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90-Day Mastery · InterviewLane

90-day plan

90-Day Mastery

Three months covering every surface: JS foundations → senior engineering → DSA patterns → behavioral. Built for engineers targeting senior or staff roles at top-tier companies.

Day 1

0/3 done

Design a responsive grid layout with CSS Grid/Flexboxcss · hard
Explain implementation of Context APIreact · medium
What’s your approach to ensure consistent rendering and layout across different browsers and devicescss · easy

Day 2

0/3 done

Implement an event emitterfrontend · easy
Design infinite scroll for millions of items.frontend · hard
Why keys matter (and how bad keys break apps)frontend · medium

Day 3

0/3 done

Implement a custom useDebounce hook from scratchreact · easy
How async/await works in JavaScriptjavascript · medium
What is currying in JavaScriptjavascript · easy

Day 4

0/3 done

Largest Contentful Paint (LCP) — what slows it down and how to fix itfrontend · medium
How to implement throttlingfrontend · medium
Word Break II — string segmentation using a dictionaryfrontend · easy

Day 5

0/3 done

Frontend architecture: Large scale React app structurereact · hard
Frontend system design: JavaScript internals (closures, event loop, promises, async/await)browser-internals · hard
How would you architect an application to support multiple devicesfrontend · medium

Day 6

0/3 done

Implement a pub-sub / event emitter from scratchjavascript · medium
Build an accessible Accordion / Expandable Listmachine-coding · easy
Build a fully functional React application from scratch (state management, forms, component architecture)react · hard

Day 7

0/3 done

How do you share common functions/libraries in a microfrontend appfrontend · medium
Write code for mul(2)(3)(4) = 24.frontend · easy
Explain async/await vs Promises. How does the event loop handle thembrowser-internals · medium

Day 8

0/3 done

Implement a Fetcher class with get(id) and post(id, x) methods that throw on missing/duplicate idsfrontend · easy
How do you implement Role-Based Access Control on the frontend?security · medium
Functional vs class components — what's the practical difference?react · easy

Day 9

0/3 done

Write a custom debounce function in JavaScriptjavascript · easy
Explain the use cases of Trees and Binary Search Trees in frontend interviewsfrontend · medium
Build a simple task management interface with:frontend · easy

Day 10

0/3 done

Write a polyfill for Function.prototype.bindjavascript · easy
Why do functional updates (setCount(prev => prev + 1)) matter inside useCallback with empty depsfrontend · medium
Why do components re-renderreact · medium

Day 11

0/3 done

Implement a basic debounce function without using any library.frontend · easy
How do you prevent XSS and CSRF attacksperformance · medium
How would you debounce user input to avoid unnecessary re-renders or API callsfrontend · medium

Day 12

0/3 done

What challenges have you faced when building for large user trafficperformance · medium
Explain memory leaks in SPAs and how to debug thembrowser-internals · hard
What is Node.js and the event loopbrowser-internals · easy

Day 13

0/3 done

How do you handle errors if multiple requests are sent simultaneouslyjavascript · medium
Second question: How would you check if a graph is connected or notfrontend · medium
Users report intermittent UI glitches in different browsers—how would you troubleshootfrontend · medium

Day 14

0/3 done

How do you handle a disagreement with a designer or backend developer on implementation detailsbehavioral · medium
Walk through a bug you shipped to production, specifically what changed afterward.frontend · easy
How is Promise.allSettled different and when would you use itjavascript · medium

Day 15

0/3 done

What is a hydration error in Next.js and why does it happenreact · medium
Implement currying for infinite sum: sum(10)(20)(30)() === 60javascript · medium
Build a reusable component with search functionalityfrontend · easy

Day 16

0/3 done

Build a Modalfrontend · easy
Populating Next Right Pointers in Each Node (binary tree)frontend · easy
How would you protect routes that require authentication and redirect users to the login page if they’re not authenticatedfrontend · medium

Day 17

0/3 done

Image optimization techniques on the frontendperformance · medium
Would you support per-component theming or global themingfrontend · easy
Build a React list with collapse/expand featurereact · easy

Day 18

0/3 done

== vs === — what does coercion actually do?javascript · easy
Scenario Based: You have an e-commerce product listing where multiple users can add items to cart simultaneously. How would you use React Query / SWR with optimistic updates to prevent stale UIreact · medium
What are devDependencies, and how are they different from dependencies?frontend · easy

Day 19

0/3 done

You notice a memory leak in a production SPA—how do you identify and fix itbrowser-internals · medium
Behavioral: motivation and reasons for joining the companybehavioral · easy
Explain React 18’s concurrent features.react · medium

Day 20

0/3 done

How do you handle asynchronous code using async/await and Promisesjavascript · medium
How do you optimize Core Web Vitals?performance · medium
Explain the difference between shallow and deep comparison in React's `shouldComponentUpdate`.react · medium

Day 21

0/3 done

CSS position values — static, relative, absolute, fixed, stickycss · easy
Build concurrent progress bars (multiple tasks running in parallel)react · easy
Build a debounced search boxfrontend · easy

Day 22

0/3 done

useRef and forwardRef — what are they for?react · medium
Performance topic: Debounced inputsperformance · easy
How do you handle asynchronous code execution and state updates in Reactreact · medium

Day 23

0/3 done

How do you run A/B tests in React appsreact · medium
Difference between shallow copy vs structural sharingjavascript · medium
Frontend architecture: Code splittingsystem-design · hard

Day 24

0/3 done

Build a Stopwatch / Timerfrontend · easy
What is the purpose of the alt attribute in imagesfrontend · easy
Code splitting strategies — route, component, vendorperformance · medium

Day 25

0/3 done

Stacks and queues — common interview applicationsfrontend · easy
CSS box model and box-sizing — content-box vs border-boxcss · easy
Web storage APIs: localStorage, sessionStorage, cookiesfrontend · easy

Day 26

0/3 done

Output-based: what does `obj.print()` log when print uses `this.value`frontend · medium
Difference between React 16 and React 18react · easy
Find the first non-repeating character in a given stringfrontend · easy

Day 27

0/3 done

Build an accessible Tabs componentmachine-coding · easy
How would you pass data between sibling components in React without using Reduxreact · medium
React 18 concurrent featuresreact · medium

Day 28

0/3 done

Where do you see yourself growing in the next few yearsfrontend · easy
What is srcset in HTMLhtml · easy
Build a virtualized list for very large datasetsfrontend · easy

Day 29

0/3 done

Build a custom hook that handles loading, error, and success statesreact · medium
How do you reduce bundle size in production?performance · medium
Behavioral: how do you handle tight deadlinesbehavioral · medium

Day 30

0/3 done

Why do we need the useRef hook in Reactreact · medium
How does useEffect lifecycle workreact · medium
Have you used React.memo , useCallback , useMemo or any other optimization technique and how you used these in your projectsreact · medium

Day 31

0/3 done

What are pseudo-classes and pseudo-elementsfrontend · easy
Build a frontend app from scratch — tested code quality, scalability, & component design.frontend · hard
How do you structure a scalable React applicationreact · medium

Day 32

0/3 done

Protected routes and auth in Reactreact · medium
Which would you prefer for displaying products in a layout similar to an e-commerce website (e.g., Amazon), with 3 columns and multiple rows—Flexbox or CSS Grid? Whycss · medium
How do concurrent rendering and transitions work?react · hard

Day 33

0/3 done

Async JS: API fetch with a timeoutnetworking · easy
Create a todo list application with an input field and an add button. On clicking the button, the entered item should be added and displayed in a list. Each item should include a delete button to remove it from the list when clicked.frontend · medium
Flatten a nested array of arbitrary depth (without Array.prototype.flat)javascript · easy

Day 34

0/3 done

Implement a polyfill for useImperativeHandlefrontend · easy
How does React handle circular dependencies in useEffectreact · medium
Why are HttpOnly cookies preferred for auth?security · medium

Day 35

0/3 done

Explain the difference between REST APIs and GraphQL.frontend · medium
How do you write unit tests in React using Jest and React Testing Libraryreact · medium
Explain the difference between npm and yarn.frontend · medium

Day 36

0/3 done

Implement memoization for an async function with callbacks (cache, deep equality, parallel calls)javascript · easy
Object reference vs primitive comparison in JavaScriptjavascript · easy
Build a Dark mode + themingfrontend · easy

Day 37

0/3 done

Implicit vs explicit type coercionfrontend · easy
Tell me about a time you delivered a frontend feature under tight deadlines. How did you prioritize and execute?behavioral · medium
How can you do caching on a websiteperformance · medium

Day 38

0/3 done

Designing a frontend feature flag systemsystem-design · hard
If checkout goes down during a high-traffic sale, what does your first 10 minutes look likefrontend · easy
How React handles re-renders and memoryreact · medium

Day 39

0/3 done

What are higher-order functionsfrontend · easy
How would you design a system to handle client-side caching, API retries, and error boundaries gracefullyperformance · hard
How do you manage state in a complex app to avoid unnecessary re-rendersfrontend · medium

Day 40

0/3 done

Build a debounced search / autocompletefrontend · easy
DSA: graph-based optimization problemfrontend · easy
How would you manage overlapping toasts or priority-based queuingfrontend · medium

Day 41

0/3 done

Context vs Redux vs Zustand — when to use what?react · medium
How would you optimize React app performance when handling large lists or gridsreact · medium
Backward compatibility for a published packagefrontend · easy

Day 42

0/3 done

Write a custom implementation of Function.prototype.bind.javascript · easy
What’s the difference between Repaint and Reflow (often confused with Rework)frontend · easy
Design a real-time collaborative text editor (like Google Docs).frontend · hard

Day 43

0/3 done

What are Promises in JavaScriptjavascript · easy
How would you enforce WCAG compliance across teamsfrontend · medium
Build a Drag & Dropfrontend · easy

Day 44

0/3 done

How would you design a reusable table component with features like sorting, filtering, pagination, and column resizingfrontend · hard
How browsers render HTML, CSS, and JScss · medium
Design the frontend for a product listing page (component structure, state, data flow, edge cases, scalability)frontend · hard

Day 45

0/3 done

Explain the use case of `useEffect()` for fetching data from an API.frontend · medium
How would you optimize React rendering performance in large lists (e.g., 10k+ rows)react · medium
Explain event loop with execution orderbrowser-internals · medium

Day 46

0/3 done

Type definitions for a JS library — strategiesjavascript · easy
How would you design a reusable component library across teams?architecture · hard
How would you prevent unnecessary re-renders in a dashboard with live updates?react · hard

Day 47

0/3 done

How do you secure your application from attackssecurity · medium
Is the app ready to be deployed on the production. If not, then tell the flaws.frontend · easy
What is the difference between React Node, Element, and Componentreact · easy

Day 48

0/3 done

Implement a polyfill for useCallbackfrontend · easy
How do you prevent a malicious parent page from spoofing a payment success eventfrontend · medium
Explain JSX and how React renders elementsreact · medium

Day 49

0/3 done

How would you separate concerns between data rendering and UI controlsfrontend · medium
What tools would you use for testing and whyfrontend · medium
What reusable components or patterns would you buildfrontend · easy

Day 50

0/3 done

Tell me about a React performance optimization you implemented that failedreact · easy
Frontend system design: Performance, SSR, and SEOperformance · hard
How to measure and improve Lighthouse scoresfrontend · medium

Day 51

0/3 done

Frontend accessibility basics — what every interview expects you to knowaccessibility · medium
Performance topic: CDN + asset optimizationperformance · easy
Cookies vs localStorage vs sessionStorage — when to use each?browser-internals · easy

Day 52

0/3 done

Implement a polyfill for Object.assignfrontend · easy
Browser storage: localStorage vs sessionStorage vs cookies vs IndexedDBfrontend · easy
What is the difference between state and propsfrontend · easy

Day 53

0/3 done

How would you improve Interaction to Next Paint (INP)frontend · medium
What triggers garbage collection in JavaScript?browser-internals · medium
Implement debounce and explain the risks of wrong timing on a payment form.frontend · medium

Day 54

0/3 done

Big list rendering: windowing and virtualizationperformance · easy
Build a Todo App machine-coding roundfrontend · easy
What algorithm does Array.prototype.sort() use? What’s the output of [1, null, 5, 2, undefined]javascript · easy

Day 55

0/3 done

How would you implement A/B testing without affecting current usersfrontend · medium
Controlled vs uncontrolled componentsreact · easy
Implement a DOM-like data structurefrontend · easy

Day 56

0/3 done

Can React Hooks fully replace Redux for state management? Explain why or why not.react · medium
Build a Shopping Cart UIfrontend · easy
When NOT to use map / filter / reducefrontend · medium

Day 57

0/3 done

How would you manage performance if the undo stack gets largeperformance · medium
Build a Code structure & state managementfrontend · easy
How does tree shaking remove unused code in bundlersfrontend · medium

Day 58

0/3 done

How do you ensure consistent rendering and layout across browsers and devices?css · medium
Build an embeddable checkout widget dropped into any merchant site via a script tag.frontend · easy
Implement Promise from scratchjavascript · hard

Day 59

0/3 done

Critical rendering path — what blocks itfrontend · easy
Error boundaries and crash recoveryreact · medium
How do you call and handle responses from AI APIs like OpenAI, Hugging Face, or Google Cloud AIsecurity · medium

Day 60

0/3 done

Implement a polyfill for useStatefrontend · easy
Event bubbling, capturing, and delegation — what's the difference?browser-internals · easy
How do you handle version control and collaboration using Gitfrontend · medium

Day 61

0/3 done

Promises and handling multiple async operationsjavascript · medium
Why Tailwind CSS over Bootstrap or traditional CSScss · medium
Event propagation in JavaScript: bubbling vs capturingjavascript · easy

Day 62

0/3 done

Build a Persist cart state across page refreshes using localStorage.frontend · easy
What are the UX challenges unique to AI interfaces (latency, hallucinations, streaming)frontend · easy
Implement Tabs component in React.react · easy

Day 63

0/3 done

Frontend testing strategytesting · medium
Build a tab-based form with two sections:frontend · easy
Behavioral: deep dive into past projects, SDLC, debugging production issues, observability, knowing when not to optimizebehavioral · medium

Day 64

0/3 done

Why is code splitting better than loading everything at oncefrontend · medium
How would you queue toasts so they don't all stack at oncefrontend · medium
Design the Send Message feature like Slack (UI + client logic)frontend · hard

Day 65

0/3 done

Write code to find frequency of elements in an arrayjavascript · easy
AI usage in your workflowbehavioral · easy
How would you design a frontend architecture that can handle 1M+ users dailysystem-design · hard

Day 66

0/3 done

Implement a Promise.allSettled polyfilljavascript · medium
Explain CSS position values: static, relative, absolute, fixed, stickycss · medium
Optimizing third-party script loadingperformance · medium

Day 67

0/3 done

Scenario Based: You're designing a dashboard where chart updates, user notifications, and data fetches must happen independently. How would you use React Context / Custom Hooks to achieve thisreact · medium
How would you manage the filtered results state and the interaction with an APIfrontend · medium
What is hydration in server-side rendering.react · easy

Day 68

0/3 done

What are the benefits of server-side rendering (SSR) in React applicationsreact · easy
Deep copy vs shallow copy — behavior and how to achieve eachjavascript · medium
How does React's rendering process work under the hoodreact · medium

Day 69

0/3 done

Explain your strategy for managing global state efficiently in a React app with multiple teams contributing to it.react · medium
How would you handle throttling, debouncing, or optimization to prevent excessive requestsfrontend · medium
Explain how state updates and re-renders work in Reactreact · medium

Day 70

0/3 done

Difference between AI and Generative AIfrontend · easy
Implement a polyfill for useMemofrontend · easy
Behavioral: leadership, ownership, teamwork, and past experiencesbehavioral · easy

Day 71

0/3 done

Preload, prefetch, and lazy loadingperformance · easy
Difference between Prototypal and Classical Inheritance in JavaScriptjavascript · easy
Search bar with debounce + async suggestions — how do you architect itjavascript · medium

Day 72

0/3 done

How to convert it into a reusable componentfrontend · medium
How to assign different widths to flex itemscss · medium
Explain Client-Side Rendering and when it's beneficialfrontend · medium

Day 73

0/3 done

Typeof, instanceof, and reliable type checkingfrontend · easy
Build a product listing page with search, multi-select filters, and a cart in React.react · easy
Explain event loop and how asynchronous tasks are executed in JavaScript.browser-internals · medium

Day 74

0/3 done

Can you create a React app without create-react-appreact · easy
How would you expose the API for parent componentsfrontend · medium
Feature flags at scalefrontend · easy

Day 75

0/3 done

Build an autocomplete like Flipkart's Search Bar (HLD)frontend · easy
Explain the Critical Rendering Path and the CSSOMfrontend · medium
A CSS animation is janky on mobile devices—how do you identify and fix the issuecss · medium

Day 76

0/3 done

Spread/rest operator and destructuring — how do they work and when do they bite?javascript · easy
How would you re-render a component when the window is resizedfrontend · medium
Implement a polyfill for useLayoutEffectfrontend · easy

Day 77

0/3 done

Frontend architecture: Scenario-based discussion on past projects, ownership, and decision-making.system-design · hard
Tell me about a time when a critical production bug broke the UI — how did you handle itbehavioral · medium
How does React’s reconciliation algorithm (Fiber) work internally, and how does it improve rendering performancereact · hard

Day 78

0/3 done

How do you measure performance in real-world projectsperformance · medium
What is closure? Where have you used itjavascript · easy
Implement a polyfill for setIntervalfrontend · easy

Day 79

0/3 done

Discuss how you would optimize the rendering of a React component that handles large datasetsreact · medium
Performance topic: Code splitting and dynamic importsperformance · easy
Error handling in async JavaScriptjavascript · easy

Day 80

0/3 done

Explain SSR vs SSG vs CSR, and how does each affect AI-generated content renderingfrontend · medium
How would you implement infinite scrolling in React?react · medium
How do you integrate an AI API (like OpenAI or Claude) into a frontend appfrontend · medium

Day 81

0/3 done

Monitoring + error tracking strategyfrontend · easy
Write a program to create a promise and an async/await function.javascript · easy
Explain how you’d reduce the initial page load time of a React application by 40%.react · medium

Day 82

0/3 done

Why does deleting object properties affect V8 optimization?browser-internals · hard
How do you prioritize technical debt vs. new featuresfrontend · medium
Promise.all vs allSettled vs race vs any — when do you use each?javascript · medium

Day 83

0/3 done

Explain Progressive Web Apps (PWAs) and their benefits.frontend · medium
Implement limitConcurrency(tasks, limit) using a worker-pool patternfrontend · easy
Describe how to optimize the filtering process for large datasets.frontend · medium

Day 84

0/3 done

What are the limitations of React in building large-scale applicationsreact · easy
How does the browser render a webpage (Critical Rendering Path)frontend · medium
How do you reduce Time to Interactive (TTI)?performance · medium

Day 85

0/3 done

Build a priority-based data fetching systemfrontend · easy
How you implemented code splitting in your project.frontend · medium
What is state colocation and why does it matterfrontend · medium

Day 86

0/3 done

How would you ensure accessibility (e.g., screen readers)accessibility · medium
Performance topic: Virtualizationperformance · easy
Design a Payment Gateway with discussion around transaction flow, consistency and ledger systems, reliability and fault tolerance.frontend · hard

Day 87

0/3 done

Behavioral: communication between different teamsbehavioral · easy
Describe a time when your production build caused a major UI issue — what was your response and fixfrontend · medium
What is lazy loading in React, and how does it improve application performancereact · medium

Day 88

0/3 done

Behavioral: how do you resolve conflict in a team using STARbehavioral · medium
How does React manage re-renders (useMemo, useCallback, React.memo)react · medium
How www Worksfrontend · medium

Day 89

0/3 done

Forms and validation in Reactreact · medium
Implement a debounce function from scratchjavascript · medium
Build a To-Do List (Vanilla JS/React) → optimize re-rendersreact · medium

Day 90

0/3 done

How would you manage styles in large-scale apps (CSS-in-JS, CSS variables, or something else)css · medium
Write a promise method that is resolving something.javascript · easy
Why does StrictMode behave differently in development?react · medium