JavaScript Top Tricks for Advanced Interviews

🚀 JavaScript Top Tricks for Advanced Interviews: Crack Any Coding Round Like a Pro 💡

JavaScript is one of the most popular programming languages, and with its dynamic nature, it offers plenty of hidden tricks that can impress interviewers. Whether you’re preparing for FAANG interviews or top product-based companies, knowing these advanced tricks can set you apart.

Let’s dive into the top JavaScript tricks that often appear in advanced interviews, with examples explained in depth. 🧑‍💻🔥

1️⃣ Trick: == vs === — The Type Coercion Magic

Many developers know the difference, but interviews often go deeper.

console.log(0 == false);  // true
console.log(0 === false); // false
console.log(null == undefined);  // true
console.log(null === undefined); // false

Why?

• == performs type coercion → converts values to the same type before comparing.
• === checks for strict equality without type conversion.

👉 Interview Insight: Interviewers often test your knowledge of JavaScript coercion rules.

✅ Pro Tip: Always prefer === to avoid bugs, unless you intentionally want coercion.

2️⃣ Trick: Closure in Loops 🌀

Closures are the most asked JavaScript concept in interviews.

for (var i = 1; i <= 3; i++) {
  setTimeout(() => console.log(i), 1000);
}
// Output: 4, 4, 4

But if we use let 👇

for (let i = 1; i <= 3; i++) {
  setTimeout(() => console.log(i), 1000);
}
// Output: 1, 2, 3

Why?

• var is function-scoped → all callbacks share the same i.
• let is block-scoped → each iteration has its own i.

👉 Interview Question: How would you fix this without let?

for (var i = 1; i <= 3; i++) {
  (function(i){
    setTimeout(() => console.log(i), 1000);
  })(i);
}

3️⃣ Trick: Debouncing and Throttling ⚡

These are essential in performance optimization.

Debounce (wait until user stops typing):

function debounce(fn, delay) {
  let timer;
  return function(...args) {
    clearTimeout(timer);
    timer = setTimeout(() => fn.apply(this, args), delay);
  }
}

const search = debounce((query) => console.log("Searching:", query), 500);

search("J"); search("Ja"); search("Jav"); search("JavaScript");
// Only last one runs

Throttle (limit execution rate):

function throttle(fn, limit) {
  let waiting = false;
  return function(...args) {
    if (!waiting) {
      fn.apply(this, args);
      waiting = true;
      setTimeout(() => waiting = false, limit);
    }
  }
}

const logScroll = throttle(() => console.log("Scroll event"), 1000);
window.addEventListener("scroll", logScroll);

👉 Interview Insight: These are real-world performance hacks often asked in system design + frontend interviews.

4️⃣ Trick: Currying 🍛

Currying transforms functions into one argument at a time.

function curry(fn) {
  return function curried(...args) {
    if (args.length >= fn.length) {
      return fn.apply(this, args);
    } else {
      return function(...next) {
        return curried.apply(this, args.concat(next));
      }
    }
  }
}

function add(a, b, c) {
  return a + b + c;
}

const curriedAdd = curry(add);
console.log(curriedAdd(1)(2)(3)); // 6

👉 Interview Use Case: Currying is used in functional programming and libraries like Redux.

5️⃣ Trick: Event Loop & Microtasks 🔄

Understanding the Event Loop is a must-have skill.

console.log("Start");

setTimeout(() => console.log("setTimeout"), 0);

Promise.resolve().then(() => console.log("Promise"));

console.log("End");

Output:

Start
End
Promise
setTimeout

👉 Why?

• Microtasks (Promise callbacks) execute before macrotasks (setTimeout, setInterval).

✅ Pro Tip: Interviewers often test if you know this ordering.

6️⃣ Trick: Object Destructuring + Renaming ✨

A quick hack for cleaner code.

const user = { id: 101, name: "Alice", age: 25 };
const { name: username, age } = user;

console.log(username); // Alice
console.log(age);      // 25

👉 Useful in React hooks, API responses, etc.

🔥 More Advanced Interview Questions & Answers

❓ Q1: What is the difference between map, forEach, and reduce?

• map → returns a new array.
• forEach → executes function but returns undefined.
• reduce → reduces array into a single value.

let nums = [1,2,3];
console.log(nums.map(x => x*2)); // [2,4,6]
console.log(nums.forEach(x => x*2)); // undefined
console.log(nums.reduce((a,b)=>a+b,0)); // 6

❓ Q2: Explain Prototypal Inheritance in JS.

function Person(name) {
  this.name = name;
}
Person.prototype.greet = function() {
  return "Hello " + this.name;
}

const user = new Person("John");
console.log(user.greet()); // Hello John

👉 Insight: Objects inherit directly from other objects via prototype chains.

❓ Q3: What’s the difference between call, apply, and bind?

function greet(greeting, punctuation) {
  return `${greeting}, ${this.name}${punctuation}`;
}

const person = { name: "Alice" };

console.log(greet.call(person, "Hello", "!")); // Hello, Alice!
console.log(greet.apply(person, ["Hi", "?"])); // Hi, Alice?
const bound = greet.bind(person);
console.log(bound("Hey", ".")); // Hey, Alice.

🎯 Final Takeaway

In advanced interviews, it’s not about memorizing everything, but about showing deep understanding of JavaScript’s quirks. Mastering closures, event loop, currying, and optimization patterns like debounce/throttle will give you a competitive edge.

🚀 Keep practicing these, and you’ll be ready to ace your next JavaScript interview!