Learn JavaScript · JS Recap

Understand what your code does

JavaScript is easy to start and surprisingly easy to misread. A copied object still changes. A function remembers a variable. A promise runs before a timer. The interesting part is understanding why.

A chapter-by-chapter JavaScript reference, from primitive values and operators to promises and modules. Read the notes, follow the code examples, and explore the interactive experiments.

Try one

Four lines of code, one surprising order. Step through them and watch where each callback waits before it runs.

console.log('A');setTimeout(() => console.log('timer'), 0);Promise.resolve().then(() => console.log('promise'));console.log('B');

Step 0 of 6: Guess the order the four logs will print, then press Next step.

Call stackrunning nowempty

Microtask queuepromises, runs firstempty

Task queuetimers, runs afterempty

Console

Illustrative model. The controls step through the behavior described above.

Figure 1. Execution order. Normal code runs first. Then promise callbacks (microtasks). Then timer callbacks (tasks).

CHAPTER 1

Fundamentals

Values, operators, conversions and control flow: the pieces every program is built from.

1.1Basis of JS 1.2The 7 primitive data types 1.3Operators 1.4Template literals 1.5Type conversion and coercion 1.6Conditional statements (if / else)1.7Switch statement 1.8Short circuiting 1.9Nullish coalescing operator 1.10Loops

CHAPTER 2

Functions

Declarations, expressions, arrows, closures, and scheduling work with timers.

2.1Functions 2.2Timers

CHAPTER 3

Data structures

Arrays, objects, sets, maps and strings, and when to reach for each.

3.1Choosing a data structure 3.2Arrays 3.3Objects 3.4Sets 3.5Maps 3.6Working with strings

CHAPTER 4

The DOM

Selecting, creating and changing elements, and reacting to events.

4.1DOM manipulation

CHAPTER 5

Object-oriented JavaScript

Classes, constructor functions, prototypes and inheritance.

5.1Object-oriented programming

CHAPTER 6

Asynchronous JavaScript

How JavaScript waits: the web, APIs, AJAX and promises.

6.1Synchronous code 6.2Asynchronous code 6.3What are AJAX calls?6.4What is an API?6.5What happens when we access a web server 6.6Promises and fetch

CHAPTER 7

Modern JavaScript and practice

Modules, tooling, and the habits that make you a better developer.

7.1An overview of modules 7.2Developer skills

Glossary

Closure: A function together with access to the variables of the scope it was created in.
Prototype: An object consulted when a property is missing on another object.
Hoisting: Declarations are set up before code runs, so some can be used above the line that declares them.
Coercion: JavaScript converting a value to another type implicitly, as in "5" * 2.
Callback: A function passed to another function, to be called later.
Promise: An object standing in for a value that will be available later, or an error.
Microtask: Work, such as a promise reaction, processed after the current stack empties and before the next task.
Scope: The region of code where a variable can be accessed.
Primitive: An immutable value that is not an object: string, number, bigint, boolean, undefined, symbol or null.
Event Loop: The mechanism that takes queued tasks and runs them once the call stack is empty.
Destructuring: Unpacking values from arrays or properties from objects into separate variables.

Learn JavaScript: an interactive guide to the language