JavaScript Tricky Questions You Must Master for Your Next Interview

🧠 JavaScript Tricky Questions You Must Master for Your Next Interview! 🚀

JavaScript may look friendly, but under the hood, it’s full of quirks that even experienced developers get stumped by! 😅 Whether you’re prepping for your next big tech interview or want to level up your JS brain, here are some tricky and mind-bending questions with real-world examples. 💡

Let’s dive deep! 🏊‍♂️

🔥 1. What is the result of [] + [] and why?

Answer: "" (an empty string)

console.log([] + []); // ""

Why?

When using + with arrays, JavaScript tries to convert both operands to strings and then concatenate them.

[].toString(); // ""

So essentially it becomes: "" + "" => ""

🤯 Mind blown? It gets crazier!

🌀 2. What is the output of [1,2,3] + [4,5,6]?

Answer: "1,2,34,5,6"

console.log([1,2,3] + [4,5,6]); // "1,2,34,5,6"

JavaScript converts both arrays to strings:

[1,2,3].toString(); // "1,2,3"
[4,5,6].toString(); // "4,5,6"

Then: "1,2,3" + "4,5,6" => "1,2,34,5,6"

📌 Pro Tip: Always double-check data types before combining!

🤖 3. What does typeof NaN return?

Answer: "number"

console.log(typeof NaN); // "number"

Even though NaN stands for Not a Number, it’s still considered a number type in JS.

🧠 Takeaway: Don’t rely solely on typeof for validation—use Number.isNaN().

🕳️ 4. What will be the result of this code?

let a = [1, 2, 3];
a[10] = 99;
console.log(a.length); // ?

Answer: 11

Sparse arrays! JavaScript sets a[10], leaving holes (undefineds) in between.

// Array looks like:
[1, 2, 3, <7 empty items>, 99]

🧨 Trap: Don’t assume sequential indexes when working with arrays.

🧊 5. Is {} + [] the same as [] + {}?

console.log({} + []); // 0
console.log([] + {}); // "[object Object]"

Why?

In the first case, {} is treated as a block scope and +[] becomes 0.

+[] // 0

In the second case:

[] + {} 
// "" + "[object Object]" => "[object Object]"

📘 Lesson: Operator precedence + expression context = head-scratcher 😵

🧪 6. What is the output of console.log(0.1 + 0.2 === 0.3)?

Answer: false

console.log(0.1 + 0.2 === 0.3); // false

Due to floating-point precision errors in JS:

0.1 + 0.2 = 0.30000000000000004

✅ Fix: Use a precision tolerance:

Math.abs((0.1 + 0.2) - 0.3) < Number.EPSILON

🪝 7. What does this return?

let foo = function() {};
console.log(foo.prototype); 

Answer: {}

Every function (not arrow function) has a .prototype object, used for inheritance when using new.

But arrow functions don’t have .prototype:

const bar = () => {};
console.log(bar.prototype); // undefined

💡 Note: This is crucial when working with constructors and classes!

🐍 8. What will let x = (1, 2, 3); return?

Answer: 3

let x = (1, 2, 3);
console.log(x); // 3

Comma operator evaluates each expression but returns only the last one.

📌 Rare but useful in minified or inline expressions.

🧟 9. Why does this return 'object'?

console.log(typeof null); // "object"

Answer: This is a legacy bug from the early days of JS.

Internally, null was represented with a null pointer, which was 0—but the type tag system also used 0 for “object.”

So…

🧠 Takeaway: Use value === null to check for null explicitly.

🔒 10. What does this return?

let a = {};
let b = { key: "b" };
let c = { key: "c" };

a[b] = 123;
a[c] = 456;

console.log(a[b]); // ??

Answer: 456

🤯 Why? JS converts object keys to strings. So:

b.toString() => "[object Object]"
c.toString() => "[object Object]"

So both a[b] and a[c] are actually a["[object Object]"].

📌 Fix: Use Map if you want objects as keys:

let map = new Map();
map.set(b, 123);
map.set(c, 456);
console.log(map.get(b)); // 123

👀 Wrapping Up…

Mastering these tricky questions gives you a huge edge in interviews! 🏆

✅ They test:

• Deep knowledge of JavaScript internals
• Awareness of quirks and bugs
• Practical debugging mindset