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