Frontend comparisons, settled
The “what's the difference between…” questions that come up in every frontend interview — answered properly, with runnable code and what the interviewer is actually testing.
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Complete Frontend Kit
Every topic on this page, plus 200+ interview questions with detailed answers, coding challenges and a revision sheet.
== vs === : Loose vs Strict Equality
=== compares without conversion; == coerces first, following rules worth knowing. The classic surprises explained, plus the one place == is correct.
JSRead →call vs apply vs bind in JavaScript
call and apply invoke now, bind returns a new function. Plus what interviewers test: why bind can't be undone and why arrow functions ignore all three.
JSRead →Class vs Functional Components in React
Hooks replaced classes. The lifecycle-to-useEffect mapping, the `this` problem, and the one thing class components are still required for.
ReactRead →Context API vs Redux
Context delivers a value down the tree; Redux manages state. The re-render problem that makes context a poor store — and when you truly need Redux.
ReactRead →Controlled vs Uncontrolled Components in React
Controlled vs uncontrolled components in React: who owns the input value, when to use each, the two warnings everyone hits, and why file inputs differ.
ReactRead →CSR vs SSR vs SSG: Rendering Strategies
Where your HTML is built decides first paint, SEO and cost. CSR, SSR, SSG and ISR compared on the metrics that matter, with the Next.js mapping.
CoreRead →Debounce vs Throttle in JavaScript
Debounce waits for the burst to stop; throttle runs at a fixed maximum rate. Both implemented from scratch, plus the React hook trap that breaks them.
JSRead →em vs rem in CSS
em is relative to the element's own font size and compounds when nested; rem is always root-relative. Which to use for text, padding and breakpoints.
CoreRead →Flexbox vs CSS Grid
Flexbox lays out one axis and sizes from content; Grid defines two axes up front. The property mapping, auto-fit vs auto-fill, and the overflow trap.
CoreRead →inline vs block vs inline-block in CSS
block fills the width, inline ignores width and vertical margin, inline-block does both. Plus the vertical-padding overflow and the mystery 4px gap fix.
CoreRead →localStorage vs sessionStorage vs Cookies
localStorage persists, sessionStorage dies with the tab, cookies go to the server on every request. Sizes, expiry, and where an auth token really belongs.
JSRead →map vs forEach in JavaScript
map returns a new array, forEach returns undefined. Plus what interviewers really ask: why you can't break out of either, and why async forEach won't wait.
JSRead →null vs undefined in JavaScript
undefined means a value was never assigned; null means you assigned emptiness. Why typeof null is "object", and when == null is the right check.
JSRead →Promise.all vs Promise.allSettled
Promise.all rejects on the first failure; Promise.allSettled always resolves with every outcome. The result shapes, when to use each, and race vs any.
JSRead →Props vs State in React
Props come from the parent and are read-only; state is owned by the component and changing it re-renders. Plus lifting state up and the derived-state trap.
ReactRead →React Query vs Redux
They solve different problems: React Query caches server data, Redux stores client state. Why most apps need far less Redux than they think.
ReactRead →REST vs GraphQL for Frontend Developers
REST gives you resources and free HTTP caching; GraphQL gives you exactly the fields you ask for. Over-fetching, N+1, caching, and when each wins.
CoreRead →Shallow Copy vs Deep Copy in JavaScript
A shallow copy duplicates the top level and shares everything nested; a deep copy duplicates all of it. structuredClone, the JSON trap, and the React bug.
JSRead →slice vs splice in JavaScript
slice copies and returns a new array; splice mutates the original and returns what it removed. Negative indexes, argument traps, and why React needs slice.
JSRead →Spread vs Rest Operator in JavaScript
Spread and rest both use ... but do opposite things. The position rule that tells them apart, the shallow-copy trap, and rest params vs arguments.
JSRead →TypeScript vs JavaScript
TypeScript is JavaScript plus a compile-time type checker that disappears at runtime. What it catches, what it costs, and whether to learn it first.
CoreRead →useEffect vs useLayoutEffect
useEffect runs after the browser paints; useLayoutEffect runs before it, blocking the screen. The flicker it fixes, the SSR warning, and when to switch.
ReactRead →useMemo vs React.memo
useMemo caches a value inside a component; React.memo skips a component's re-render. Why memo usually does nothing, and how the two work together.
ReactRead →useMemo vs useCallback: What's the Difference?
useMemo caches a value, useCallback caches a function — and useCallback is literally useMemo under the hood. When each actually helps, with runnable code.
ReactRead →useRef vs useState in React
useState re-renders when it changes, useRef doesn't. Which to use for timers, DOM nodes, previous values and the stale closure bug that catches everyone.
ReactRead →useState vs useReducer in React
useState for independent values, useReducer when several fields change together. The signals to switch, impossible states, and why dispatch is stable.
ReactRead →var vs let vs const in JavaScript
Scope, hoisting, the temporal dead zone and reassignment, with the loop-closure classic and why const objects are still mutable. Interview-ready.
JSRead →
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Complete Frontend Kit
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