While loop in Javascript - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of a simple while loop

What will be the output of the following JavaScript code?

Javascript

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

A "0-1-2-3-"

B "0-1-2-"

C "1-2-3-"

D "-0-1-2"

💡 Hint

Remember the loop runs while count is less than 3, and count starts at 0.

The loop runs while count is 0, 1, and 2. Each time it adds the current count and a dash. When count reaches 3, the loop stops, so '0-1-2-' is printed.

❓ Predict Output

intermediate

While loop with break statement

What will be printed by this code?

Javascript

let i = 1;
let output = '';
while (i <= 5) {
  if (i === 3) break;
  output += i;
  i++;
}
console.log(output);

A "12345"

B "123"

C ""

D "12"

💡 Hint

The loop stops when i equals 3.

The loop adds '1' and '2' to output. When i becomes 3, the break stops the loop before adding '3'. So output is '12'.

❓ Predict Output

advanced

While loop with nested condition

What is the final value of variable sum after running this code?

Javascript

let n = 5;
let sum = 0;
while (n > 0) {
  if (n % 2 === 0) {
    sum += n;
  }
  n--;
}

A 8

B 10

C 6

D 0

💡 Hint

Add only even numbers from 5 down to 1.

Even numbers between 5 and 1 are 4 and 2. Their sum is 6.

❓ Predict Output

advanced

While loop with post-increment in condition

What will this code output?

Javascript

let x = 0;
let output = '';
while (x++ < 3) {
  output += x;
}
console.log(output);

A "123"

B "234"

C "1234"

D "012"

💡 Hint

Remember x++ returns the value before incrementing.

The condition checks x before incrementing. Loop runs while x is 0,1,2. Inside loop, x is incremented, so output adds 1,2,3.

❓ Predict Output

expert

While loop with complex condition and variable update

What is the output of this code snippet?

Javascript

let a = 1;
let b = 10;
let result = '';
while (a < b) {
  result += a + ',';
  a *= 2;
  b -= 3;
}
console.log(result);

A "1,2,"

B "1,2,4,8,"

C "1,2,4,"

D "1,2,4,8,16,"

💡 Hint

Check how a and b change each loop and when the condition fails.

Loop 1: a=1,b=10 (1<10) add '1,'; a=2,b=7 Loop 2: a=2,b=7 (2<7) add '2,'; a=4,b=4 Condition: a=4,b=4 (4<4) false, stop. So output is '1,2,'

Practice

1. What does a while loop do in JavaScript?

easy

A. Repeats code while a condition is true

B. Runs code only once

C. Runs code a fixed number of times

D. Stops the program immediately

Solution

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

   A while loop runs repeatedly as long as its condition stays true.

2. Step 2: Compare options with the definition

   Only Repeats code while a condition is true correctly describes this behavior.

3. Final Answer:

   Repeats code while a condition is true -> Option A

4. Quick Check:

   while loop = repeats while true [OK]

Hint: Remember: while loops run only if condition is true [OK]

Common Mistakes:

• Thinking while loops run a fixed number of times
• Confusing while with if statement
• Assuming while loops run at least once

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

easy

A. while condition { /* code */ }

B. while (condition) { /* code */ }

C. while: (condition) { /* code */ }

D. while (condition); { /* code */ }

Solution

1. Step 1: Recall JavaScript while loop syntax

   The correct syntax uses parentheses around the condition and curly braces for the code block: while (condition) { ... }.

2. Step 2: Check each option

   while (condition) { /* code */ } matches the correct syntax exactly.

3. Final Answer:

   while (condition) { /* code */ } -> Option B

4. Quick Check:

   Correct syntax = while (condition) { } [OK]

Hint: Look for parentheses around condition and braces for code [OK]

Common Mistakes:

• Omitting parentheses around the condition
• Using a semicolon after while condition
• Adding a colon after while keyword

3. What will be the output of this code?

let count = 1;
while (count <= 3) {
  console.log(count);
  count++;
}

medium

A. 0 1 2 3

B. 1 2 3 4

C. 3 2 1

D. 1 2 3

Solution

1. Step 1: Analyze the loop condition and increments

   The loop starts with count = 1 and runs while count is less than or equal to 3. Each time, it prints count and then adds 1.

2. Step 2: Trace the output values

   It prints 1, then 2, then 3. When count becomes 4, the condition fails and loop stops.

3. Final Answer:

   1 2 3 -> Option D

4. Quick Check:

   Prints 1 to 3 inclusive [OK]

Hint: Count starts at 1 and stops after 3 [OK]

Common Mistakes:

• Including 4 in output
• Starting count at 0 instead of 1
• Printing in reverse order

4. Find the error in this code snippet:

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

medium

A. Missing semicolon after console.log

B. Variable i is not declared

C. Missing parentheses around the condition

D. Increment operator is incorrect

Solution

1. Step 1: Check while loop syntax

   JavaScript requires parentheses around the condition in a while loop.

2. Step 2: Identify the error in the code

   The code has while i < 5 { but it should be while (i < 5) {.

3. Final Answer:

   Missing parentheses around the condition -> Option C

4. Quick Check:

   while needs (condition) [OK]

Hint: Always put condition inside parentheses in while [OK]

Common Mistakes:

• Forgetting parentheses around condition
• Adding semicolon after while condition
• Misusing increment operator

5. You want to print all even numbers from 2 to 10 using a while loop. Which code correctly does this?

hard

A. let num = 2;
while (num <= 10) {
console.log(num);
num += 2;
}

B. let num = 1;
while (num < 10) {
if (num % 2 === 0) console.log(num);
num++;
}

C. let num = 2;
while (num < 10) {
console.log(num);
num += 2;
}

D. let num = 0;
while (num <= 10) {
console.log(num);
num += 2;
}

Solution

1. Step 1: Understand the goal and loop conditions

   We want to print even numbers from 2 to 10 inclusive. So the loop must start at 2 and run while number is less than or equal to 10.

2. Step 2: Check each option for correctness

   let num = 2; while (num <= 10) { console.log(num); num += 2; } starts at 2, prints number, then adds 2, stopping at 10. This prints 2,4,6,8,10 correctly. let num = 1; while (num < 10) { if (num % 2 === 0) console.log(num); num++; } starts at 1, uses modulo check, but since num < 10 stops after 9, printing 2,4,6,8 missing 10. let num = 2; while (num < 10) { console.log(num); num += 2; } stops before 10, missing 10. let num = 0; while (num <= 10) { console.log(num); num += 2; } starts at 0, which is not in the required range.

3. Final Answer:

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

4. Quick Check:

   Start 2, add 2, stop at 10 [OK]

Hint: Start at 2, add 2 each loop, stop at 10 [OK]

Common Mistakes:

• Starting at 0 or 1 instead of 2
• Stopping before 10
• Not incrementing by 2