Do–while loop in Javascript - Time & Space Complexity

Time Complexity: Do-while loop

O(n)

Understanding Time Complexity

We want to understand how the time it takes to run a do-while loop changes as the input size grows.

Specifically, how many times the loop runs affects the total work done.

Scenario Under Consideration

Analyze the time complexity of the following code snippet.

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

This code prints numbers from 0 up to n-1 using a do-while loop.

Identify Repeating Operations

Identify the loops, recursion, array traversals that repeat.

• Primary operation: The do-while loop runs the block inside repeatedly.
• How many times: It runs exactly n times, once for each number from 0 to n-1.

How Execution Grows With Input

As n grows, the number of times the loop runs grows directly with n.

Input Size (n)	Approx. Operations
10	10
100	100
1000	1000

Pattern observation: The work grows in a straight line with the input size.

Final Time Complexity

Time Complexity: O(n)

This means the time to run the loop grows directly in proportion to the input size n.

Common Mistake

[X] Wrong: "The do-while loop always runs at least once, so its time is constant regardless of n."

[OK] Correct: While the loop runs at least once, the total number of runs depends on n, so time grows with n.

Interview Connect

Understanding how loops like do-while scale helps you explain how your code handles bigger inputs clearly and confidently.

Self-Check

"What if the loop condition was based on a fixed number instead of n? How would the time complexity change?"

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