Why loops are needed in Javascript - Test Your Understanding

Practice - 5 Tasks

1fill in blank

easy

Complete the code to print numbers from 1 to 3 using a loop.

Javascript

for(let i = 1; i [1] 4; i++) {
  console.log(i);
}

Drag options to blanks, or click blank then click option'

A>

B<

C<=

D==

Attempts:

3 left

💡 Hint

Common Mistakes

Using '>' will never run the loop.

Using '==' will only run when i equals 4.

The loop should run while i is less than 4 to print numbers 1, 2, and 3.

2fill in blank

medium

Complete the code to sum numbers from 1 to 5 using a loop.

Javascript

let sum = 0;
for(let num = 1; num [1] 6; num++) {
  sum += num;
}
console.log(sum);

Drag options to blanks, or click blank then click option'

A<=

B<

C>

D==

Attempts:

3 left

💡 Hint

Common Mistakes

Using '<' will exclude 5, which is needed.

Using '>' will not run the loop.

The loop should run while num is less than or equal to 5 to include numbers 1 through 5.

3fill in blank

hard

Fix the error in the loop condition to print 'Hello' 3 times.

Javascript

for(let count = 0; count [1] 3; count++) {
  console.log('Hello');
}

Drag options to blanks, or click blank then click option'

A<

B>=

C==

D>

Attempts:

3 left

💡 Hint

Common Mistakes

Using '>=' will cause the loop to never run.

Using '==' will only run once.

The loop should run while count is less than 3 to print 'Hello' three times.

4fill in blank

hard

Fill both blanks to create a loop that prints even numbers from 2 to 10.

Javascript

for(let i = [1]; i [2] 11; i += 2) {
  console.log(i);
}

Drag options to blanks, or click blank then click option'

A2

B<

C<=

D1

Attempts:

3 left

💡 Hint

Common Mistakes

Starting from 1 will print odd numbers.

Using '<=' will include 12 which is not needed.

Start from 2 and loop while i is less than 11 to print even numbers up to 10.

5fill in blank

hard

Fill all three blanks to create a loop that prints numbers from 5 down to 1.

Javascript

for(let num = [1]; num [2] 0; num [3]) {
  console.log(num);
}

Drag options to blanks, or click blank then click option'

A5

B>

C--

D<

Attempts:

3 left

💡 Hint

Common Mistakes

Using '<' will cause the loop to never run.

Using '++' will increase instead of decrease.

Start from 5, loop while num is greater than 0, and decrease num by 1 each time.

Practice

1. Why do we use loops in JavaScript?
for or while loops help us to:

easy

A. Repeat a task multiple times without writing it again

B. Write code only once and never run it

C. Make the program run slower

D. Stop the program from running

Solution

1. Step 1: Understand the purpose of loops

   Loops allow repeating the same action many times without rewriting the code.

2. Step 2: Compare options with loop purpose

   Only Repeat a task multiple times without writing it again matches this purpose by saying loops repeat tasks multiple times.

3. Final Answer:

   Repeat a task multiple times without writing it again -> Option A

4. Quick Check:

   Loops repeat tasks = B [OK]

Hint: Loops repeat code to save writing it many times [OK]

Common Mistakes:

• Thinking loops slow down the program
• Believing loops stop the program
• Confusing loops with one-time code

2. Which of these is the correct syntax to start a for loop in JavaScript?

easy

A. for (let i = 0; i < 5; i++) { }

B. for i in range(5) { }

C. loop (i = 0; i < 5; i++) { }

D. for i = 0 to 5 { }

Solution

1. Step 1: Recall JavaScript for loop syntax

   The correct syntax uses parentheses with initialization, condition, and increment separated by semicolons.

2. Step 2: Match options to syntax

   for (let i = 0; i < 5; i++) { } matches this exactly: for (let i = 0; i < 5; i++) { }.

3. Final Answer:

   for (let i = 0; i < 5; i++) { } -> Option A

4. Quick Check:

   JS for loop syntax = D [OK]

Hint: JS for loops use semicolons inside parentheses [OK]

Common Mistakes:

• Using Python or other language syntax
• Missing parentheses or semicolons
• Using wrong keywords like 'loop'

3. What will this code print?

for (let i = 1; i <= 3; i++) {
  console.log(i);
}

medium

A. 0 1 2

B. 1 2 3

C. 1 2 3 4

D. 3 2 1

Solution

1. Step 1: Analyze loop start and end values

   The loop starts at i=1 and runs while i is less than or equal to 3.

2. Step 2: List printed values

   It prints 1, then 2, then 3, then stops because i becomes 4.

3. Final Answer:

   1 2 3 -> Option B

4. Quick Check:

   Loop prints 1 to 3 = C [OK]

Hint: Check loop start, end, and condition carefully [OK]

Common Mistakes:

• Starting count from 0 instead of 1
• Including number 4 by mistake
• Printing in reverse order

4. Find the error in this loop:

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

medium

A. Missing opening brace { after for statement

B. Incorrect loop condition

C. Variable i is not declared

D. Extra closing brace } without matching {

Solution

1. Step 1: Check braces in the code

   The loop has no opening brace { but has a closing brace } at the end.

2. Step 2: Identify mismatch

   Extra closing brace } without a matching opening brace causes syntax error.

3. Final Answer:

   Extra closing brace } without matching { -> Option D

4. Quick Check:

   Brace mismatch error = A [OK]

Hint: Count opening and closing braces carefully [OK]

Common Mistakes:

• Forgetting to add opening brace {
• Assuming missing semicolon causes error here
• Not noticing extra closing brace

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

hard

A. for (let i = 2; i < 10; i++) { console.log(i); }

B. for (let i = 0; i <= 10; i++) { if (i % 2 === 0) console.log(i); }

C. for (let i = 2; i <= 10; i += 2) { console.log(i); }

D. for (let i = 0; i <= 10; i += 3) { console.log(i); }

Solution

1. Step 1: Understand the goal

   We want to print even numbers from 2 to 10 inclusive.

2. Step 2: Check each option

   for (let i = 2; i <= 10; i += 2) { console.log(i); } starts at 2, increments by 2, and stops at 10, printing 2,4,6,8,10 correctly.

3. Final Answer:

   for (let i = 2; i <= 10; i += 2) { console.log(i); } -> Option C

4. Quick Check:

   Increment by 2 from 2 to 10 = A [OK]

Hint: Use step of 2 to print even numbers directly [OK]

Common Mistakes:

• Printing odd numbers by mistake
• Stopping before 10
• Using wrong increment step