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.

Practice

1. What does a for loop do in JavaScript?

easy

A. Stops the program immediately

B. Runs code only once

C. Creates a new variable

D. Repeats code a set number of times

Solution

1. Step 1: Understand the purpose of a for loop

   A for loop is designed to repeat code multiple times based on a condition.

2. Step 2: Identify the correct description

   Among the options, only repeating code a set number of times matches the for loop behavior.

3. Final Answer:

   Repeats code a set number of times -> Option D

4. Quick Check:

   for loop = repeat fixed times [OK]

Hint: For loops repeat code a fixed number of times [OK]

Common Mistakes:

• Thinking for loops run only once
• Confusing for loops with if statements
• Believing for loops stop the program

2. Which of the following is the correct syntax for a for loop in JavaScript?

easy

A. for i = 0 to 5 { }

B. for (let i = 0; i < 5; i++) { }

C. loop (i = 0; i < 5; i++) { }

D. for (i < 5; i = 0; i++) { }

Solution

1. Step 1: Recall JavaScript for loop syntax

   The correct syntax uses for (initialization; condition; increment) { }.

2. Step 2: Compare options to syntax

   Only for (let i = 0; i < 5; i++) { } matches the correct syntax with let i = 0; i < 5; i++.

3. Final Answer:

   for (let i = 0; i < 5; i++) { } -> Option B

4. Quick Check:

   for loop syntax = initialization; condition; increment [OK]

Hint: Remember for loop uses three parts inside parentheses [OK]

Common Mistakes:

• Using incorrect keywords like 'loop'
• Swapping initialization and condition
• Missing semicolons inside for parentheses

3. What will be the output of this code?

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

medium

A. 6

B. 3

C. 0

D. Error

Solution

1. Step 1: Trace the loop iterations

   The loop runs with i = 1, 2, 3. Each time, sum adds i: 0+1=1, 1+2=3, 3+3=6.

2. Step 2: Determine final sum value

   After the loop, sum is 6, which is printed by console.log.

3. Final Answer:

   6 -> Option A

4. Quick Check:

   Sum 1+2+3 = 6 [OK]

Hint: Add numbers step-by-step inside the loop [OK]

Common Mistakes:

• Stopping loop early
• Adding wrong values
• Confusing sum initialization

4. Find the error in this loop:

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

medium

A. Console.log is incorrect

B. Syntax error in for loop

C. The loop will run infinitely because i decreases

D. Variable i is not declared

Solution

1. Step 1: Analyze the loop increment/decrement

   The loop uses i--, so i decreases each time starting from 0.

2. Step 2: Check loop condition

   The condition is i < 5, which is true for all i less than 5, including negative numbers. Since i decreases, it never reaches 5 or more, so loop never ends.

3. Final Answer:

   The loop will run infinitely because i decreases -> Option C

4. Quick Check:

   Decrement with < condition causes infinite loop [OK]

Hint: Check if increment matches loop condition direction [OK]

Common Mistakes:

• Confusing i-- with i++
• Ignoring loop condition logic
• Assuming loop stops automatically

5. How many times will this loop run?

let count = 0;
let i = 10;
while (i > 0) {
  i -= 3;
  count++;
}
console.log(count);

hard

A. 4

B. 3

C. 5

D. 6

Solution

1. Step 1: Trace the while loop iterations

   Start i=10, count=0. Each loop: i decreases by 3, count increases by 1.
   Iterations:
   1) i=7, count=1
   2) i=4, count=2
   3) i=1, count=3
   4) i=-2, count=4 (loop stops after this because i > 0 is false)

2. Step 2: Count total iterations

   The loop runs 4 times before i becomes -2 and condition fails.

3. Final Answer:

   4 -> Option A

4. Quick Check:

   Subtract 3 until i ≤ 0 counts 4 times [OK]

Hint: Count how many times i decreases by 3 until ≤ 0 [OK]

Common Mistakes:

• Counting one less or more iteration
• Not updating count correctly
• Misunderstanding while loop condition