JavaScript Interview Prep
Machine Coding Patterns — JavaScript
Master frontend machine coding interview rounds with essential patterns: Toast Notifications, Accordion, Virtualized List, Auto-complete Search, and Star Rating.
Machine Coding Patterns — JavaScript
Machine Coding is a hands-on technical interview format used by top tech companies (Uber, Atlassian, Flipkart, Swiggy, Razorpay) where candidates are asked to build a fully functional, self-contained UI feature or component in vanilla JavaScript/DOM within 60–90 minutes.1. Core Principles of Machine Coding Success
1. Clean Separation of Concerns: Keep business/state logic decoupled from DOM rendering.
2. Event Delegation: Attach single listeners to parent containers rather than binding handlers to every dynamic child element.
3. Accessibility (a11y): Use ARIA attributes (aria-expanded, role="alert", keyboard accessibility).
4. Cleanup & Memory Safety: Always clean up event listeners and timers (clearTimeout, clearInterval) to prevent memory leaks.
2. Pattern 1: Toast Notification System
Requirement
Build a global notification toast queue that stacks alerts, supports auto-dismissal timers, and provides custom types (success, error, info).3. Pattern 2: Auto-Complete Search Bar (Debounced & Cached)
Requirement
Implement an auto-complete input with API debouncing, response caching, keyboard navigation (Arrow Up/Down, Enter), and highlight matching text.4. Pattern 3: Star Rating Component (Interactive & Accessible)
5. Interactive Code Playground
Test the dynamic implementation of the Star Rating Component below:
6. Machine Coding Checklist for Interviews
querySelectorAll('.item').forEach(el => el.addEventListener(...)).scroll, resize, or keyup events.aria-expanded, aria-label, and tabindex="0".7. Summary Matrix
| Pattern | Primary Key Challenge | Recommended Strategy |
|---|---|---|
| Toast Notifications | Stacking animations & timer cleanup | Closure timers + CSS transition events |
| Auto-Complete | Race conditions & server overload | Debouncing + Map cache + AbortController |
| Star Rating | State sync (hover vs selected rating) | State-driven re-render via single event listener |
| Accordion | Collapsible animations & a11y | CSS grid grid-template-rows: 0fr \rightarrow 1fr |
| Infinite Scroll | High DOM node memory footprint | IntersectionObserver with pagination guard |