Do–while loop in Javascript - Mini Project: Build & Apply

Using a Do-while Loop in JavaScript

📖 Scenario: You are creating a simple program that counts up from 1 to a certain number. This is like counting steps while walking.

🎯 Goal: You will build a program that uses a do-while loop to count from 1 up to a number you choose, and then show the count.

📋 What You'll Learn

Create a variable called count starting at 1

Create a variable called maxCount set to 5

Use a do-while loop to increase count until it is greater than maxCount

Print the final value of count

💡 Why This Matters

🌍 Real World

Counting steps, retries, or attempts in programs often uses loops like <code>do-while</code> to ensure the action happens at least once.

💼 Career

Understanding <code>do-while</code> loops helps in writing programs that need to repeat tasks until a condition changes, common in many software jobs.

1

Create the starting count variable

Create a variable called count and set it to 1.

Javascript

# Your code here

Hint

Use let to create the variable count and set it to 1.

2

Set the maximum count limit

Create a variable called maxCount and set it to 5.

Javascript

let count = 1;
# Your code here

Hint

Use let to create maxCount and set it to 5.

3

Use a do-while loop to count up

Write a do-while loop that increases count by 1 each time, and continues while count is less than or equal to maxCount.

Javascript

let count = 1;
let maxCount = 5;
# Your code here

Hint

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

4

Print the final count

Write console.log(count) to display the final value of count.

Javascript

let count = 1;
let maxCount = 5;
do {
    count++;
} while (count <= maxCount);
# Your code here

Hint

Use console.log(count) to show the number after the loop ends.

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