For loop in Javascript

Introduction

A for loop helps you repeat a task many times without writing the same code again and again.

When you want to count from 1 to 10 and do something each time.

When you have a list of names and want to greet each person.

When you want to add up numbers in a list.

When you want to repeat an action a fixed number of times.

When you want to process items in an array one by one.

Syntax

Javascript

for (initialization; condition; update) {
  // code to run each time
}

initialization sets a starting point, like let i = 0.

condition is checked before each loop; if true, the loop runs.

update changes the loop variable, usually increasing it.

Examples

This counts from 0 to 4 and prints each number.

Javascript

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

This counts down from 10 to 1 and prints each number.

Javascript

for (let count = 10; count > 0; count--) {
  console.log(count);
}

This goes through a list of fruits and prints each one.

Javascript

const fruits = ['apple', 'banana', 'cherry'];
for (let i = 0; i < fruits.length; i++) {
  console.log(fruits[i]);
}

Sample Program

This program prints numbers from 1 to 5 with a message each time.

Javascript

for (let i = 1; i <= 5; i++) {
  console.log(`Number is ${i}`);
}

Important Notes

Remember the loop stops when the condition becomes false.

You can use break to stop the loop early if needed.

Using let inside the loop keeps the variable local to the loop.

Summary

A for loop repeats code a set number of times.

It has three parts: start, condition, and change.

Use it to work with lists or repeat tasks easily.

Practice

1.

What does a for loop do in JavaScript?

easy

A. Changes the value of a variable once

B. Stops the program immediately

C. Repeats a block of code a set number of times

D. Creates a new variable

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: Compare options with the for loop behavior

   Only Repeats a block of code a set number of times correctly describes repeating code multiple times.

3. Final Answer:

   Repeats a block of code a set number of times -> Option C

4. Quick Check:

   For loop = repeat code [OK]

Hint: For loops repeat code multiple times [OK]

Common Mistakes:

• Thinking for loop stops program
• Confusing variable creation with looping
• Assuming for loop runs only once

2.

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

for (___; ___; ___) {
  // code
}

easy

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

B. for i = 0; i < 5; i++

C. for (i < 5; i = 0; i++)

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

Solution

1. Step 1: Recall the for loop syntax

   The correct syntax is: for (start; condition; change) { ... }

2. Step 2: Check each option against syntax

   for (i = 0; i < 5; i++) matches the correct syntax with initialization, condition, and increment.

3. Final Answer:

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

4. Quick Check:

   Syntax = for (init; cond; change) [OK]

Hint: For loop needs three parts inside parentheses [OK]

Common Mistakes:

• Missing parentheses around for loop parts
• Wrong order of parts inside for loop
• Using semicolons incorrectly

3.

What will be the output of this code?

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

medium

A. "3 2 1"

B. "1 2 3"

C. "123"

D. "1 2 3 4"

Solution

1. Step 1: Understand the loop iterations

   The loop runs with i = 1, 2, 3 because i <= 3.

2. Step 2: Track the string concatenation

   Each iteration adds the number and a space to result: "1 ", then "1 2 ", then "1 2 3 ". trim() removes the last space.

3. Final Answer:

   "1 2 3" -> Option B

4. Quick Check:

   Loop 1 to 3 adds numbers with spaces [OK]

Hint: Check loop range and string building carefully [OK]

Common Mistakes:

• Ignoring the spaces added
• Forgetting trim() removes trailing space
• Assuming loop includes 4

4.

Find the error in this for loop code:

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

medium

A. Missing curly braces around loop body

B. Incorrect variable declaration 'let i = 0'

C. Wrong loop condition 'i < 5'

D. Missing increment operator 'i++' in the loop

Solution

1. Step 1: Check the loop increment part

   The loop has 'i' instead of 'i++' which means i never changes.

2. Step 2: Understand the effect of missing increment

   Without increment, the loop runs forever or does not progress, causing an error or infinite loop.

3. Final Answer:

   Missing increment operator 'i++' in the loop -> Option D

4. Quick Check:

   Increment needed to change loop variable [OK]

Hint: Check all three parts of for loop syntax carefully [OK]

Common Mistakes:

• Forgetting to increment loop variable
• Using wrong condition syntax
• Missing braces but allowed in single line

5.

You want to create a for loop that prints only even numbers from 2 to 10 inclusive. Which code snippet does this correctly?

hard

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

D. for (let i = 2; i <= 10; i++) { if (i % 2 === 1) console.log(i); }

Solution

1. Step 1: Identify the range and condition for even numbers

   We want numbers 2, 4, 6, 8, 10 inclusive.

2. Step 2: Check each option for correctness

   for (let i = 2; i <= 10; i += 2) { console.log(i); } increments by 2 starting at 2, printing only even numbers up to 10. for (let i = 0; i <= 10; i++) { if (i % 2 === 0) console.log(i); } starts from 0 and prints 0 which is outside the 2-10 range. for (let i = 2; i < 10; i++) { if (i % 2 === 0) console.log(i); } misses 10 because it uses i < 10. for (let i = 2; i <= 10; i++) { if (i % 2 === 1) console.log(i); } prints odd numbers, which is wrong.

3. Final Answer:

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

4. Quick Check:

   Increment by 2 prints evens directly [OK]

Hint: Use i += 2 to skip odd numbers [OK]

Common Mistakes:

• Using wrong loop condition (i < 10)
• Printing odd numbers by mistake
• Not incrementing properly to skip odds