out there. For example, the following code overwrites whatever is in the element with ID "holder" with the string "Hello World":

document.getElementById("holder").textContent = "Hello World";

If you wanted to instead append the text, simply use += instead of =:

document.getElementById("holder").textContent += " -  Later, he said Hello World  - ";

Note that textContent displays strings literally; that is, it does not allow for HTML. Setting the textContent property also removes any HTML that may have already been in that element.

innerHTML

If you need to use HTML in your output but don't want to use the method described earlier in this section, a straightforward strategy is to use the innerHTML property of an element. For example, the following code would add a paragraph saying "Hello World" to the element with ID "holder":

document.getElementById("holder").innerHTML += "<p>Hello World</p>";

Important: You need to be careful to prevent XSS-type attacks if you display information using innerHTML. For example, be sure to escape a user-supplied string before appending it to innerHTML.

JavaScript Inspectors

Watch on DOCTYPE

Using Firebug, WebKit Inspector, and IE Developer Tools

Various browsers and browser plugins give you, the developer, tools to debug your code, inspect the DOM, and much more. Using these tools is essential for an efficient workflow.

  • Firebug. Firebug is a plugin for Firefox that is a toolkit full of web developer tools, one of which is a DOM inspector. To inspect the DOM of a web page, simply open the Firebug window while viewing the web page.
  • WebKit Inspector. The WebKit Inspector, which is available in both Chrome and Safari, enables you to see the DOM tree and relationships between elements. However, its DOM inspection tools are not as robust as those of Firebug.
  • Opera Dragonfly. The web browser Opera comes with a web developer toolkit similar to the WebKit Inspector called Dragonfly. It is important that you test your sites in Opera, because although Opera has only a small fraction of the desktop browser market, it is second only to WebKit on the mobile market. Opera also has extremely strong CSS support, so it is good for testing for bugs in your CSS.

All of the above-mentioned tools enable you to look at the DOM of your web page and find what properties and methods are associated with each node. This is helpful because if you have JavaScript that manipulates HTML elements, you cannot tell which properties are associated with which elements by looking at the source code alone.