The DOM and Browser APIs

How browsers turn raw HTML into a living tree of objects you can manipulate — nodes, elements, the document object, and why the DOM is not your HTML.

querySelector, getElementById, DOM traversal — how to find any element on the page and navigate the tree like you own it.

createElement, DocumentFragment, innerHTML vs textContent, classList, dataset — everything you need to build and change the DOM programmatically.

addEventListener, bubbling vs capturing, event delegation, stopPropagation, preventDefault — the event system that powers every interactive feature on the web.

FormData, the Constraint Validation API, custom validity, input events — how to handle forms properly without reinventing what the browser already gives you.

The Web Storage API — when to use localStorage vs sessionStorage, storage events, JSON serialization, size limits, and when to reach for cookies or IndexedDB instead.

fetch(), Request/Response objects, headers, AbortController, error handling — the modern way to talk to servers from JavaScript.

setTimeout, setInterval, requestAnimationFrame, requestIdleCallback — how JavaScript schedules work in the future, and why 0ms doesn't mean instant.

IntersectionObserver for lazy loading and infinite scroll, ResizeObserver for responsive components, MutationObserver for DOM changes — the observer APIs that replaced scroll/resize event listeners.

pushState, replaceState, popstate events, URL and URLSearchParams — how SPA routers work under the hood and why the back button is harder than you think.

10 questions testing what DOM methods actually return, how events behave, storage quirks, and fetch gotchas. Know the APIs, not just the tutorials.