For loop in Javascript - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of a simple for loop

What will be the output of the following code?

Javascript

let result = '';
for (let i = 0; i < 3; i++) {
  result += i + '-';
}
console.log(result);

A "0-1-2-"

B "0-1-2"

C "1-2-3-"

D "0-1-2-3-"

💡 Hint

Remember the loop runs while i is less than 3, starting from 0.

The loop runs with i values 0, 1, and 2. Each time it adds i and a dash to the string. So the final string is "0-1-2-".

❓ Predict Output

intermediate

For loop with break statement

What will be printed by this code?

Javascript

let sum = 0;
for (let i = 1; i <= 5; i++) {
  if (i === 3) break;
  sum += i;
}
console.log(sum);

A 0

B 6

C 10

D 3

💡 Hint

The loop stops when i equals 3.

The loop adds 1 and 2 to sum, then stops before adding 3. So sum is 1 + 2 = 3.

❓ Predict Output

advanced

Nested for loops output

What is the output of this nested loop code?

Javascript

let output = '';
for (let i = 1; i <= 2; i++) {
  for (let j = 1; j <= 3; j++) {
    output += `${i}${j} `;
  }
}
console.log(output.trim());

A "11 12 13 21 22 23"

B "11 21 12 22 13 23"

C "123 123"

D "1 2 3 1 2 3"

💡 Hint

The outer loop runs twice, inner loop runs three times each.

For i=1, j runs 1 to 3 producing 11 12 13. For i=2, j runs 1 to 3 producing 21 22 23. Combined output is "11 12 13 21 22 23".

❓ Predict Output

advanced

For loop with continue statement

What will this code print?

Javascript

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

A "024"

B "1234"

C "135"

D "0246"

💡 Hint

Continue skips even numbers.

The loop skips i when it is even (0,2,4). It adds only odd numbers 1, 3, and 5 to text, so output is "135".

❓ Predict Output

expert

For loop with asynchronous behavior

What will be logged to the console when this code runs?

Javascript

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

A 3 3 3

B 0 1 2

C 0 0 0

D 1 2 3

💡 Hint

The variable i is declared with let inside the loop.

Using let creates a new binding for each loop iteration, so the logged values are 0, 1, and 2 as expected.

Practice

1.

What does a for loop do in JavaScript?

easy

A. Changes the value of a variable once

B. Stops the program immediately

C. Repeats a block of code a set number of times

D. Creates a new variable

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: Compare options with the for loop behavior

   Only Repeats a block of code a set number of times correctly describes repeating code multiple times.

3. Final Answer:

   Repeats a block of code a set number of times -> Option C

4. Quick Check:

   For loop = repeat code [OK]

Hint: For loops repeat code multiple times [OK]

Common Mistakes:

• Thinking for loop stops program
• Confusing variable creation with looping
• Assuming for loop runs only once

2.

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

for (___; ___; ___) {
  // code
}

easy

A. for (i = 0; i < 5; i++)

B. for i = 0; i < 5; i++

C. for (i < 5; i = 0; i++)

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

Solution

1. Step 1: Recall the for loop syntax

   The correct syntax is: for (start; condition; change) { ... }

2. Step 2: Check each option against syntax

   for (i = 0; i < 5; i++) matches the correct syntax with initialization, condition, and increment.

3. Final Answer:

   for (i = 0; i < 5; i++) -> Option A

4. Quick Check:

   Syntax = for (init; cond; change) [OK]

Hint: For loop needs three parts inside parentheses [OK]

Common Mistakes:

• Missing parentheses around for loop parts
• Wrong order of parts inside for loop
• Using semicolons incorrectly

3.

What will be the output of this code?

let result = '';
for (let i = 1; i <= 3; i++) {
  result += i + ' ';
}
console.log(result.trim());

medium

A. "3 2 1"

B. "1 2 3"

C. "123"

D. "1 2 3 4"

Solution

1. Step 1: Understand the loop iterations

   The loop runs with i = 1, 2, 3 because i <= 3.

2. Step 2: Track the string concatenation

   Each iteration adds the number and a space to result: "1 ", then "1 2 ", then "1 2 3 ". trim() removes the last space.

3. Final Answer:

   "1 2 3" -> Option B

4. Quick Check:

   Loop 1 to 3 adds numbers with spaces [OK]

Hint: Check loop range and string building carefully [OK]

Common Mistakes:

• Ignoring the spaces added
• Forgetting trim() removes trailing space
• Assuming loop includes 4

4.

Find the error in this for loop code:

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

medium

A. Missing curly braces around loop body

B. Incorrect variable declaration 'let i = 0'

C. Wrong loop condition 'i < 5'

D. Missing increment operator 'i++' in the loop

Solution

1. Step 1: Check the loop increment part

   The loop has 'i' instead of 'i++' which means i never changes.

2. Step 2: Understand the effect of missing increment

   Without increment, the loop runs forever or does not progress, causing an error or infinite loop.

3. Final Answer:

   Missing increment operator 'i++' in the loop -> Option D

4. Quick Check:

   Increment needed to change loop variable [OK]

Hint: Check all three parts of for loop syntax carefully [OK]

Common Mistakes:

• Forgetting to increment loop variable
• Using wrong condition syntax
• Missing braces but allowed in single line

5.

You want to create a for loop that prints only even numbers from 2 to 10 inclusive. Which code snippet does this correctly?

hard

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

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

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

D. for (let i = 2; i <= 10; i++) { if (i % 2 === 1) console.log(i); }

Solution

1. Step 1: Identify the range and condition for even numbers

   We want numbers 2, 4, 6, 8, 10 inclusive.

2. Step 2: Check each option for correctness

   for (let i = 2; i <= 10; i += 2) { console.log(i); } increments by 2 starting at 2, printing only even numbers up to 10. for (let i = 0; i <= 10; i++) { if (i % 2 === 0) console.log(i); } starts from 0 and prints 0 which is outside the 2-10 range. for (let i = 2; i < 10; i++) { if (i % 2 === 0) console.log(i); } misses 10 because it uses i < 10. for (let i = 2; i <= 10; i++) { if (i % 2 === 1) console.log(i); } prints odd numbers, which is wrong.

3. Final Answer:

   for (let i = 2; i <= 10; i += 2) { console.log(i); } -> Option A

4. Quick Check:

   Increment by 2 prints evens directly [OK]

Hint: Use i += 2 to skip odd numbers [OK]

Common Mistakes:

• Using wrong loop condition (i < 10)
• Printing odd numbers by mistake
• Not incrementing properly to skip odds