JS Interview Prep & Master Reference Guide
The ultimate graduation handbook: ace technical frontend interviews at top tech companies, master the top 50 architectural interview questions, polyfill complex APIs, and reference the complete JavaScript language standard.
Senior Frontend Technical Competency Hierarchy
Senior engineers are evaluated not merely on syntax familiarity, but on their deep mental models of memory allocation, browser event dispatching, and asynchronous concurrency.
1. Classic FAANG Polyfill Challenge: Promise.all()
Building your own polyfills demonstrates a profound understanding of asynchronous concurrency, index mapping, and Promise resolution invariants.
Production Polyfill: Promise.all()
function promiseAll(promises) {
return new Promise((resolve, reject) => {
// Handle empty iterable
if (!Array.isArray(promises)) {
return reject(new TypeError('Input must be an array'));
}
const results = [];
let completedCount = 0;
const total = promises.length;
if (total === 0) return resolve([]);
promises.forEach((promise, index) => {
// Coerce primitives into Promises via Promise.resolve()
Promise.resolve(promise).then(value => {
results[index] = value; // Preserve exact input order!
completedCount++;
if (completedCount === total) {
resolve(results);
}
}).catch(reject); // Short-circuit on first rejection!
});
});
}Interactive Lab: Senior Technical Interview Flashcards
Top 50 Q&AClick any interview card to reveal the comprehensive technical answer that interviewers at Google, Amazon, and Meta look for.
let or const variable declaration is evaluated. Accessing the variable during this window throws a ReferenceError. Unlike var, let and const are not initialized to undefined during hoisting.
queueMicrotask, MutationObserver) have priority over standard Macrotasks (setTimeout, setInterval, I/O). The Event Loop completely drains the entire Microtask queue after each task and before rendering frames.
[[Prototype]] chain without copying.
localStorage has no access restrictions from JavaScript running on the page. Any Cross-Site Scripting (XSS) vulnerability allows malicious scripts to exfiltrate tokens. Authentication tokens should be stored in HttpOnly, Secure, SameSite=Strict cookies.
Build a Deep Object Flattener (LeetCode Hard)
Implement a flattenObject(obj) utility that takes a deeply nested object and returns a flat key-value dictionary with dot-delimited path keys (e.g. { "user.address.city": "San Francisco" }).
function flattenObject(obj, prefix = '', result = {}) {
for (const [key, value] of Object.entries(obj)) {
const fullPath = prefix ? `${prefix}.${key}` : key;
if (typeof value === 'object' && value !== null && !Array.isArray(value)) {
flattenObject(value, fullPath, result);
} else {
result[fullPath] = value;
}
}
return result;
}
// Example:
const nested = { user: { profile: { name: 'Alex' }, role: 'admin' } };
console.log(flattenObject(nested));
// { "user.profile.name": "Alex", "user.role": "admin" }Chapter 40 Knowledge Check
Validate your mastery of interview architecture, execution contexts, and standard paradigms.