Loop execution flow 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 JavaScript 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 "2 3 4"

B "2 3 4 5 6 7"

C "2 3 4 5 6"

D "2 4 6"

💡 Hint

Remember that the inner loop breaks when j equals 2, so only j=1 runs each time.

The inner loop runs only once per outer loop iteration because it breaks when j === 2. So for each i, j=1 runs, adding i+j to result. i=1: 1+1=2, i=2: 2+1=3, i=3: 3+1=4. So result is "2 3 4".

❓ Predict Output

intermediate

Loop with continue statement output

What will be printed by this code?

let output = '';
for (let k = 0; k < 5; k++) {
  if (k % 2 === 0) continue;
  output += k + ',';
}
console.log(output.slice(0, -1));

Javascript

let output = '';
for (let k = 0; k < 5; k++) {
  if (k % 2 === 0) continue;
  output += k + ',';
}
console.log(output.slice(0, -1));

A "1,3"

B "1,3,4"

C "1,3,5"

D "1,3,5,7"

💡 Hint

The continue skips even numbers, so only odd k values are added.

The loop runs k from 0 to 4. When k is even (0,2,4), continue skips adding. So only k=1 and k=3 are added, resulting in "1,3".

❓ Predict Output

advanced

Output of labeled break in nested loops

What is the output of this code?

let res = '';
outer: for (let a = 1; a <= 3; a++) {
  for (let b = 1; b <= 3; b++) {
    if (a * b > 3) break outer;
    res += a + b + ' ';
  }
}
console.log(res.trim());

Javascript

let res = '';
outer: for (let a = 1; a <= 3; a++) {
  for (let b = 1; b <= 3; b++) {
    if (a * b > 3) break outer;
    res += a + b + ' ';
  }
}
console.log(res.trim());

A "2 3 3"

B "2 3"

C "2 3 4 3"

D "2 3 4 5"

💡 Hint

The labeled break stops both loops when a*b > 3.

The loops add a+b to res until a*b > 3. Iterations: a=1,b=1(1), add 2; a=1,b=2(2), add 3; a=1,b=3(3), add 4; a=2,b=1(2), add 3; a=2,b=2(4 > 3 triggers break). So res is "2 3 4 3".

❓ Predict Output

advanced

Final value after loop with post-increment

What is the value of variable x after this code runs?

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

Javascript

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

A 10

B 6

C 8

D 12

💡 Hint

Remember that i++ increases i after its value is used in the addition.

Loop iterations: i=0, x+=0, i becomes 1, then i++ in for loop makes i=2; i=2, x+=2, i=3, then i=4; i=4, x+=4, i=5, loop ends. x=0+2+4=6.

🧠 Conceptual

expert

Understanding loop execution with async/await

Consider this code snippet:

async function test() {
  for (let i = 0; i < 3; i++) {
    await new Promise(resolve => setTimeout(resolve, 100));
    console.log(i);
  }
}
test();

Which statement best describes the output behavior?

Javascript

async function test() {
  for (let i = 0; i < 3; i++) {
    await new Promise(resolve => setTimeout(resolve, 100));
    console.log(i);
  }
}
test();

A Throws a syntax error because await cannot be used in for loops.

B Logs 0, 1, 2 immediately without delay.

C Logs 0, 1, 2 all after approximately 300ms at once.

D Logs 0, 1, 2 with 100ms delay between each log sequentially.

💡 Hint

Await pauses the loop until the promise resolves before continuing.

The await inside the for loop pauses each iteration until the 100ms timeout resolves, so logs happen one by one with 100ms delay each.