Do–while loop in Javascript - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of a simple do–while loop

What is the output of this JavaScript code?

Javascript

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

A 1 2 3

B 0 1 2

C 0 1 2 3

D 1 2

💡 Hint

Remember, do–while runs the block first, then checks the condition.

The loop starts with count = 0, prints it, then increments. It repeats while count < 3, so it prints 0, 1, and 2.

❓ Predict Output

intermediate

Do–while loop with break statement

What will this code print to the console?

Javascript

let i = 5;
do {
  console.log(i);
  if (i === 3) break;
  i--;
} while (i > 0);

A 5 4 3

B 5 4 3 2 1

C 5 4

D 3 2 1

💡 Hint

The break stops the loop when i equals 3.

The loop prints 5, then 4, then 3. When i is 3, break stops the loop immediately.

🧠 Conceptual

advanced

Understanding do–while loop execution

Which statement about do–while loops is true?

A Do–while loops cannot be used to run code multiple times.

B The loop body may not execute if the condition is false at the start.

C The condition is checked before the loop body runs.

D The loop body executes at least once, even if the condition is false initially.

💡 Hint

Think about when the condition is checked in a do–while loop.

In a do–while loop, the code inside the loop runs first, then the condition is checked. So it always runs at least once.

❓ Predict Output

advanced

Output with nested do–while loops

What is the output of this nested do–while loop code?

Javascript

let a = 1;
do {
  let b = 1;
  do {
    console.log(a * b);
    b++;
  } while (b <= 2);
  a++;
} while (a <= 2);

A 1 2 2 4

B 1 2 1 2

C 2 4 2 4

D 1 2 3 4

💡 Hint

Trace the values of a and b carefully through both loops.

Outer loop runs for a=1 and a=2. Inner loop runs for b=1 and b=2 each time. Outputs: 1*1=1, 1*2=2, 2*1=2, 2*2=4.

🔧 Debug

expert

Identify the error in this do–while loop

What error will this code cause when run?

Javascript

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

A ReferenceError

B TypeError

C SyntaxError

D No error, outputs 0 1 2

💡 Hint

Check the syntax of the while condition in a do–while loop.

The while condition must be inside parentheses. Missing parentheses causes SyntaxError.

Practice

1. What is the main difference between a do-while loop and a while loop in JavaScript?

easy

A. A while loop always runs the code block at least once.

B. A do-while loop runs the code block at least once before checking the condition.

C. A do-while loop never checks the condition.

D. A while loop runs the code block only if the condition is false.

Solution

1. Step 1: Understand do-while loop behavior

   A do-while loop executes the code block first, then checks the condition to decide if it should repeat.

2. Step 2: Compare with while loop

   A while loop checks the condition before running the code block, so it may not run at all if the condition is false initially.

3. Final Answer:

   A do-while loop runs the code block at least once before checking the condition. -> Option B

4. Quick Check:

   do-while runs once before condition [OK]

Hint: Remember: do-while runs code first, then checks condition [OK]

Common Mistakes:

• Thinking while loop always runs once
• Believing do-while skips condition check
• Confusing order of execution and condition check

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

easy

A. do (condition) { /* code */ };

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

C. do { /* code */ } while (condition);

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

Solution

1. Step 1: Recall correct do-while syntax

   The do-while loop starts with do, then the code block in braces, followed by while with the condition in parentheses and a semicolon.

2. Step 2: Check each option

   do { /* code */ } while (condition); matches the correct syntax exactly. Others have wrong order or missing parts.

3. Final Answer:

   do { /* code */ } while (condition); -> Option C

4. Quick Check:

   Syntax: do { } while(condition); [OK]

Hint: Syntax always: do { } while(condition); ends with semicolon [OK]

Common Mistakes:

• Forgetting the semicolon after while(condition)
• Placing while before do
• Using parentheses incorrectly around code block

3. What will be the output of the following code?

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

medium

A. 3 2 1

B. 3 2 1 0

C. 2 1 0

D. 1 2 3

Solution

1. Step 1: Trace the loop iterations

   Initial count is 3. The loop prints count, then decreases it by 1. It repeats while count > 0.

2. Step 2: List printed values

   First print: 3, count becomes 2
   Second print: 2, count becomes 1
   Third print: 1, count becomes 0
   Loop stops because count is not > 0.

3. Final Answer:

   3 2 1 -> Option A

4. Quick Check:

   Prints 3,2,1 then stops [OK]

Hint: Count down from 3, print before decrement, stop when zero [OK]

Common Mistakes:

• Including 0 in output
• Printing after decrement instead of before
• Confusing loop condition direction

4. Identify the error in this do-while loop code:

let i = 0;
do {
  console.log(i);
  i++
} while i < 3;

medium

A. Missing semicolon after i++ inside the loop.

B. The loop condition should be inside curly braces.

C. Variable i is not initialized.

D. Missing parentheses around the condition in while statement.

Solution

1. Step 1: Check the while condition syntax

   The condition after while must be inside parentheses. Here, it is written as while i < 3; without parentheses.

2. Step 2: Verify other parts

   Missing semicolon after i++ is allowed in JavaScript (automatic semicolon insertion). Variable i is initialized. Condition does not go inside braces, only parentheses.

3. Final Answer:

   Missing parentheses around the condition in while statement. -> Option D

4. Quick Check:

   Condition must be in ( ) after while [OK]

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

Common Mistakes:

• Omitting parentheses around while condition
• Thinking semicolon inside loop is mandatory
• Putting condition inside curly braces

5. You want to prompt a user to enter a number greater than 10. Which do-while loop code correctly repeats the prompt until the user enters a valid number?

hard

A. let num;
do {
num = prompt('Enter a number > 10');
} while (num <= 10);

B. let num = 0;
do {
num = prompt('Enter a number > 10');
} while (num < 10);

C. let num;
do {
num = prompt('Enter a number > 10');
} while (num < 10);

D. let num;
do {
num = prompt('Enter a number > 10');
} while (num > 10);

Solution

1. Step 1: Understand the condition needed

   The loop should continue prompting while the entered number is less than or equal to 10. Since prompt returns a string, comparison with 10 works if user enters a number string.

2. Step 2: Check each option

   let num; do { num = prompt('Enter a number > 10'); } while (num <= 10);: while (num <= 10) - correct condition to reject 10 or less; uninitialized num fine in do-while.
   let num = 0; do { num = prompt('Enter a number > 10'); } while (num < 10);: while (num < 10) - accepts 10 incorrectly; unnecessary init.
   let num; do { num = prompt('Enter a number > 10'); } while (num < 10);: while (num < 10) - accepts 10 incorrectly.
   let num; do { num = prompt('Enter a number > 10'); } while (num > 10);: while (num > 10) - loops when input is valid.

3. Final Answer:

   let num; do { num = prompt('Enter a number > 10'); } while (num <= 10); -> Option A

4. Quick Check:

   Repeat while input <= 10 [OK]

Hint: Repeat prompt while input <= 10 [OK]

Common Mistakes:

• Using wrong comparison operator
• Loop condition reversed
• Initializing variable unnecessarily