Why loops are needed in Javascript - Challenge Your Understanding

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of a simple for loop

What will be the output of this JavaScript code?

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

Javascript

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

A 1 2 3

B 1 2 3 4

C 3 2 1

D 123

💡 Hint

Look at how the loop counts from 1 to 3 and adds each number with a space.

The loop runs from 1 to 3, adding each number followed by a space to the string. The trim() removes the last space, so the output is '1 2 3'.

🧠 Conceptual

intermediate

Why use loops instead of repeating code?

Why do programmers use loops instead of writing the same code many times?

Alice wants to print numbers 1 to 5. Which reason best explains why she should use a loop?

A Loops are harder to read than repeating code.

B Loops make the computer run slower.

C Loops make code shorter and easier to change if needed.

D Loops only work with numbers, not text.

💡 Hint

Think about what happens if Alice wants to print numbers 1 to 100 instead of 1 to 5.

Loops let you write code once and repeat it many times, making code shorter and easier to update.

❓ Predict Output

advanced

Output of nested loops

What will this JavaScript code print?

let output = '';
for (let i = 1; i <= 2; i++) {
  for (let j = 1; j <= 2; j++) {
    output += i + '-' + j + ' ';
  }
}
console.log(output.trim());

Javascript

let output = '';
for (let i = 1; i <= 2; i++) {
  for (let j = 1; j <= 2; j++) {
    output += i + '-' + j + ' ';
  }
}
console.log(output.trim());

A 1-1 2-2 1-2 2-1

B 1-1 2-1 1-2 2-2

C 1-1 1-2 2-2 2-1

D 1-1 1-2 2-1 2-2

💡 Hint

The outer loop controls i, inner loop controls j. Inner loop runs fully for each i.

For i=1, j=1 and j=2 run; then for i=2, j=1 and j=2 run. So output is '1-1 1-2 2-1 2-2'.

❓ Predict Output

advanced

Loop with break statement output

What will this code print?

let text = '';
for (let i = 1; i <= 5; i++) {
  if (i === 3) break;
  text += i + ' ';
}
console.log(text.trim());

Javascript

let text = '';
for (let i = 1; i <= 5; i++) {
  if (i === 3) break;
  text += i + ' ';
}
console.log(text.trim());

A 1 2 3

B 1 2

C 3 4 5

D 1 2 4 5

💡 Hint

The loop stops when i equals 3 before adding it to text.

When i is 3, break stops the loop immediately, so only 1 and 2 are added.

🧠 Conceptual

expert

Why loops help with repetitive tasks

Imagine you want to send a greeting message to 100 friends. Which statement best explains why using a loop is the best choice?

A Loops let you write one message and send it 100 times automatically.

B Loops make the program slower because it repeats the message many times.

C Loops only work if all friends have the same name.

D Loops are only useful for counting numbers, not sending messages.

💡 Hint

Think about how much work it would be to write 100 lines of code to send messages.

Loops let you repeat actions many times without writing the same code again and again.