Common Javascript Interview Questions and Examples

Table of Contents

This list contains a longer list of important JavaScript questions. Not all of them will be asked during Front End Engineer interviews but they provide a well-rounded review of JavaScript concepts ranging from beginner to advanced topics.

No.	Questions	Level
1	Question 1: Predict the Output	Basic
2	Question 2: Private Counter Using Closures	Basic

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Question 1: Predict the Output

for (let i = 0; i < 5; i++) {
  setTimeout(() => console.log(i), 0);
}

Answer:

The let declaration creates a new block-scoped variable for each iteration, so the output will be:

0, 1, 2, 3, 4

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Question 2: Private Counter Using Closures

function privateCounter() {
  let count = 0;
  return {
    increment: () => ++count,
    decrement: () => --count,
    getCount: () => count,
  };
}

const counter = privateCounter();
console.log(counter.increment()); // Output: 1
console.log(counter.decrement()); // Output: 0
console.log(counter.getCount()); // Output: 0

Answer:

The privateCounter function creates a closure with a private count variable, which is manipulated through the increment, decrement, and getCount methods.

Output:

1
0
0

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Question 3: Identify and Fix Closure Issue

Problematic Code:

for (var i = 1; i <= 5; i++) {
  setTimeout(() => console.log(`n = ${i}`), 0);
}

Answer:

The var keyword does not create block-scoped variables. By the time the callback executes, i is already 6.

Fixed Code Using IIFE:

for (var i = 1; i <= 5; i++) {
  (function (n) {
    setTimeout(() => console.log(`n = ${n}`), 0);
  })(i);
}

Fixed Code Using let:

for (let i = 1; i <= 5; i++) {
  setTimeout(() => console.log(`n = ${i}`), 0);
}

Expected Output:

n = 1
n = 2
n = 3
n = 4
n = 5

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Question 4: Implicit Type Coercion

What will the following code output, and why?

console.log([] + {}); // ?
console.log({} + []); // ?

Answer:

• [] + {} → “[object Object]”
  • An empty array is coerced into an empty string, and an empty object is coerced into “[object Object]”.
• {} + [] → 0
  • Here, {} is treated as an empty block, and the result is +[], which is coerced to 0.

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Question 5: NaN Comparison

Explain the output of this comparison:

console.log(NaN === NaN); // ?
console.log(Object.is(NaN, NaN)); // ?

Answer:

• NaN === NaN → false
  • NaN is the only value in JavaScript that is not equal to itself .
• Object.is(NaN, NaN) → true
  • Object.is performs a strict equality check that considers NaN equal to itself.

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Question 6: Hoisting Oddities

What will the following code output, and why?

function foo() {
  console.log(a);
  var a = 10;
  console.log(a);
}
foo();

Answer:

• Output:

  undefined
  10
  

  • Due to hoisting, var a is declared at the top of the function but not initialized, so the first console.log(a) outputs undefined.

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Question 7: Function and Block Scope

What is the output of this code?

if (true) {
  function test() {
    console.log('Inside function');
  }
}

test();

Answer:

• In non-strict mode , the function declaration is hoisted to the global scope, so it works, and the output is:

  Inside function
  

• In strict mode , function declarations inside blocks are scoped to the block, and it will throw a ReferenceError.

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Question 8: typeof null

Why does typeof null return "object"?

console.log(typeof null); // ?

Answer:

• typeof null → "object"
  • This is a historical bug in JavaScript that stems from its early implementation. Null was meant to signify a pointer to an object in C-like languages, and this behavior has persisted.

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Question 9: Array Holes

What will the following code output?

const arr = [1, , 3];
console.log(arr.length); // ?
console.log(arr[1]); // ?

Answer:

• arr.length → 3
  • The length of an array includes empty slots (holes).
• arr[1] → undefined
  • The slot exists but is uninitialized.

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