Merge pull request #42 from adambrenecki/javascript

JavaScript

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+---
+language: javascript
+author: Adam Brenecki
+author_url: http://adam.brenecki.id.au
+---
+
+Javascript was created by Netscape's Brendan Eich in 1995. It was originally
+intended as a simpler scripting language for websites, complimenting the use of
+Java for more complex web applications, but its tight integration with Web pages
+and built-in support in browsers has caused it to become far more common than
+Java in web frontends.
+
+JavaScript isn't just limited to web browsers, though: Node.js, a project that
+provides a standalone runtime for Google Chrome's V8 JavaScript engine, is
+becoming more and more popular.
+
+Feedback would be highly appreciated! You can reach me at
+[@adambrenecki](https://twitter.com/adambrenecki), or
+[adam@brenecki.id.au](mailto:adam@brenecki.id.au).
+
+```javascript
+// Comments are like C. Single-line comments start with two slashes,
+/* and multiline comments start with slash-star
+   and end with star-slash */
+
+// Statements can be terminated by ;
+doStuff();
+
+// ... but they don't have to be, as semicolons are automatically inserted
+// wherever there's a newline, except in certain cases.
+doStuff()
+
+// We'll leave semicolons off here; whether you do or not will depend on your
+// personal preference or your project's style guide.
+
+/***********
+ * 1. Numbers, Strings and Operators
+ ***********/
+
+// Javascript has one number type (which is a 64-bit IEEE 754 double).
+3 // = 3
+1.5 // = 1.5
+
+// All the basic arithmetic works as you'd expect.
+1 + 1 // = 2
+8 - 1 // = 7
+10 * 2 // = 20
+35 / 5 // = 7
+
+// Including uneven division.
+5 / 2 // = 2.5
+
+// Bitwise operations also work; when you perform a bitwise operation your float
+// is converted to a signed int *up to* 32 bits.
+1 << 2 // = 4
+
+// Precedence is enforced with parentheses.
+(1 + 3) * 2 // = 8
+
+// There are three special not-a-real-number values:
+Infinity // result of e.g. 1/0
+-Infinity // result of e.g. -1/0
+NaN // result of e.g. 0/0
+
+// There's also a boolean type.
+true
+false
+
+// Strings are created with ' or ".
+'abc'
+"Hello, world"
+
+// Negation uses the ! symbol
+!true // = false
+!false // = true
+
+// Equality is ==
+1 == 1 // = true
+2 == 1 // = false
+
+// Inequality is !=
+1 != 1 // = false
+2 != 1 // = true
+
+// More comparisons
+1 < 10 // = true
+1 > 10 // = false
+2 <= 2 // = true
+2 >= 2 // = true
+
+// Strings are concatenated with +
+"Hello " + "world!" // = "Hello world!"
+
+// and are compared with < and >
+"a" < "b" // = true
+
+// Type coercion is performed for comparisons...
+"5" == 5 // = true
+
+// ...unless you use ===
+"5" === 5 // = false
+
+// You can access characters in a string with charAt
+"This is a string".charAt(0)
+
+// There's also null and undefined
+null // used to indicate a deliberate non-value
+undefined // used to indicate a value that hasn't been set yet
+
+// null, undefined, NaN, 0 and "" are falsy, and everything else is truthy.
+// Note that 0 is falsy and "0" is truthy, even though 0 == "0".
+
+/***********
+ * 2. Variables, Arrays and Objects
+ ***********/
+
+// Variables are declared with the var keyword. Javascript is dynamically typed,
+// so you don't need to specify type. Assignment uses a single = character.
+var someVar = 5
+
+// if you leave the var keyword off, you won't get an error...
+someOtherVar = 10
+
+// ...but your variable will be created in the global scope, not in the scope
+// you defined it in.
+
+// Variables declared without being assigned to are set to undefined.
+var someThirdVar // = undefined
+
+// There's shorthand for performing math operations on variables:
+someVar += 5 // equivalent to someVar = someVar + 5; someVar is 10 now
+someVar *= 10 // now someVar is 100
+
+// and an even-shorter-hand for adding or subtracting 1
+someVar++ // now someVar is 101
+someVar-- // back to 100
+
+// Arrays are ordered lists of values, of any type.
+var myArray = ["Hello", 45, true]
+
+// Their members can be accessed using the square-brackets subscript syntax.
+// Array indices start at zero.
+myArray[1] // = 45
+
+// JavaScript's objects are equivalent to 'dictionaries' or 'maps' in other
+// languages: an unordered collection of key-value pairs.
+{key1: "Hello", key2: "World"}
+
+// Keys are strings, but quotes aren't required if they're a valid
+// JavaScript identifier. Values can be any type.
+var myObj = {myKey: "myValue", "my other key": 4}
+
+// Object attributes can also be accessed using the subscript syntax,
+myObj["my other key"] // = 4
+
+// ... or using the dot syntax, provided the key is a valid identifier.
+myObj.myKey // = "myValue"
+
+// Objects are mutable; values can be changed and new keys added.
+myObj.myThirdKey = true
+
+// If you try to access a value that's not yet set, you'll get undefined.
+myObj.myFourthKey // = undefined
+
+/***********
+ * 3. Logic and Control Structures
+ ***********/
+
+// The if structure works as you'd expect.
+var count = 1
+if (count == 3){
+    // evaluated if count is 3
+} else if (count == 4) {
+    // evaluated if count is 4
+} else {
+    // evaluated if it's not either 3 or 4
+}
+
+// As does while.
+while (true) {
+    // An infinite loop!
+}
+
+// Do-while loops are like while loops, except they always run at least once.
+var input
+do {
+    input = getInput()
+} while (!isValid(input))
+
+// the for loop is the same as C and Java: 
+// initialisation; continue condition; iteration.
+for (var i = 0; i < 5; i++){
+    // will run 5 times
+}
+
+// && is logical and, || is logical or
+if (house.size == "big" && house.colour == "blue"){
+    house.contains = "bear"
+}
+if (colour == "red" || colour == "blue"){
+    // colour is either red or blue
+}
+
+// && and || "short circuit", which is useful for setting default values.
+var name = otherName || "default"
+
+/***********
+ * 5. Functions, Scope and Closures
+ ***********/
+
+// JavaScript functions are declared with the function keyword.
+function myFunction(thing){
+    return thing.toUpperCase()
+}
+myFunction("foo") // = "FOO"
+
+// Functions can also be defined "anonymously" - without a name:
+function(thing){
+    return thing.toLowerCase()
+}
+// (we can't call our function, since we don't have a name to refer to it with)
+
+// JavaScript functions are first class objects, so they can be reassigned to
+// different variable names and passed to other functions as arguments - for
+// example, when supplying an event handler:
+function myFunction(){
+    // this code will be called in 5 seconds' time
+}
+setTimeout(myFunction, 5000)
+
+// You can even write the function statement directly in the call to the other
+// function.
+
+setTimeout(function myFunction(){
+    // this code will be called in 5 seconds' time
+}, 5000)
+
+// JavaScript has function scope; functions get their own scope but other blocks
+// do not.
+if (true){
+    var i = 5
+}
+i // = 5 - not undefined as you'd expect in a block-scoped language
+
+// This has led to a common pattern of "immediately-executing anonymous
+// functions", which prevent temporary variables from leaking into the global
+// scope.
+function(){
+    var temporary = 5
+    // We can access the global scope by assiging to the 'global object', which
+    // in a web browser is always 'window'. The global object may have a
+    // different name in non-browser environments such as Node.js.
+    window.permanent = 10
+    // Or, as previously mentioned, we can just leave the var keyword off.
+    permanent2 = 15
+}()
+temporary // raises ReferenceError
+permanent // = 10
+permanent2 // = 15
+
+// One of JavaScript's most powerful features is closures. If a function is
+// defined inside another function, the inner function has access to all the
+// outer function's variables.
+function sayHelloInFiveSeconds(name){
+    var prompt = "Hello, " + name + "!"
+    function inner(){
+        alert(prompt)
+    }
+    setTimeout(inner, 5000)
+    // setTimeout is asynchronous, so this function will finish without waiting
+    // 5 seconds. However, once the 5 seconds is up, inner will still have
+    // access to the value of prompt.
+}
+sayHelloInFiveSeconds("Adam") // will open a popup with "Hello, Adam!" in 5s
+
+/***********
+ * 6. More about Objects; Constructors and Prototypes
+ ***********/
+
+// Objects can contain functions.
+var myObj = {
+    myFunc: function(){
+        return "Hello world!"
+    }
+}
+myObj.myFunc() // = "Hello world!"
+
+// When functions attached to an object are called, they can access the object
+// they're attached to using the this keyword.
+myObj = {
+    myString: "Hello world!",
+    myFunc: function(){
+        return this.myString
+    }
+}
+myObj.myFunc() // = "Hello world!"
+
+// What this is set to has to do with how the function is called, not where
+// it's defined. So, our function doesn't work if it isn't called in the
+// context of the object.
+var myFunc = myObj.myFunc
+myFunc() // = undefined
+
+// Inversely, a function can be assigned to the object and gain access to it
+// through this, even if it wasn't attached when it was defined.
+var myOtherFunc = function(){
+    return this.myString.toUpperCase()
+}
+myObj.myOtherFunc = myOtherFunc
+myObj.myOtherFunc() // = "HELLO WORLD!"
+
+// When you call a function with the new keyword, a new object is created, and
+// made available to the function via this. Functions designed to be called
+// like this are called constructors.
+
+var MyConstructor = function(){
+    this.myNumber = 5
+}
+myNewObj = new MyConstructor() // = {myNumber: 5}
+myNewObj.myNumber // = 5
+
+// Every JavaScript object has a 'prototype'. When you go to access a property
+// on an object that doesn't exist on the actual object, the interpreter will
+// look at its prototype.
+
+// Some JS implementations let you access an object's prototype on the magic
+// property __proto__. While this is useful for explaining prototypes it's not
+// part of the standard; we'll get to standard ways of using prototypes later.
+var myObj = {
+    myString: "Hello world!",
+}
+var myPrototype = {
+    meaningOfLife: 42,
+    myFunc: function(){
+        return this.myString.toLowerCase()
+    }
+}
+myObj.__proto__ = myPrototype
+myObj.meaningOfLife // = 42
+
+// This works for functions, too.
+myObj.myFunc() // = "hello world!"
+
+// Of course, if your property isn't on your prototype, the prototype's
+// prototype is searched, and so on.
+myPrototype.__proto__ = {
+    myBoolean: true
+}
+myObj.myBoolean // = true
+
+// There's no copying involved here; each object stores a reference to its
+// prototype. This means we can alter the prototype and our changes will be
+// reflected everywhere.
+myPrototype.meaningOfLife = 43
+myObj.meaningOfLife // = 43
+
+// We mentioned that __proto__ was non-standard, and there's no standard way to
+// change the prototype of an existing object. However, there's two ways to
+// create a new object with a given prototype.
+
+// The first is Object.create, which is a recent addition to JS, and therefore
+// not available in all implementations yet.
+var myObj = Object.create(myPrototype)
+myObj.meaningOfLife // = 43
+
+// The second way, which works anywhere, has to do with constructors.
+// Constructors have a property called prototype. This is *not* the prototype of
+// the constructor function itself; instead, it's the prototype that new objects
+// are given when they're created with that constructor and the new keyword.
+myConstructor.prototype = {
+    getMyNumber: function(){
+        return self.myNumber
+    }
+}
+var myNewObj2 = new myConstructor()
+myNewObj2.getMyNumber() // = 5
+
+// Built-in types like strings and numbers also have constructors that create
+// equivalent wrapper objects.
+var myNumber = 12
+var myNumberObj = new Number(12)
+myNumber == myNumberObj // = true
+
+// Except, they aren't exactly equivalent.
+typeof(myNumber) // = 'number'
+typeof(myNumberObj) // = 'object'
+myNumber === myNumberObj // = false
+if (0){
+    // This code won't execute, because 0 is falsy.
+}
+if (Number(0)){
+    // This code *will* execute, because Number(0) is truthy.
+}
+
+// However, the wrapper objects and the regular builtins share a prototype, so
+// you can actually add functionality to a string, for instance.
+String.prototype.firstCharacter = function(){
+    return this.charAt(0)
+}
+"abc".firstCharacter() // = "a"
+
+// This fact is often used in "polyfilling", which is implementing newer
+// features of JavaScript in an older subset of JavaScript, so that they can be
+// used in older environments such as outdated browsers.
+
+// For instance, we mentioned that Object.create isn't yet available in all
+// implementations, but we can still use it with this polyfill:
+if (Object.create === undefined){ // don't overwrite it if it exists
+    Object.create = function(proto){
+        // make a temporary constructor with the right prototype
+        var Constructor = function(){}
+        Constructor.prototype = proto
+        // then use it to create a new, appropriately-prototyped object
+        return new Constructor()
+    }
+}
+```
+
+## Further Reading
+
+The [Mozilla Developer
+Network](https://developer.mozilla.org/en-US/docs/Web/JavaScript) provides
+excellent documentation for JavaScript as it's used in browsers. Plus, it's a
+wiki, so as you learn more you can help others out by sharing your own
+knowledge.
+
+MDN's [A re-introduction to
+JavaScript](https://developer.mozilla.org/en-US/docs/Web/JavaScript/A_re-introduction_to_JavaScript)
+covers much of the concepts covered here in more detail. This guide has quite
+deliberately only covered the JavaScript language itself; if you want to learn
+more about how to use JavaScript in web pages, start by learning about the
+[Document Object
+Model](https://developer.mozilla.org/en-US/docs/Using_the_W3C_DOM_Level_1_Core)
+
+In addition to direct contributors to this article, some content is adapted
+from Louie Dinh's Python tutorial on this site, and the [JS
+Tutorial](https://developer.mozilla.org/en-US/docs/Web/JavaScript/A_re-introduction_to_JavaScript)
+on the Mozilla Developer Network.