Chapter 6 Beginner 60 Questions

Practice Questions — Loops (for, while, for...of, for...in)

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11 Easy

12 Medium

10 Hard

Topic-Specific Questions

Question 1

Easy

What is the output of the following code?

for (let i = 0; i < 4; i++) {
  console.log(i);
}

The loop starts at 0 and runs while i is less than 4.

0
1
2
3

Question 2

Easy

What is the output?

for (let i = 1; i <= 5; i++) {
  console.log(i * 2);
}

Multiply each value of i by 2.

2
4
6
8
10

Question 3

Easy

What is the output?

const fruits = ["apple", "mango", "banana"];
for (const fruit of fruits) {
  console.log(fruit);
}

for...of gives you each value in the array.

apple
mango
banana

Question 4

Easy

What is the output?

let count = 3;
while (count > 0) {
  console.log(count);
  count--;
}

count starts at 3 and decreases by 1 each time.

3
2
1

Question 5

Easy

What is the output?

for (const char of "Hi!") {
  console.log(char);
}

for...of on a string gives you each character.

H
i
!

Question 6

Easy

What is the output?

for (let i = 10; i >= 6; i--) {
  console.log(i);
}

The loop counts down from 10, stopping when i goes below 6.

10
9
8
7
6

Question 7

Medium

What is the output?

for (let i = 1; i <= 5; i++) {
  if (i === 3) continue;
  console.log(i);
}

continue skips the rest of the current iteration.

1
2
4
5

Question 8

Medium

What is the output?

for (let i = 1; i <= 10; i++) {
  if (i === 4) break;
  console.log(i);
}

break exits the loop entirely.

1
2
3

Question 9

Medium

What is the output?

let x = 1;
while (x < 20) {
  x *= 2;
}
console.log(x);

Trace x: 1, 2, 4, 8, 16, ... When does x first reach 20 or more?

32

Question 10

Medium

What is the output?

let num = 10;
do {
  console.log(num);
  num += 10;
} while (num < 5);

do...while always runs the body at least once before checking the condition.

10

Question 11

Medium

What is the output?

const obj = { a: 1, b: 2, c: 3 };
for (const key in obj) {
  console.log(key);
}

for...in gives you the property names (keys), not the values.

a
b
c

Question 12

Medium

What is the output?

let total = 0;
for (let i = 1; i <= 5; i++) {
  total += i;
}
console.log(total);

Add 1 + 2 + 3 + 4 + 5.

15

Question 13

Hard

What is the output?

for (let i = 1; i <= 3; i++) {
  for (let j = 1; j <= 2; j++) {
    console.log(i + "," + j);
  }
}

The inner loop runs completely (j=1, j=2) for each value of i.

1,1
1,2
2,1
2,2
3,1
3,2

Question 14

Hard

What is the output?

for (let i = 0; i < 3; i++) {
  for (let j = 0; j <= i; j++) {
    process.stdout.write("* ");
  }
  console.log();
}

The inner loop runs i+1 times for each row.

*
* *
* * *

Question 15

Hard

What is the output?

const arr = [10, 20, 30, 40];
let result = 0;
for (const val of arr) {
  if (val === 30) break;
  result += val;
}
console.log(result);

The loop adds values until it hits 30, then breaks.

30

Question 16

Hard

What is the output?

let count = 0;
for (let i = 0; i < 4; i++) {
  for (let j = 0; j < 4; j++) {
    if (i === j) count++;
  }
}
console.log(count);

How many times does i equal j in a 4x4 grid?

4

Question 17

Medium

What is the difference between for...of and for...in? When should you use each one?

Think about what each loop gives you: values vs keys.

for...of iterates over values of iterable objects like arrays and strings. for...in iterates over property names (keys) of objects. Use for...of for arrays and strings. Use for...in for plain objects. Never use for...in on arrays because it gives string indices and can include inherited properties.

Question 18

Easy

What is the difference between break and continue?

One stops the loop entirely, the other skips one iteration.

break exits the loop completely - no more iterations run. continue skips the rest of the current iteration and moves on to the next one. With break, the loop is done. With continue, the loop keeps going.

Question 19

Hard

What is the output?

const word = "JavaScript";
let vowels = 0;
for (const ch of word) {
  if ("aeiouAEIOU".includes(ch)) {
    vowels++;
  }
}
console.log(vowels);

Count the vowels in "JavaScript": J-a-v-a-S-c-r-i-p-t.

3

Question 20

Easy

What is the output?

for (let i = 0; i < 3; i++) {
  console.log("Hello");
}

How many times does the loop run? i goes 0, 1, 2.

Hello
Hello
Hello

Mixed & Application Questions

Question 1

Easy

What is the output?

const nums = [5, 10, 15];
for (const n of nums) {
  console.log(n + 1);
}

Add 1 to each element.

6
11
16

Question 2

Easy

What is the output?

let sum = 0;
for (let i = 2; i <= 10; i += 2) {
  sum += i;
}
console.log(sum);

i takes values 2, 4, 6, 8, 10. Add them up.

30

Question 3

Medium

What is the output?

for (let i = 1; i <= 5; i++) {
  if (i % 2 === 0) continue;
  console.log(i);
}

continue skips even numbers.

1
3
5

Question 4

Medium

What is the output?

const scores = { maths: 95, science: 88, english: 76 };
for (const subject in scores) {
  if (scores[subject] >= 90) {
    console.log(subject);
  }
}

Only print subjects where the score is 90 or above.

maths

Question 5

Medium

What is the output?

let result = "";
for (let i = 3; i >= 1; i--) {
  for (let j = 1; j <= i; j++) {
    result += "*";
  }
  result += "\n";
}
console.log(result);

The inner loop runs i times. i goes 3, 2, 1.

***
**
*

Question 6

Medium

What is the output?

let i = 5;
while (i > 0) {
  i -= 2;
}
console.log(i);

Trace: 5, 3, 1, -1. What stops the loop?

-1

Question 7

Hard

What is the output?

const matrix = [[1, 2], [3, 4], [5, 6]];
for (const row of matrix) {
  for (const val of row) {
    console.log(val);
  }
}

The outer loop gets each sub-array. The inner loop gets each value within that sub-array.

1
2
3
4
5
6

Question 8

Hard

What is the output?

let n = 1;
for (let i = 0; i < 4; i++) {
  n *= 3;
}
console.log(n);

Multiply n by 3 four times: 1 * 3 * 3 * 3 * 3.

81

Question 9

Hard

What is the output?

const text = "hello";
let reversed = "";
for (let i = text.length - 1; i >= 0; i--) {
  reversed += text[i];
}
console.log(reversed);

Start from the last character and go backwards.

olleh

Question 10

Hard

What is the output?

outer:
for (let i = 0; i < 3; i++) {
  for (let j = 0; j < 3; j++) {
    if (j === 1) continue outer;
    console.log(i, j);
  }
}

continue outer skips to the next iteration of the outer loop, not the inner one.

0 0
1 0
2 0

Question 11

Medium

Why should you use let instead of var in a for loop? What problem does var cause?

Think about variable scope and closures.

var is function-scoped, meaning there is only one variable shared across all iterations. let is block-scoped, creating a new variable for each iteration. With var, if you use the loop variable inside a callback (like setTimeout), all callbacks will see the final value of the variable. With let, each callback captures its own copy of the variable.

Question 12

Easy

What is the output?

for (let i = 0; i < 5; i += 2) {
  console.log(i);
}

i starts at 0 and increases by 2 each time.

0
2
4

Question 13

Hard

What is the output?

let a = 0;
let b = 1;
for (let i = 0; i < 7; i++) {
  let temp = a + b;
  a = b;
  b = temp;
}
console.log(a);

This generates Fibonacci numbers. Trace a few iterations.

13

Multiple Choice Questions

MCQ 1

What are the three parts of a for loop in JavaScript?

A. start, end, step
B. initialization, condition, update
C. begin, check, increment
D. declare, test, change

Answer: B
B is correct. A for loop has three parts separated by semicolons: initialization (runs once), condition (checked before each iteration), and update (runs after each iteration). The other options use informal terms that are not standard.

MCQ 2

Which loop always runs its body at least once?

A. for loop
B. while loop
C. do...while loop
D. for...of loop

Answer: C
C is correct. The do...while loop executes the body first and then checks the condition. This guarantees at least one execution. The other loops check the condition before running the body, so they might not run at all.

MCQ 3

Which loop is best for iterating over array values?

A. for...in
B. for...of
C. while
D. do...while

Answer: B
B is correct. for...of is designed to iterate over values of iterable objects like arrays and strings. for...in (option A) iterates over keys and should not be used on arrays. while and do...while work but require manual index management.

MCQ 4

What does the break statement do inside a loop?

A. Skips the current iteration
B. Restarts the loop
C. Exits the loop immediately
D. Pauses the loop

Answer: C
C is correct. break exits the loop completely, stopping all further iterations. Option A describes continue. Options B and D are not real loop behaviors in JavaScript.

MCQ 5

How many times does this loop run? for (let i = 0; i < 6; i++)

A. 5 times
B. 6 times
C. 7 times
D. Infinite times

Answer: B
B is correct. The loop starts at i=0 and runs while i < 6. The values of i are 0, 1, 2, 3, 4, 5 - that is 6 iterations. When i becomes 6, the condition fails and the loop stops.

MCQ 6

What does for...in loop over?

A. Array values
B. Object property names (keys)
C. String characters
D. Number sequences

Answer: B
B is correct. for...in iterates over the enumerable property names (keys) of an object. While it technically works on arrays (giving string indices), it is designed for objects. Use for...of for array values and string characters.

MCQ 7

What is the output of: let i = 10; while (i < 5) { console.log(i); i++; }

A. 10
B. 5 6 7 8 9
C. Nothing is printed
D. Infinite loop

Answer: C
C is correct. The while loop checks the condition before running the body. Since 10 < 5 is false from the start, the body never executes. Nothing is printed.

MCQ 8

What causes an infinite loop?

A. Using too many variables
B. The loop condition never becoming false
C. Using nested loops
D. Using break inside a loop

Answer: B
B is correct. An infinite loop occurs when the condition never becomes false, usually because the loop variable is not updated correctly. Nested loops (C) and multiple variables (A) are perfectly fine. break (D) actually stops a loop.

MCQ 9

What is the difference between continue and break?

A. continue exits the loop, break skips an iteration
B. continue skips the current iteration, break exits the loop
C. They do the same thing
D. continue works in for loops only, break works in while loops only

Answer: B
B is correct. continue skips the rest of the current iteration and goes to the next one. break exits the loop entirely. Both work in all loop types (for, while, do...while, for...of, for...in).

MCQ 10

Which loop type should you NOT use with arrays?

A. for loop
B. for...of loop
C. for...in loop
D. while loop

Answer: C
C is correct. for...in should not be used with arrays because it iterates over indices as strings and can include inherited properties. Use a regular for loop or for...of for arrays.

MCQ 11

How many times does the inner console.log execute?

for (let i = 0; i < 5; i++) {
  for (let j = 0; j < 3; j++) {
    console.log(i, j);
  }
}

A. 5
B. 3
C. 8
D. 15

Answer: D
D is correct. The outer loop runs 5 times and the inner loop runs 3 times for each outer iteration. Total: 5 x 3 = 15. Option C (8) incorrectly adds instead of multiplying.

MCQ 12

What is the output?

for (let i = 0; i < 3; i++) {
  // empty body
}
console.log(typeof i);

A. "number"
B. "undefined"
C. 3
D. ReferenceError

Answer: D
D is correct. When you use let in a for loop, the variable is scoped to the loop block. After the loop ends, i does not exist in the outer scope. Trying to access it throws a ReferenceError. This is different from var, which would make i accessible.

MCQ 13

What does the following code create on a webpage?

const items = ["A", "B", "C"];
const ul = document.createElement("ul");
for (const item of items) {
  const li = document.createElement("li");
  li.textContent = item;
  ul.appendChild(li);
}

A. A table with 3 rows
B. An unordered list with 3 items
C. 3 separate paragraphs
D. 3 div elements

Answer: B
B is correct. The code creates a <ul> element and loops through the array, creating a <li> for each item and appending it to the ul. The result is an unordered list with 3 list items: A, B, C.

MCQ 14

What is a labeled statement used for with loops?

A. Adding comments to loops
B. Naming loops so break/continue can target a specific outer loop
C. Making loops run faster
D. Creating loop variables

Answer: B
B is correct. Labels like outer: allow break outer or continue outer to affect the labeled loop instead of just the innermost loop. This is useful in nested loops when you want to break out of or continue an outer loop from inside an inner loop.

MCQ 15

What does continue do?

A. Stops the loop entirely
B. Restarts the loop from the beginning
C. Skips the rest of the current iteration and goes to the next
D. Pauses the loop for 1 second

Answer: C
C is correct. continue skips the remaining code in the current iteration and immediately starts the next iteration (after running the update expression in a for loop). It does not stop the loop or restart it.

MCQ 16

What is the correct syntax for a do...while loop?

A. do { } while (condition)
B. do (condition) { }
C. while (condition) do { }
D. do { } (condition)

Answer: A
A is correct. The syntax is do { body } while (condition);. The body comes first, then the while keyword with the condition. Note the semicolon after the closing parenthesis.

MCQ 17

What happens when you use for...of on a regular object?

A. It loops over the values
B. It loops over the keys
C. It throws a TypeError
D. It returns undefined

Use nested for loops. Create div elements and style them dynamically.

const container = document.createElement("div");
container.style.display = "grid";
container.style.gridTemplateColumns = "repeat(5, 60px)";
container.style.gap = "4px";

for (let row = 0; row < 5; row++) {
  for (let col = 0; col < 5; col++) {
    const div = document.createElement("div");
    const blue = (row * 5 + col) * 10 + 50;
    div.style.width = "60px";
    div.style.height = "60px";
    div.style.backgroundColor = `rgb(0, 0, ${blue})`;
    div.style.borderRadius = "4px";
    container.appendChild(div);
  }
}

document.body.appendChild(container);

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