JavaScript output-based interview questions

🙌 Like this project?

If you found this helpful, support the project by clicking the ⭐ button at the top of the repo! Your star motivates us to add more high-quality content 🌟✨

Contribution Guidelines

If you want to contribute, improve, or suggest changes to this repo, then check out the Contributing Guide

1. What will be the output

console.log(0 || 3);
console.log(1 || 2);
console.log(0 && 1);
console.log(1 && 2);

🔍 Click to View Answer

🧾 Output:

3
1
0
2

🧠 Explanation:

• 0 || 3 → returns 3 because 0 is falsy, so it evaluates to the second operand (3).
• 1 || 2 → returns 1 because 1 is truthy, so it evaluates to the first operand.
• 0 && 1 → returns 0 because 0 is falsy, so && returns the first falsy operand.
• 1 && 2 → returns 2 because both are truthy, so && returns the last operand.

2. What will be the output

let f = "2";
let a = 4;
console.log(+f + a + 1);

🔍 Click to View Answer

🧾 Output:

7

🧠 Explanation:

• let f = "2"; — f is a string "2".
• +f — the unary plus converts the string "2" to the number 2.
• +f + a + 1 → 2 + 4 + 1 → 7.

3. What will be the output

const a = (1, 2, 3);
console.log(a);

🔍 Click to View Answer

🧾 Output:

3

🧠 Explanation:

• The comma operator evaluates each of its operands (from left to right) and returns the value of the last operand.
• (1, 2, 3) evaluates to 3.
• So, a is assigned the value 3, and console.log(a); prints 3.

4. What will be the output

function multiple(x, y) {
  return x * y;
}
let multi = multiple.bind(this, 7);
console.log(multi(6));

🔍 Click to View Answer

🧾 Output:

42

🧠 Explanation:

bind(this, 7) creates a new function where:

• this is explicitly set (in this case to the current this)
• First argument x is pre-filled with 7
• You need to pass only the second argument y
• this inside multiple is not used, so its value doesn't matter for the output.

5. What will be the output

let i = 0;
for (i = 0; i < 3; i++) {
  const log = () => {
    console.log(i);
  };
  setTimeout(log, 100);
}

🔍 Click to View Answer

🧾 Output:

3
3
3

🧠 Explanation:

• let i = 0; for (i = 0; i < 3; i++) {...} runs the loop 3 times with i = 0, 1, 2.

• Inside the loop, a new function log is created in each iteration, capturing the same i from outer scope.

• The function log is scheduled using setTimeout(..., 100), which executes after the loop has finished.

• By the time all three timeouts fire (after ~100ms), the value of i has already become 3. So each call to log() prints 3.

If you wanted it to print 0 1 2, you'd need to capture the value of i for each iteration using let inside the loop or IIFE:

for (let i = 0; i < 3; i++) {
  const log = () => {
    console.log(i);
  };
  setTimeout(log, 100);
}
// Output: 0 1 2

6. What will be the output

console.log({} == {});
console.log({} === {});

🔍 Click to View Answer

🧾 Output:

false
false

🧠 Explanation:

In JavaScript, objects are compared by reference, not by value.

• {} creates a new object each time. So, {} == {} → comparing two different object references → false.

• {} === {} → strict equality also compares reference still false.

• They're not the same object in memory, even though they have the same structure (i.e., empty).

7. What will be the output

console.log(typeof isNaN);
console.log(isNaN === isNaN);

console.log(typeof NaN);
console.log(NaN === NaN);

🔍 Click to View Answer

🧾 Output:

"function"
true
"number"
false

🧠 Explanation:

• typeof isNaN returns "function" because isNaN is a built-in function.
• isNaN === isNaN is true because both refer to the same function.
• typeof NaN returns "number" because NaN is a special numeric value.
• NaN === NaN is false because NaN is not equal to anything, even itself.

8. What will be the output

let a = [1, 2, 3];
let b = a;
b.push(4);
console.log(a);
console.log(b);

🔍 Click to View Answer

🧾 Output:

[1, 2, 3, 4]
[1, 2, 3, 4]

🧠 Explanation:

• Arrays are reference types. b points to the same array as a.
• Pushing to b also changes a.

9. What will be the output

console.log(x);
var x = 10;
if (true) {
  console.log(x);
  let y = 5;
}
console.log(y);

🔍 Click to View Answer

🧾 Output:

undefined
10
ReferenceError: y is not defined

🧠 Explanation:

• var x is hoisted, so console.log(x) before assignment is undefined.
• Inside the block, x is 10.
• let y is block-scoped, so console.log(y) outside the block throws an error.

10. What will be the output

console.log(typeof null);
console.log(null instanceof Object);

🔍 Click to View Answer

🧾 Output:

"object"
false

🧠 Explanation:

• typeof null returns "object" (quirk in JS).
• null is not an instance of Object.

11. What will be the output

const obj = {
  name: "Heer",
  greet: function () {
    console.log(this.name);
    console.log(`My name is ${this.name}`);
  },
};

const greet = obj.greet;
greet();

obj.greet();

🔍 Click to View Answer

🧾 Output:

undefined
My name is undefined
Heer
My name is Heer

🧠 Explanation:

• When greet is called standalone, this is undefined (or global object in non-strict mode).
• When called as obj.greet(), this refers to obj.

12. What will be the output

// Class based output
class Person {
  constructor(name) {
    this.name = name;
  }
  greet() {
    console.log(`Hello, my name is ${this.name}`);
  }
}

const p = new Person("Heer");
p.greet();

🔍 Click to View Answer

🧾 Output:

Hello, my name is Heer

🧠 Explanation:

• The class constructor sets the name.
• greet() prints the name.

13. What will be the output

<div id="parent">
  <button id="child">Child</button>
</div>

const parentEle = document.getElementById("parent");
const childEle = document.getElementById("child");

parentEle.onclick = function () {
  console.log("Hello from parent");
};

childEle.onclick = function () {
  console.log("Hello from child");
};

🔍 Click to View Answer

🧾 Output:

Clicking the button logs "Hello from child".
Clicking elsewhere in the parent logs "Hello from parent".

🧠 Explanation:

• Each element has its own click handler.
• Clicking the child triggers only the child's handler.

14. What will be the output

function multiple(x, y) {
  return x * y;
}

let multi = multiple.bind(this, 7);
console.log(multi(6));

🔍 Click to View Answer

🧾 Output:

42

🧠 Explanation:

• The bind() method creates a new function with a fixed this value and initial arguments
• multiple.bind(this, 7) creates a new function where:
  • First parameter x is permanently set to 7
  • Second parameter y will be provided when calling multi()
• When we call multi(6):
  • x is 7 (bound value)
  • y is 6 (passed argument)
  • Returns 7 * 6 = 42