JavaScript

JavaScript was created by Netscape’s Brendan Eich in 1995. It was originally intended as a simpler scripting language for websites, complementing 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.

JavaScript has a C-like syntax, so if you’ve used languages like C or Java, a lot of the basic syntax will already be familiar. Despite this, and despite the similarity in name, JavaScript’s object model is significantly different to Java’s.

// Single-line comments start with two slashes.
/* 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()

// Because those cases can cause unexpected results, we'll keep on using
// semicolons in this guide.

///////////////////////////////////
// 1. Numbers, Strings and Operators

// JavaScript has one number type (which is a 64-bit IEEE 754 double).
// Doubles have a 52-bit mantissa, which is enough to store integers
// up to about 9✕10¹⁵ precisely.
3; // = 3
1.5; // = 1.5

// Some basic arithmetic works as you'd expect.
1 + 1; // = 2
0.1 + 0.2; // = 0.30000000000000004
8 - 1; // = 7
10 * 2; // = 20
35 / 5; // = 7

// Including uneven division.
5 / 2; // = 2.5

// And modulo division.
10 % 2; // = 0
30 % 4; // = 2
18.5 % 7; // = 4.5

// 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, stands for 'Not a Number'

// 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!"

// ... which works with more than just strings
"1, 2, " + 3; // = "1, 2, 3"
"Hello " + ["world", "!"]; // = "Hello world,!"

// ...which can result in some weird behaviour...
13 + !0; // 14
"13" + !0; // '13true'

// and are compared with < and >
"a" < "b"; // = true

// Type coercion is performed for comparisons with double equals...
"5" == 5; // = true
null == undefined; // = true

// ...unless you use ===
"5" === 5; // = false
null === undefined; // = false

// You can access characters in a string with `charAt`
"This is a string".charAt(0);  // = 'T'

// ...or use `substring` to get larger pieces.
"Hello world".substring(0, 5); // = "Hello"

// `length` is a property, so don't use ().
"Hello".length; // = 5

// There's also `null` and `undefined`.
null;      // used to indicate a deliberate non-value
undefined; // used to indicate a value is not currently present (although
           // `undefined` is actually a value itself)

// false, null, undefined, NaN, 0 and "" are falsy; 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

// If you want to declare a couple of variables, then you could use a comma
// separator
var someFourthVar = 2, someFifthVar = 4;

// 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

// Arrays are mutable and of variable length.
myArray.push("World");
myArray.length; // = 4

// Add/Modify at specific index
myArray[3] = "Hello";

// Add and remove element from front or back end of an array
myArray.unshift(3); // Add as the first element
someVar = myArray.shift(); // Remove first element and return it
myArray.push(3); // Add as the last element
someVar = myArray.pop(); // Remove last element and return it

// Join all elements of an array with semicolon
var myArray0 = [32,false,"js",12,56,90];
myArray0.join(";"); // = "32;false;js;12;56;90"

// Get subarray of elements from index 1 (include) to 4 (exclude)
myArray0.slice(1,4); // = [false,"js",12]

// Remove 4 elements starting from index 2, and insert there strings
// "hi","wr" and "ld"; return removed subarray
myArray0.splice(2,4,"hi","wr","ld"); // = ["js",12,56,90]
// myArray0 === [32,false,"hi","wr","ld"]

// JavaScript's objects are equivalent to "dictionaries" or "maps" in other
// languages: an unordered collection of key-value pairs.
var myObj = {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:
// initialization; continue condition; iteration.
for (var i = 0; i < 5; i++){
    // will run 5 times
}

// Breaking out of labeled loops is similar to Java
outer:
for (var i = 0; i < 10; i++) {
    for (var j = 0; j < 10; j++) {
        if (i == 5 && j ==5) {
            break outer;
            // breaks out of outer loop instead of only the inner one
        }
    }
}

// The for/in statement allows iteration over properties of an object.
var description = "";
var person = {fname:"Paul", lname:"Ken", age:18};
for (var x in person){
    description += person[x] + " ";
} // description = 'Paul Ken 18 '

// The for/of statement allows iteration over iterable objects (including the built-in String, 
// Array, e.g. the Array-like arguments or NodeList objects, TypedArray, Map and Set, 
// and user-defined iterables).
var myPets = "";
var pets = ["cat", "dog", "hamster", "hedgehog"];
for (var pet of pets){
    myPets += pet + " ";
} // myPets = 'cat dog hamster hedgehog '

// && 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";

// The `switch` statement checks for equality with `===`.
// Use 'break' after each case
// or the cases after the correct one will be executed too.
grade = 'B';
switch (grade) {
  case 'A':
    console.log("Great job");
    break;
  case 'B':
    console.log("OK job");
    break;
  case 'C':
    console.log("You can do better");
    break;
  default:
    console.log("Oy vey");
    break;
}


///////////////////////////////////
// 4. Functions, Scope and Closures

// JavaScript functions are declared with the `function` keyword.
function myFunction(thing){
    return thing.toUpperCase();
}
myFunction("foo"); // = "FOO"

// Note that the value to be returned must start on the same line as the
// `return` keyword, otherwise you'll always return `undefined` due to
// automatic semicolon insertion. Watch out for this when using Allman style.
function myFunction(){
    return // <- semicolon automatically inserted here
    {thisIsAn: 'object literal'};
}
myFunction(); // = undefined

// 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);
// Note: setTimeout isn't part of the JS language, but is provided by browsers
// and Node.js.

// Another function provided by browsers is setInterval
function myFunction(){
    // this code will be called every 5 seconds
}
setInterval(myFunction, 5000);

// Function objects don't even have to be declared with a name - you can write
// an anonymous function definition directly into the arguments of another.
setTimeout(function(){
    // 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 assigning 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;
})();
temporary; // raises ReferenceError
permanent; // = 10

// 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, even after the outer function exits.
function sayHelloInFiveSeconds(name){
    var prompt = "Hello, " + name + "!";
    // Inner functions are put in the local scope by default, as if they were
    // declared with `var`.
    function inner(){
        alert(prompt);
    }
    setTimeout(inner, 5000);
    // setTimeout is asynchronous, so the sayHelloInFiveSeconds function will
    // exit immediately, and setTimeout will call inner afterwards. However,
    // because inner is "closed over" sayHelloInFiveSeconds, inner still has
    // access to the `prompt` variable when it is finally called.
}
sayHelloInFiveSeconds("Adam"); // will open a popup with "Hello, Adam!" in 5s

///////////////////////////////////
// 5. 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!"

// We can also specify a context for a function to execute in when we invoke it
// using `call` or `apply`.

var anotherFunc = function(s){
    return this.myString + s;
};
anotherFunc.call(myObj, " And Hello Moon!"); // = "Hello World! And Hello Moon!"

// The `apply` function is nearly identical, but takes an array for an argument
// list.

anotherFunc.apply(myObj, [" And Hello Sun!"]); // = "Hello World! And Hello Sun!"

// This is useful when working with a function that accepts a sequence of
// arguments and you want to pass an array.

Math.min(42, 6, 27); // = 6
Math.min([42, 6, 27]); // = NaN (uh-oh!)
Math.min.apply(Math, [42, 6, 27]); // = 6

// But, `call` and `apply` are only temporary. When we want it to stick, we can
// use `bind`.

var boundFunc = anotherFunc.bind(myObj);
boundFunc(" And Hello Saturn!"); // = "Hello World! And Hello Saturn!"

// `bind` can also be used to partially apply (curry) a function.

var product = function(a, b){ return a * b; };
var doubler = product.bind(this, 2);
doubler(8); // = 16

// When you call a function with the `new` keyword, a new object is created, and
// made available to the function via the `this` keyword. Functions designed to be
// called like that are called constructors.

var MyConstructor = function(){
    this.myNumber = 5;
};
myNewObj = new MyConstructor(); // = {myNumber: 5}
myNewObj.myNumber; // = 5

// Unlike most other popular object-oriented languages, JavaScript has no
// concept of 'instances' created from 'class' blueprints; instead, JavaScript
// combines instantiation and inheritance into a single concept: a 'prototype'.

// 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

// The for/in statement allows iteration over properties of an object,
// walking up the prototype chain until it sees a null prototype.
for (var x in myObj){
    console.log(myObj[x]);
}
///prints:
// Hello world!
// 43
// [Function: myFunc]
// true

// To only consider properties attached to the object itself
// and not its prototypes, use the `hasOwnProperty()` check.
for (var x in myObj){
    if (myObj.hasOwnProperty(x)){
        console.log(myObj[x]);
    }
}
///prints:
// Hello world!

// We mentioned that `__proto__` was non-standard, and there's no standard way to
// change the prototype of an existing object. However, there are 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 = {
    myNumber: 5,
    getMyNumber: function(){
        return this.myNumber;
    }
};
var myNewObj2 = new MyConstructor();
myNewObj2.getMyNumber(); // = 5
myNewObj2.myNumber = 6;
myNewObj2.getMyNumber(); // = 6

// 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 (new Number(0)){
   // This code will execute, because wrapped numbers are objects, and objects
   // are always 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();
    };
}

// ES6 Additions

// The "let" keyword allows you to define variables in a lexical scope, 
// as opposed to a function scope like the var keyword does.
let name = "Billy";

// Variables defined with let can be reassigned new values.
name = "William";

// The "const" keyword allows you to define a variable in a lexical scope
// like with let, but you cannot reassign the value once one has been assigned.

const pi = 3.14;

pi = 4.13; // You cannot do this.

// There is a new syntax for functions in ES6 known as "lambda syntax".
// This allows functions to be defined in a lexical scope like with variables
// defined by const and let. 

const isEven = (number) => {
    return number % 2 === 0;
};

isEven(7); // false

// The "equivalent" of this function in the traditional syntax would look like this:

function isEven(number) {
    return number % 2 === 0;
};

// I put the word "equivalent" in double quotes because a function defined
// using the lambda syntax cannot be called before the definition.
// The following is an example of invalid usage:

add(1, 8);

const add = (firstNumber, secondNumber) => {
    return firstNumber + secondNumber;
};

Further Reading

The Mozilla Developer Network 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 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.

Learn Javascript by Example and with Challenges is a variant of this reference with built-in challenges.

JavaScript Garden is an in-depth guide of all the counter-intuitive parts of the language.

JavaScript: The Definitive Guide is a classic guide and reference book.

Eloquent Javascript by Marijn Haverbeke is an excellent JS book/ebook with attached terminal

Javascript: The Right Way is a guide intended to introduce new developers to JavaScript and help experienced developers learn more about its best practices.

Javascript:Info is a modern javascript tutorial covering the basics (core language and working with a browser) as well as advanced topics with concise explanations.

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 on the Mozilla Developer Network.