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Introduction

Overview: Introduction

• About this guide
• About JavaScript
• JavaScript and Java
• ECMAScript
• Tools
• Hello World

Grammar and types

Overview: Grammar and types

• Basic syntax & comments
• Declarations
• Variable scope
• Variable hoisting
• Data structures and types
• Literals

Control flow and error handling

Overview: Control flow and error handling

• if...else
• switch
• try/catch/throw
• Error objects

Loops and iteration

Overview: Loops and iteration

• for
• while
• do...while
• continue
• break
• for...in
• for...of

Functions

Overview: Functions

• Defining functions
• Calling functions
• Function scope
• Closures
• Arguments & parameters
• Arrow functions

Expressions and operators

Overview: Expressions and operators

• Assignment & Comparisons
• Arithmetic operators
• Bitwise & logical operators
• Conditional [ternary] operator

Numbers and dates

Overview: Numbers and dates

• Number literals
• Number object
• Math object
• Date object

Text formatting

Overview: Text formatting

• String literals
• String object
• Template literals
• Internationalization
• Regular Expressions

Indexed collections

Overview: Indexed collections

• Arrays
• Typed arrays

Keyed collections

Overview: Keyed collections

• Map
• WeakMap
• Set
• WeakSet

Working with objects

Overview: Working with objects

• Objects and properties
• Creating objects
• Defining methods
• Getter and setter

Using classes

Overview: Using classes

• Declaring a class
• Various class features
• Extends and inheritance
• Why classes?

Promises

Overview: Promises

• Guarantees
• Chaining
• Error propagation
• Composition
• Timing

Iterators and generators

Overview: Iterators and generators

• Iterators
• Iterables
• Generators

Meta programming

Overview: Meta programming

• Proxy
• Handlers and traps
• Revocable Proxy
• Reflect

JavaScript modules

Overview: JavaScript modules

• Exporting
• Importing
• Default exports
• Renaming features
• Aggregating modules
• Dynamic module loading

• Next »

Data types

Let's start off by looking at the building blocks of any language: the types. JavaScript programs manipulate values, and those values all belong to a type. JavaScript offers seven primitive types:

• Number: used for all number values [integer and floating point] except for very big integers.
• BigInt: used for arbitrarily large integers.
• String: used to store text.
• Boolean: true and false — usually used for conditional logic.
• Symbol: used for creating unique identifiers that won't collide.
• Undefined: indicating that a variable has not been assigned a value.
• Null: indicating a deliberate non-value.

Everything else is known as an Object. Common object types include:

• Function
• Array
• Date
• RegExp
• Error

Functions aren't special data structures in JavaScript — they are just a special type of object that can be called.

Numbers

JavaScript has two built-in numeric types: Number and BigInt.

The Number type is a IEEE 754 64-bit double-precision floating point value, which means integers can be safely represented between -[253 − 1] and 253 − 1 without loss of precision, and floating point numbers can be stored all the way up to 1.79 × 10308. Within numbers, JavaScript does not distinguish between floating point numbers and integers.

console.log[3 / 2]; // 1.5, not 1

So an apparent integer is in fact implicitly a float. Because of IEEE 754 encoding, sometimes floating point arithmetic can be imprecise.

console.log[0.1 + 0.2]; // 0.30000000000000004

For operations that expect integers, such as bitwise operations, the number will be converted to a 32-bit integer.

Number literals can also have prefixes to indicate the base [binary, octal, decimal, or hexadecimal], or an exponent suffix.

console.log[0b111110111]; // 503
console.log[0o767]; // 503
console.log[0x1f7]; // 503
console.log[5.03e2]; // 503

The BigInt type is an arbitrary length integer. Its behavior is similar to C's integer types [e.g. division truncates to zero], except it can grow indefinitely. BigInts are specified with a number literal and an n suffix.

console.log[-3n / 2n]; // -1n

The standard arithmetic operators are supported, including addition, subtraction, remainder arithmetic, etc. BigInts and numbers cannot be mixed in arithmetic operations.

The Math object provides standard mathematical functions and constants.

Math.sin[3.5];
const circumference = 2 * Math.PI * r;

There are three ways to convert a string to a number:

• parseInt[], which parses the string for an integer.
• parseFloat[], which parses the string for a floating-point number.
• The Number[] function, which parses a string as if it's a number literal and supports many different number representations.

You can also use the unary plus + as a shorthand for Number[].

Number values also include NaN [short for "Not a Number"] and Infinity. Many "invalid math" operations will return NaN — for example, if attempting to parse a non-numeric string, or using Math.log[] on a negative value. Division by zero produces Infinity [positive or negative].

NaN is contagious: if you provide it as an operand to any mathematical operation, the result will also be NaN. NaN is the only value in JavaScript that's not equal to itself [per IEEE 754 specification].

Strings

Strings in JavaScript are sequences of Unicode characters. This should be welcome news to anyone who has had to deal with internationalization. More accurately, they are UTF-16 encoded.

console.log["Hello, world"];
console.log["你好，世界！"]; // Nearly all Unicode characters can be written literally in string literals

Strings can be written with either single or double quotes — JavaScript does not have the distinction between characters and strings. If you want to represent a single character, you just use a string consisting of that single character.

console.log["Hello"[1] === "e"]; // true

To find the length of a string [in code units], access its length property.

Strings have utility methods to manipulate the string and access information about the string. Because all primitives are immutable by design, these methods return new strings.

The + operator is overloaded for strings: when one of the operands is a string, it performs string concatenation instead of number addition. A special template literal syntax allows you to write strings with embedded expressions more succinctly. Unlike Python's f-strings or C#'s interpolated strings, template literals use backticks [not single or double quotes].

const age = 25;
console.log["I am " + age + " years old."]; // String concatenation
console.log[`I am ${age} years old.`]; // Template literal

Other types

JavaScript distinguishes between null, which indicates a deliberate non-value [and is only accessible through the null keyword], and undefined, which indicates absence of value. There are many ways to obtain undefined:

• A return statement with no value [return;] implicitly returns undefined.
• Accessing a nonexistent object property [obj.iDontExist] returns undefined.
• A variable declaration without initialization [let x;] will implicitly initialize the variable to undefined.

JavaScript has a Boolean type, with possible values true and false — both of which are keywords. Any value can be converted to a boolean according to the following rules:

1. false, 0, empty strings [""], NaN, null, and undefined all become false.
2. All other values become true.

You can perform this conversion explicitly using the Boolean[] function:

Boolean[""]; // false
Boolean[234]; // true

However, this is rarely necessary, as JavaScript will silently perform this conversion when it expects a boolean, such as in an if statement [see Control structures]. For this reason, we sometimes speak of "truthy" and "falsy", meaning values that become true and false, respectively, when used in boolean contexts.

Boolean operations such as && [logical and], || [logical or], and ! [logical not] are supported; see Operators.

The Symbol type is often used to create unique identifiers. Every symbol created with the Symbol[] function is guaranteed to be unique. In addition, there are registered symbols, which are shared constants, and well-known symbols, which are utilized by the language as "protocols" for certain operations. You can read more about them in the symbol reference.

Variables

Variables in JavaScript are declared using one of three keywords: let, const, or var.

let allows you to declare block-level variables. The declared variable is available from the block it is enclosed in.

let a;
let name = "Simon";

// myLetVariable is *not* visible out here

for [let myLetVariable = 0; myLetVariable  {
  let sum = 0;
  for [const item of args] {
    sum += item;
  }
  return sum / args.length;
};

// You can omit the `return` when simply returning an expression
const sum = [a, b, c] => a + b + c;

Arrow functions are not semantically equivalent to function expressions — for more information, see its reference page.

There's another way that anonymous functions can be useful: it can be simultaneously declared and invoked in a single expression, called an Immediately invoked function expression [IIFE]:

[function [] {
  // …
}][];

For use-cases of IIFEs, you can read emulating private methods with closures.

Recursive functions

JavaScript allows you to call functions recursively. This is particularly useful for dealing with tree structures, such as those found in the browser DOM.

function countChars[elm] {
  if [elm.nodeType === 3] {
    // TEXT_NODE
    return elm.nodeValue.length;
  }
  let count = 0;
  for [let i = 0, child; [child = elm.childNodes[i]]; i++] {
    count += countChars[child];
  }
  return count;
}

Function expressions can be named as well, which allows them to be recursive.

const charsInBody = [function counter[elm] {
  if [elm.nodeType === 3] {
    // TEXT_NODE
    return elm.nodeValue.length;
  }
  let count = 0;
  for [let i = 0, child; [child = elm.childNodes[i]]; i++] {
    count += counter[child];
  }
  return count;
}][document.body];

The name provided to a function expression as above is only available to the function's own scope. This allows more optimizations to be done by the engine and results in more readable code. The name also shows up in the debugger and some stack traces, which can save you time when debugging.

If you are used to functional programming, beware of the performance implications of recursion in JavaScript. Although the language specification specifies tail-call optimization, only JavaScriptCore [used by Safari] has implemented it, due to the difficulty of recovering stack traces and debuggability. For deep recursion, consider using iteration instead to avoid stack overflow.

Functions are first-class objects

JavaScript functions are first-class objects. This means that they can be assigned to variables, passed as arguments to other functions, and returned from other functions. In addition, JavaScript supports closures out-of-the-box without explicit capturing, allowing you to conveniently apply functional programming styles.

// Function returning function
const add = [x] => [y] => x + y;
// Function accepting function
const babies = ["dog", "cat", "hen"].map[[name] => `baby ${name}`];

Note that JavaScript functions are themselves objects — like everything else in JavaScript — and you can add or change properties on them just like we've seen earlier in the Objects section.

Inner functions

JavaScript function declarations are allowed inside other functions. An important detail of nested functions in JavaScript is that they can access variables in their parent function's scope:

function parentFunc[] {
  const a = 1;

  function nestedFunc[] {
    const b = 4; // parentFunc can't use this
    return a + b;
  }
  return nestedFunc[]; // 5
}

This provides a great deal of utility in writing more maintainable code. If a called function relies on one or two other functions that are not useful to any other part of your code, you can nest those utility functions inside it. This keeps the number of functions that are in the global scope down.

This is also a great counter to the lure of global variables. When writing complex code, it is often tempting to use global variables to share values between multiple functions, which leads to code that is hard to maintain. Nested functions can share variables in their parent, so you can use that mechanism to couple functions together without polluting your global namespace.

Classes

JavaScript offers the class syntax that's very similar to languages like Java.

class Person {
  constructor[name] {
    this.name = name;
  }
  sayHello[] {
    return `Hello, I'm ${this.name}!`;
  }
}

const p = new Person["John"];
console.log[p.sayHello[]];

JavaScript classes are just functions that must be instantiated with the new operator. Every time a class is instantiated, it returns an object containing the methods and properties that the class specified. Classes don't enforce any code organization — for example, you can have functions returning classes, or you can have multiple classes per file. Here's an example of how ad-hoc the creation of a class can be: it's just an expression returned from an arrow function. This pattern is called a mixin.

const withAuthentication = [cls] =>
  class extends cls {
    authenticate[] {
      // …
    }
  };

class Admin extends withAuthentication[Person] {
  // …
}

Static properties are created by prepending static. Private properties are created by prepending a hash # [not private]. The hash is an integral part of the property name. [Think about # as _ in Python.] Unlike most other languages, there's absolutely no way to read a private property outside the class body — not even in derived classes.

For a detailed guide on various class features, you can read the guide page.

Asynchronous programming

JavaScript is single-threaded by nature. There's no paralleling; only concurrency. Asynchronous programming is powered by an event loop, which allows a set of tasks to be queued and polled for completion.

There are three idiomatic ways to write asynchronous code in JavaScript:

• Callback-based [such as setTimeout[]]
• Promise-based
• async/await, which is a syntactic sugar for Promises

For example, here's how a file-read operation may look like in JavaScript:

// Callback-based
fs.readFile[filename, [err, content] => {
  // This callback is invoked when the file is read, which could be after a while
  if [err] {
    throw err;
  }
  console.log[content];
}];
// Code here will be executed while the file is waiting to be read

// Promise-based
fs.readFile[filename]
  .then[[content] => {
    // What to do when the file is read
    console.log[content];
  }].catch[[err] => {
    throw err;
  }];
// Code here will be executed while the file is waiting to be read

// Async/await
async function readFile[filename] {
  const content = await fs.readFile[filename];
  console.log[content];
}

The core language doesn't specify any asynchronous programming features, but it's crucial when interacting with the external environment — from asking user permissions, to fetching data, to reading files. Keeping the potentially long-running operations async ensures that other processes can still run while this one waits — for example, the browser will not freeze while waiting for the user to click a button to grant permission.

If you have an async value, it's not possible to get its value synchronously. For example, if you have a promise, you can only access the eventual result via the then[] method. Similarly, await can only be used in an async context, which is usually an async function or a module. Promises are never blocking — only the logic depending on the promise's result will be deferred; everything else continues to execute in the meantime. If you are a functional programmer, you may recognize promises as monads which can be mapped with then[] [however, they are not proper monads because they auto-flatten; i.e. you can't have a Promise].

In fact, the single-threaded model has made Node.js a popular choice for server-side programming due to its non-blocking IO, making handling a large number of database or file-system requests very performant. However, CPU-bound [computationally intensive] tasks that's pure JavaScript will still block the main thread. To achieve real paralleling, you may need to use workers.

To learn more about asynchronous programming, you can read about using promises or follow the asynchronous JavaScript tutorial.

Modules

JavaScript also specifies a module system supported by most runtimes. A module is usually a file, identified by it's file path or URL. You can use the import and export statements to exchange data between modules:

import { foo } from "./foo.js";

// Unexported variables are local to the module
const b = 2;

export const a = 1;

Unlike Haskell, Python, Java, etc., JavaScript module resolution is entirely host-defined — it's usually based on URLs or file paths, so relative file paths "just work" and are relative to the current module's path instead of some project root path.

However, the JavaScript language doesn't offer standard library modules — all core functionalities are powered by global variables like Math and Intl instead. This is due to the long history of JavaScript lacking a module system, and the fact that opting into the module system involves some changes to the runtime setup.

Different runtimes may use different module systems. For example, Node.js uses the package manager npm and is mostly file-system based, while Deno and browsers are fully URL-based and modules can be resolved from HTTP URLs.

For more information, see the modules guide page.

Language and runtime

Throughout this page, we've constantly mentioned that certain features are language-level while others are runtime-level.

JavaScript is a general-purpose scripting language. The core language specification focuses on pure computational logic. It doesn't deal with any input/output — in fact, without extra runtime-level APIs [most notably console.log[]], a JavaScript program's behavior is entirely unobservable.

A runtime, or a host, is something that feeds data to the JavaScript engine [the interpreter], provides extra global properties, and provides hooks for the engine to interact with the outside world. Module resolution, reading data, printing messages, sending network requests, etc. are all runtime-level operations. Since its inception, JavaScript has been adopted in various environments, such as browsers [which provide APIs like DOM], Node.js [which provides APIs like file system access], etc. JavaScript has been successfully integrated in web [which was its primary purpose], mobile apps, desktop apps, server-side apps, serverless, embedded systems, and more. While you learn about JavaScript core features, it's also important to understand host-provided features in order to put the knowledge to use. For example, you can read about all web platform APIs, which are implemented by browsers, and sometimes non-browsers.

Further exploration

This page offers a very basic insight into how various JavaScript features compare with other languages. If you want to learn more about the language itself and the nuances with each feature, you can read the JavaScript guide and the JavaScript reference.

There are some essential parts of the language that we have omitted due to space and complexity, but you can explore on your own:

• Inheritance and the prototype chain
• Closures
• Regular expressions
• Iteration