Nested loops in Javascript - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of nested loops with break

What is the output of the following code?

let result = '';
for (let i = 1; i <= 3; i++) {
  for (let j = 1; j <= 3; j++) {
    if (j === 2) break;
    result += i + '-' + j + ' ';
  }
}
console.log(result.trim());

Javascript

let result = '';
for (let i = 1; i <= 3; i++) {
  for (let j = 1; j <= 3; j++) {
    if (j === 2) break;
    result += i + '-' + j + ' ';
  }
}
console.log(result.trim());

A "1-1 1-2 1-3 2-1 2-2 2-3 3-1 3-2 3-3"

B "1-1 1-2 2-1 2-2 3-1 3-2"

C "1-1 2-1 3-1"

D "1-1 2-1 3-1 3-2"

💡 Hint

Remember that the inner loop breaks when j equals 2, so it only adds the first pair for each i.

The inner loop stops when j is 2, so only j=1 is added for each i from 1 to 3, resulting in "1-1 2-1 3-1".

❓ Predict Output

intermediate

Counting iterations in nested loops

How many times will the inner loop run in this code?

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

Javascript

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

A 10

B 6

C 4

D 12

💡 Hint

Count how many times j runs for each i: when i=0, j runs 0 times; i=1, j runs 1 time; etc.

The inner loop runs 0 + 1 + 2 + 3 = 6 times in total.

❓ Predict Output

advanced

Output of nested loops with continue and labels

What is the output of this code?

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

Javascript

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

A "12 13 23 31 32"

B "12 13 21 23 31 32 33"

C "12 13 21 23 31 32"

D "21 31 32"

💡 Hint

The continue with label skips the rest of the outer loop iteration when i equals j.

When i equals j, continue outer skips the remaining iterations of the inner loop and proceeds to the next i. i=1 skipped after first j=1, i=2 adds only '21 ', i=3 adds '31 32 '. Output: "21 31 32".

❓ Predict Output

advanced

Nested loops with array modification

What is the output of this code?

const arr = [1, 2, 3];
let result = [];
for (let i = 0; i < arr.length; i++) {
  for (let j = i; j < arr.length; j++) {
    result.push(arr[i] + arr[j]);
  }
}
console.log(result.join(','));

Javascript

const arr = [1, 2, 3];
let result = [];
for (let i = 0; i < arr.length; i++) {
  for (let j = i; j < arr.length; j++) {
    result.push(arr[i] + arr[j]);
  }
}
console.log(result.join(','));

A "2,3,4,4,5,6"

B "2,3,4,3,4,5"

C "2,3,4,5,6,7"

D "1,3,5,2,4,6"

💡 Hint

Add arr[i] + arr[j] starting with j = i, so pairs include sums of same and later elements.

The sums are: 1+1=2, 1+2=3, 1+3=4, 2+2=4, 2+3=5, 3+3=6, so output is "2,3,4,4,5,6".

🧠 Conceptual

expert

Time complexity of nested loops

Consider this code:

for (let i = 1; i <= n; i++) {
  for (let j = 1; j <= i; j++) {
    // constant time operation
  }
}

What is the time complexity in Big O notation?

A O(n^2)

B O(log n)

C O(n log n)

D O(n)

💡 Hint

Sum the inner loop counts: 1 + 2 + 3 + ... + n.

The total operations are 1 + 2 + ... + n = n(n+1)/2, which is O(n^2).