learnxinyminutes-docs/javascript.html.markdown

---
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 web apps, complimenting Java for
more complex ones, but has become far more widely used than Java on the web.

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()

// Semicolons are a heated topic in the JavaScript world, but they're really a
// matter of personal or style-guide preference. We'll leave them off here.

/***********
 * 1. Primitive Datatypes and Operators
 ***********/

// Javascript has one number type that covers ints and floats.
3 // = 3
1.5 // = 1.5

// which support all the operations you'd expect.
1 + 1 // = 2
8 - 1 // = 7
10 * 2 // = 20
35 / 5 // = 7

// Uneven division works how you'd expect, too.
5 / 2 // = 2.5

// Enforce precedence with parentheses
(1 + 3) * 2 // = 8

// 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

// You can also compare strings with numbers
"5" == 5 // = true

// but this is almost always not what you want, so use === to stop this
"5" === 5 // = false

// You can access characters in a string with charAt
"This is a string".charAt(0)

// There's also a null keyword
null // = null

/***********
 * 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 some_var = 5

// if you leave the var keyword off, you won't get an error...
some_other_var = 10

// but your variable will always end up with the global scope, even if it wasn't
// defined there, so don't do it.

// Arrays are ordered lists of values, of any type.
["Hello", 45, true]

// 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 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

/***********
 * 3. Control Structures
 ***********/

/***********
 * 5. Functions, Scope and Closures
 ***********/

/***********
 * 6. More about Objects; Constructors and Prototypes
 ***********/

// Objects can contain functions, which can be called using the dot syntax.
myObj = {
    myFunc: function(){
        return "Hello world!"
    }
}
myObj.myFunc() // = "Hello world!"

// When functions are called like this, 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!"

// The value of this has to do with how the function is called, not where it's
// defined. So, that doesn't work if the function 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 defined as such.
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

// JavaScript objects aren't defined in terms of classes like other languages,
// but you can use prototypes to do many of the same things. When you try to
// access a property of an object that isn't present, its prototype is searched.
var myObj = {}
var myPrototype = {
    meaningOfLife: 42,
    myThirdFunc: function(){
        return this.myString.toLowerCase()
    }
}
myObj.__proto__ = myPrototype
myObj.myThirdFunc() // = "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.
myObj.

// The __proto__ magic property we've used to access prototypes isn't standard,
// and shouldn't be used in real-world code. There is a way to create a new
// object with another given object as its prototype, though:
var myObj = Object.create(myPrototype)
myObj.meaningOfLife // = 42

// Unfortunately, Object.create is quite recent and isn't available in many
// browsers, so you often can't use that, either. The most reliable way to set
// prototypes involves constructors.

// TODO: write about the .prototype property on constructors

// Built-in types' prototypes work like this too, so you can actually change
// the prototype of a string, for instance (although whether you should is
// another matter).
String.prototype.firstCharacter = function(){
    return this.charAt(0)
}
"abc".firstCharacter() // = "a"

```