Loop execution flow in Javascript - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to print numbers from 0 to 4 using a for loop.

Javascript

for (let i = 0; i [1] 5; 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 <= 5 will print 0 to 5, which is one extra number.

Using > or >= will not run the loop as expected.

The loop should run while i is less than 5 to print numbers 0 to 4.

2fill in blank

medium

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

Javascript

let sum = 0;
for (let num = 1; num [1] 5; 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 < 5 will exclude 5 from the sum.

Using > or >= will not run the loop correctly.

The loop should include 5, so use num <= 5 to sum 1 through 5.

3fill in blank

hard

Fix the error in the loop condition to print even numbers from 2 to 10.

Javascript

for (let i = 2; i [1] 10; i += 2) {
  console.log(i);
}

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

A>

B<=

C<

D>=

Attempts:

3 left

💡 Hint

Common Mistakes

Using < 10 excludes 10 from the output.

Using > or >= will cause the loop not to run.

The loop should run while i is less than or equal to 10 to include 10.

4fill in blank

hard

Fill both blanks to create an object with numbers as keys and their squares as values for numbers 1 to 4.

Javascript

const squares = {};
for (let n = 1; n <= 4; n++) {
  squares[[1]] = [2];
}

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

An

Bn * n

Cn ** 2

Dn + n

Attempts:

3 left

💡 Hint

Common Mistakes

Using n + n gives double the number, not square.

Using n * n is correct but not the expected answer here.

The keys should be the number n, and the values should be the square using n ** 2.

5fill in blank

hard

Fill all three blanks to create an object with uppercase keys and values greater than 2 from the data object.

Javascript

const filtered = {};
for (const [key, value] of Object.entries(data)) {
  if (value [3] 2) {
    filtered[[1]] = [2];
  }
}

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

Akey.toUpperCase()

Bvalue

C>

D<=

Attempts:

3 left

💡 Hint

Common Mistakes

Using <= includes values less than or equal to 2, which is wrong.

Not converting keys to uppercase.

Keys are uppercase using key.toUpperCase(), values are value, and condition checks if value > 2.

Practice

1. What does a for loop do in JavaScript?

easy

A. Stops the program immediately

B. Runs code only once

C. Creates a new variable

D. Repeats code a set number of times

Solution

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

   A for loop is designed to repeat code multiple times based on a condition.

2. Step 2: Identify the correct description

   Among the options, only repeating code a set number of times matches the for loop behavior.

3. Final Answer:

   Repeats code a set number of times -> Option D

4. Quick Check:

   for loop = repeat fixed times [OK]

Hint: For loops repeat code a fixed number of times [OK]

Common Mistakes:

• Thinking for loops run only once
• Confusing for loops with if statements
• Believing for loops stop the program

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

easy

A. for i = 0 to 5 { }

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

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

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

Solution

1. Step 1: Recall JavaScript for loop syntax

   The correct syntax uses for (initialization; condition; increment) { }.

2. Step 2: Compare options to syntax

   Only for (let i = 0; i < 5; i++) { } matches the correct syntax with let i = 0; i < 5; i++.

3. Final Answer:

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

4. Quick Check:

   for loop syntax = initialization; condition; increment [OK]

Hint: Remember for loop uses three parts inside parentheses [OK]

Common Mistakes:

• Using incorrect keywords like 'loop'
• Swapping initialization and condition
• Missing semicolons inside for parentheses

3. What will be the output of this code?

let sum = 0;
for (let i = 1; i <= 3; i++) {
  sum += i;
}
console.log(sum);

medium

A. 6

B. 3

C. 0

D. Error

Solution

1. Step 1: Trace the loop iterations

   The loop runs with i = 1, 2, 3. Each time, sum adds i: 0+1=1, 1+2=3, 3+3=6.

2. Step 2: Determine final sum value

   After the loop, sum is 6, which is printed by console.log.

3. Final Answer:

   6 -> Option A

4. Quick Check:

   Sum 1+2+3 = 6 [OK]

Hint: Add numbers step-by-step inside the loop [OK]

Common Mistakes:

• Stopping loop early
• Adding wrong values
• Confusing sum initialization

4. Find the error in this loop:

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

medium

A. Console.log is incorrect

B. Syntax error in for loop

C. The loop will run infinitely because i decreases

D. Variable i is not declared

Solution

1. Step 1: Analyze the loop increment/decrement

   The loop uses i--, so i decreases each time starting from 0.

2. Step 2: Check loop condition

   The condition is i < 5, which is true for all i less than 5, including negative numbers. Since i decreases, it never reaches 5 or more, so loop never ends.

3. Final Answer:

   The loop will run infinitely because i decreases -> Option C

4. Quick Check:

   Decrement with < condition causes infinite loop [OK]

Hint: Check if increment matches loop condition direction [OK]

Common Mistakes:

• Confusing i-- with i++
• Ignoring loop condition logic
• Assuming loop stops automatically

5. How many times will this loop run?

let count = 0;
let i = 10;
while (i > 0) {
  i -= 3;
  count++;
}
console.log(count);

hard

A. 4

B. 3

C. 5

D. 6

Solution

1. Step 1: Trace the while loop iterations

   Start i=10, count=0. Each loop: i decreases by 3, count increases by 1.
   Iterations:
   1) i=7, count=1
   2) i=4, count=2
   3) i=1, count=3
   4) i=-2, count=4 (loop stops after this because i > 0 is false)

2. Step 2: Count total iterations

   The loop runs 4 times before i becomes -2 and condition fails.

3. Final Answer:

   4 -> Option A

4. Quick Check:

   Subtract 3 until i ≤ 0 counts 4 times [OK]

Hint: Count how many times i decreases by 3 until ≤ 0 [OK]

Common Mistakes:

• Counting one less or more iteration
• Not updating count correctly
• Misunderstanding while loop condition