Why Do–while loop in Javascript? - Purpose & Use Cases

The Big Idea

What if you could ask a question once and automatically repeat it only when needed, without extra code?

The Scenario

Imagine you want to ask a friend if they want to play a game, and you keep asking until they say yes. Doing this by repeating the question manually over and over is tiring and easy to mess up.

The Problem

Manually repeating the same question or code again and again is slow and boring. You might forget to ask the question at least once or make mistakes in the order. It's hard to keep track of when to stop.

The Solution

The do-while loop lets you ask the question once first, then keep asking as long as the answer is no. It makes sure the question is always asked at least once, and stops automatically when the answer is yes.

Before vs After

✗ Before

let answer;
answer = prompt('Play game?');
while(answer !== 'yes') {
  answer = prompt('Play game?');
}

✓ After

let answer;
do {
  answer = prompt('Play game?');
} while(answer !== 'yes');

What It Enables

It enables you to run a task at least once and repeat it easily until a condition is met, without extra checks before the first run.

Real Life Example

When filling out a form, you want to show the form once and keep asking for corrections until all fields are valid. The do-while loop helps by showing the form first, then repeating if needed.

Key Takeaways

Do-while loops run code at least once before checking a condition.

They simplify repeating tasks that must happen first, then repeat if needed.

They prevent mistakes from forgetting the first run or repeating too much.

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