What is the difference between localStorage and IndexedDB?

localStorage is a simple synchronous key-value store limited to about 5 MB of strings, ideal for small flags and preferences. IndexedDB is an asynchronous transactional database that stores large structured data and binary blobs, suited to offline apps and caches. Use localStorage for tiny values and IndexedDB for real data.

When should I use the Fetch API versus XMLHttpRequest?

Fetch is the modern promise-based API for HTTP requests and supports streaming, AbortController cancellation, and a clean Request/Response model. XMLHttpRequest is the older callback API kept mainly for legacy code and upload-progress events. Prefer fetch for new code.

What does IntersectionObserver do?

IntersectionObserver asynchronously reports when an element enters or leaves the viewport or a scroll container, without the jank of scroll-event listeners. It powers lazy-loading images, infinite scroll, and visibility analytics efficiently because the browser batches the work off the main thread.

What is a Web Worker?

A Web Worker runs JavaScript on a background thread so heavy computation does not block the UI. It communicates with the main thread by posting serializable messages and cannot touch the DOM directly. Use it for parsing, image processing, or any CPU-heavy task that would otherwise freeze the page.

Which APIs require a secure context?

Many powerful APIs only work over HTTPS or localhost, including Service Workers, the Geolocation API, the Clipboard API, Web Crypto's SubtleCrypto, and getUserMedia. Browsers gate them behind a secure context to protect users, so they silently fail or reject on plain HTTP pages.

Browser Web APIs Quick Reference

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The browser exposes hundreds of global APIs and remembering which object is the entry point for a given task is half the battle. This quick reference groups the Web APIs you reach for most — networking, storage, DOM, observers, workers, media, and device — with the global name, its purpose, and a usage note.

How it works

Each entry names a browser global (a constructor like IntersectionObserver, a namespace like navigator.geolocation, or a function like fetch) and places it in a category:

Networking — fetch, WebSocket, EventSource, XMLHttpRequest.
Storage — localStorage, sessionStorage, indexedDB, caches, Cookie Store.
DOM & events — document, CustomEvent, MutationObserver.
Observers — IntersectionObserver, ResizeObserver, PerformanceObserver.
Workers — Worker, SharedWorker, ServiceWorker.
Media & graphics — Canvas, WebGL, Web Audio, getUserMedia.
Device & misc — Geolocation, Clipboard, Notifications, Web Crypto.

Notes and tips

Mind the secure-context rule: Service Workers, Geolocation, Clipboard, Web Crypto’s SubtleCrypto, and getUserMedia only work on HTTPS or localhost. Prefer the asynchronous, non-blocking APIs — fetch over XMLHttpRequest, IntersectionObserver/ResizeObserver over scroll/resize listeners, and a Web Worker for any CPU-heavy task — so the main thread stays responsive.