Glossary · 24 terms

JavaScript

The language, async code and the runtime underneath.

Beginner · Students and graduates

Variable: A name bound to a value. In modern JavaScript you declare one with let, or const when it will not be reassigned. Docs
Function: A callable unit of code that takes parameters and can return a value. Functions are values too, so you can pass them around. Docs
Array: A list-like object with numeric indexes and methods such as map, filter and reduce for working with its items. Docs
Object: The basic structure of JavaScript: properties keyed by strings or symbols. Arrays, functions and dates are all objects. Docs
Boolean: The primitive type with two values. Conditions convert other values to it, which is where “truthy” and “falsy” come from. Docs
String: An immutable text value. Template literals with backticks allow ${} interpolation and multiple lines. Docs
DOM: The Document Object Model: the live, scriptable tree of nodes that represents the page. JavaScript reads and changes the page through it. Docs
Callback: A function handed to another function, which calls it when something happens, such as an event firing or a timer ending. Docs

Closure: A function bundled with its surrounding scope. It keeps access to those variables even after the outer function has returned. Docs
Promise: Represents the eventual result of an asynchronous operation: pending, then fulfilled or rejected. Chain it with then or await it. Docs
Async: An async function always returns a promise, and can pause at each await until a promise settles, so async code reads top to bottom. Docs
Hoisting: Declarations are processed before code runs. Function declarations are fully available; let and const exist but can’t be used before their line. Docs
Event loop: JavaScript runs one task at a time. The event loop picks the next task, then runs all pending microtasks, then lets the browser render. Docs
Prototype: Every object links to a prototype. Property lookups walk this chain, which is how methods are shared and how class works under the hood. Docs
Module: ES modules give each file its own scope and explicit imports and exports, which lets bundlers drop unused code. Docs
Debounce: A technique that delays a function until no new calls arrive for a set time, e.g. searching only after the user stops typing. Docs

Microtask: Promise reactions and queueMicrotask callbacks run after the current task and before rendering or the next task, all in one go. Docs
Generator: A function* returns an iterator. Each next() call runs it to the next yield, which makes lazy sequences easy. Docs
Proxy: A Proxy wraps a target with traps for get, set and more. Reactive state libraries use it to track reads and writes. Docs
WeakMap: Keys must be objects and are held weakly, so entries disappear when the key is no longer used. Good for caches and private data. Docs
Temporal: A built-in namespace with immutable types like PlainDate, ZonedDateTime and Duration, and real time zone and calendar support. Docs
Iterator: An object with a next() method returning { value, done }. Iterator helpers add map, filter and take that work lazily. Docs
Memoize: Storing results keyed by arguments so repeated calls skip the work. It only suits pure functions. Docs
Symbol: Every Symbol() is unique. Well-known symbols such as Symbol.iterator let objects plug into built-in language behaviour. Docs