While loop in Javascript - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is a while loop in JavaScript?

A while loop repeats a block of code as long as a specified condition is true. It checks the condition before each repetition.

beginner

How does the condition in a while loop affect its execution?

The loop runs only if the condition is true. If the condition is false at the start, the loop body does not run at all.

intermediate

What happens if the condition in a while loop never becomes false?

The loop will run forever, causing an infinite loop. This can freeze or crash the program.

beginner

Write the basic syntax of a while loop in JavaScript.

while (condition) { // code to run repeatedly }

beginner

Why is it important to update variables inside a while loop?

Updating variables inside the loop helps change the condition so the loop can eventually stop. Without this, the loop might run forever.

What does a while loop do in JavaScript?

A Runs code once then stops

B Runs code only if condition is false

C Repeats code while a condition is true

D Repeats code a fixed number of times

A while loop repeats the code block as long as the condition remains true.

When is the condition in a while loop checked?

A Before each loop iteration

B After each loop iteration

C Only once before the loop starts

D Only after the loop ends

The condition is checked before running the loop body each time.

What happens if the while loop condition is false at the start?

A The loop runs twice

B The loop body does not run at all

C The loop runs once

D The loop runs infinitely

If the condition is false initially, the loop body is skipped completely.

Which of these can cause an infinite while loop?

A Not updating the condition inside the loop

B Updating the condition to false

C Using a for loop instead

D Using a break statement

If the condition never changes to false, the loop never stops.

What is the correct syntax to start a while loop?

A while condition ( ) { }

B while { condition }

C while: condition { }

D while (condition) { }

The correct syntax uses parentheses around the condition and curly braces for the loop body.

Explain how a while loop works and why updating the condition inside the loop is important.

Think about what happens if the condition never changes.

Describe a real-life situation where a while loop could be useful.

Imagine waiting for something to happen before moving on.

Practice

1. What does a while loop do in JavaScript?

easy

A. Repeats code while a condition is true

B. Runs code only once

C. Runs code a fixed number of times

D. Stops the program immediately

Solution

1. Step 1: Understand the purpose of a while loop

   A while loop runs repeatedly as long as its condition stays true.

2. Step 2: Compare options with the definition

   Only Repeats code while a condition is true correctly describes this behavior.

3. Final Answer:

   Repeats code while a condition is true -> Option A

4. Quick Check:

   while loop = repeats while true [OK]

Hint: Remember: while loops run only if condition is true [OK]

Common Mistakes:

• Thinking while loops run a fixed number of times
• Confusing while with if statement
• Assuming while loops run at least once

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

easy

A. while condition { /* code */ }

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

C. while: (condition) { /* code */ }

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

Solution

1. Step 1: Recall JavaScript while loop syntax

   The correct syntax uses parentheses around the condition and curly braces for the code block: while (condition) { ... }.

2. Step 2: Check each option

   while (condition) { /* code */ } matches the correct syntax exactly.

3. Final Answer:

   while (condition) { /* code */ } -> Option B

4. Quick Check:

   Correct syntax = while (condition) { } [OK]

Hint: Look for parentheses around condition and braces for code [OK]

Common Mistakes:

• Omitting parentheses around the condition
• Using a semicolon after while condition
• Adding a colon after while keyword

3. What will be the output of this code?

let count = 1;
while (count <= 3) {
  console.log(count);
  count++;
}

medium

A. 0 1 2 3

B. 1 2 3 4

C. 3 2 1

D. 1 2 3

Solution

1. Step 1: Analyze the loop condition and increments

   The loop starts with count = 1 and runs while count is less than or equal to 3. Each time, it prints count and then adds 1.

2. Step 2: Trace the output values

   It prints 1, then 2, then 3. When count becomes 4, the condition fails and loop stops.

3. Final Answer:

   1 2 3 -> Option D

4. Quick Check:

   Prints 1 to 3 inclusive [OK]

Hint: Count starts at 1 and stops after 3 [OK]

Common Mistakes:

• Including 4 in output
• Starting count at 0 instead of 1
• Printing in reverse order

4. Find the error in this code snippet:

let i = 0;
while i < 5 {
  console.log(i);
  i++;
}

medium

A. Missing semicolon after console.log

B. Variable i is not declared

C. Missing parentheses around the condition

D. Increment operator is incorrect

Solution

1. Step 1: Check while loop syntax

   JavaScript requires parentheses around the condition in a while loop.

2. Step 2: Identify the error in the code

   The code has while i < 5 { but it should be while (i < 5) {.

3. Final Answer:

   Missing parentheses around the condition -> Option C

4. Quick Check:

   while needs (condition) [OK]

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

Common Mistakes:

• Forgetting parentheses around condition
• Adding semicolon after while condition
• Misusing increment operator

5. You want to print all even numbers from 2 to 10 using a while loop. Which code correctly does this?

hard

A. let num = 2;
while (num <= 10) {
console.log(num);
num += 2;
}

B. let num = 1;
while (num < 10) {
if (num % 2 === 0) console.log(num);
num++;
}

C. let num = 2;
while (num < 10) {
console.log(num);
num += 2;
}

D. let num = 0;
while (num <= 10) {
console.log(num);
num += 2;
}

Solution

1. Step 1: Understand the goal and loop conditions

   We want to print even numbers from 2 to 10 inclusive. So the loop must start at 2 and run while number is less than or equal to 10.

2. Step 2: Check each option for correctness

   let num = 2; while (num <= 10) { console.log(num); num += 2; } starts at 2, prints number, then adds 2, stopping at 10. This prints 2,4,6,8,10 correctly. let num = 1; while (num < 10) { if (num % 2 === 0) console.log(num); num++; } starts at 1, uses modulo check, but since num < 10 stops after 9, printing 2,4,6,8 missing 10. let num = 2; while (num < 10) { console.log(num); num += 2; } stops before 10, missing 10. let num = 0; while (num <= 10) { console.log(num); num += 2; } starts at 0, which is not in the required range.

3. Final Answer:

   let num = 2; while (num <= 10) { console.log(num); num += 2; } -> Option A

4. Quick Check:

   Start 2, add 2, stop at 10 [OK]

Hint: Start at 2, add 2 each loop, stop at 10 [OK]

Common Mistakes:

• Starting at 0 or 1 instead of 2
• Stopping before 10
• Not incrementing by 2