Do–while loop in Javascript - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is a do–while loop in JavaScript?

A do–while loop runs a block of code once, then repeats it as long as a specified condition is true. It always runs the code at least once.

beginner

How does a do–while loop differ from a while loop?

A do–while loop runs the code block first, then checks the condition. A while loop checks the condition first before running the code.

beginner

What is the syntax of a do–while loop?

The syntax is:

do {
  // code to run
} while (condition);

The code runs once before the condition is checked.

intermediate

When is it useful to use a do–while loop?

Use a do–while loop when you want the code to run at least once, like asking a user for input and then repeating if the input is invalid.

beginner

What happens if the condition in a do–while loop is false the first time it is checked?

The code inside the loop still runs once before the condition is checked. If the condition is false, the loop stops after that first run.

What is guaranteed about the code inside a do–while loop?

A It runs at least once

B It may not run at all

C It runs only if the condition is true initially

D It runs twice before checking the condition

A do–while loop always runs the code block once before checking the condition.

Which part of the do–while loop is checked after the code runs?

A The code block

B The condition in the <code>while</code> statement

C The initialization

D The loop counter

After running the code block, the loop checks the condition in the while statement to decide if it should run again.

What will happen if the condition in a do–while loop is always true?

A The loop will run twice

B The loop will run once

C The loop will not run

D The loop will run forever (infinite loop)

If the condition never becomes false, the do–while loop will continue running endlessly.

Which of these is the correct syntax for a do–while loop?

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

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

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

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

The correct syntax is do { /* code */ } while (condition); with a semicolon at the end.

Why might you choose a do–while loop over a while loop?

A To ensure the code runs at least once

B To skip the first iteration

C To run the loop only if the condition is false

D To avoid checking the condition

A do–while loop guarantees the code runs at least once, unlike a while loop.

Explain how a do–while loop works and when you would use it.

Think about the difference from a <code>while</code> loop and real-life situations where you want to try something first.

Write the syntax of a do–while loop and describe each part.

Remember the order: do, code, while(condition);

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