The JavaScript glossary

A

API

A way for programs to talk to each other. A web API is usually a URL that returns data (often JSON) instead of a web page.

Argument

The actual value you pass into a function when you call it, like "Mia" in greet("Mia").

Array

An ordered list of values, written with square brackets: [1, 2, 3].

Arrow function

A short way to write a function: (x) => x * 2.

Assignment

Putting a value into a variable with =.

Async / await

Keywords that let you write asynchronous code that reads like normal top-to-bottom code.

Asynchronous

Code that starts a slow task and carries on, dealing with the result later.

B

Back end

The part of an app that runs on a server, behind the scenes.

Block

A group of statements inside curly braces { }.

Boolean

A value that is either true or false.

Bubbling

How an event travels up from the element where it happened, through all its ancestors.

Bug

A mistake in a program that makes it behave wrongly.

C

Callback

A function passed into another function, to be called later.

Camel case

Writing names like firstName, with each new word starting with a capital letter.

Class

A blueprint for creating objects with the same properties and methods.

Closure

A function that remembers the variables from the place where it was created.

Comment

A note in code for humans, which JavaScript ignores (// or /* */).

Console

A panel where you can see output and run code.

Constant

A variable created with const, which can’t be reassigned.

Constructor

The special method in a class that sets up each new instance.

D

Debugging

Finding and fixing bugs.

Destructuring

Unpacking values from arrays or objects into variables: const { name } = user;

DOM

The Document Object Model: the browser’s live, tree-shaped model of a web page.

E

Element

A node in the DOM representing an HTML tag, like a <p> or a <button>.

Encapsulation

Hiding an object’s internal details and exposing only safe ways to use it.

Event

Something that happens on a page, like a click or a key press.

Event delegation

Using one listener on a parent element to handle events from many children.

Event listener

A function that runs when a specific event happens.

Event loop

The mechanism that runs waiting callbacks whenever the call stack is empty.

Expression

Any piece of code that produces a value, like 2 + 3 or name.length.

F

Falsy

A value that counts as false in a condition: false, 0, -0, 0n, "", null, undefined, NaN.

Fetch

The built-in function for requesting data from a URL.

Front end

The part of an app that runs in the browser, which people see and use.

Function

A named, reusable block of code that can take inputs and return an output.

G

Global scope

The outermost scope. Variables here can be seen everywhere.

H

Higher-order function

A function that takes a function as an argument, or returns one.

Hoisting

JavaScript setting up declarations at the start of their scope, before the code runs.

I

Immutable

Unable to be changed. Strings are immutable.

Index

A position in an array or string, counting from 0.

Infinite loop

A loop whose condition never becomes false, so it runs forever.

Inheritance

One class getting the properties and methods of another, using extends.

Instance

An object created from a class with new.

Iteration

One trip through a loop.

J

JSON

JavaScript Object Notation: a text format for storing and sending data.

K

Keyword

A word with a special meaning in JavaScript, like let, if or function.

L

localStorage

Browser storage that keeps strings saved between visits to a website.

Loop

Code that repeats.

M

Method

A function that belongs to an object.

Module

A separate file of code that can export things and import from other modules.

N

NaN

“Not a Number”: the result of math that makes no sense.

Node.js

A program that runs JavaScript outside the browser.

npm

The tool (and registry) for installing JavaScript packages.

null

A value meaning “deliberately empty”.

O

Object

A collection of key-value pairs (properties).

Operator

A symbol that does something to values, like +, === or &&.

P

Parameter

A placeholder in a function’s definition, which receives an argument.

Primitive

A simple, single value: string, number, boolean, undefined, null, bigint or symbol.

Promise

An object representing a value that will be ready in the future (or an error).

Property

A key-value pair inside an object.

Prototype

The object another object links to, where it looks for properties it doesn’t have.

R

Reference

How objects and arrays are stored: two variables can point to the same one.

Render

To draw (or redraw) the page from the app’s data.

Return value

The value a function gives back, using return.

S

Scope

The region of code where a variable can be seen and used.

Spread / rest

The ... syntax, which unpacks (spread) or packs up (rest) values.

State

The data an app keeps track of, which decides what’s shown.

Statement

A single instruction, usually ending with a semicolon.

String

A piece of text, written in quotes.

Syntax

The grammar rules of a programming language.

T

Template literal

A string in backticks that can include values with ${ }.

Ternary operator

A one-line if/else: condition ? a : b.

this

A keyword for the object a function was called on. It depends on how the function is called.

Truthy

Any value that counts as true in a condition (everything that isn’t falsy).

Type coercion

JavaScript automatically converting a value from one type to another.

U

undefined

The value of a variable that hasn’t been given a value.

V

Variable

A named box that holds a value.