JavaScript for Loop – In Details

🚀 What is a for Loop?

A for loop is a control structure in JavaScript that allows you to repeat a block of code a specific number of times. It's one of the most commonly used loops in programming.

✅ Syntax:

for (initialization; condition; increment/decrement) {
  // code to execute
}

• Initialization: Set a variable (e.g., let i = 0)

• Condition: Checked before each iteration (e.g., i < 5)

• Increment: Runs after each iteration (e.g., i++)

🎯 Use when you know how many times you want to loop (like counting from 0 to 4).

📝 Key Features of for Loop

📚 Step-by-Step Dry Run of for Loops

🔍 Dry Run = Simulate each step of the loop execution — helps understand what happens under the hood.

🔁 Example 1: Basic Loop — for(let i=0; i<5; i++)

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

✅ Dry Run (Step-by-Step)

✅ Output:

Hello, Sagar
Hello, Sagar
Hello, Sagar
Hello, Sagar
Hello, Sagar

🚨 Total: 5 iterations (0 to 4)

🔁 Example 2: Custom Increment — i = i + 1

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

✅ Dry Run

✅ Output:

4
5
6
7
8
9
10
11

🚨 Loop runs from 2 to 9 (inclusive), prints i+2

🔁 Example 3: Step-by-2 — i = i + 2

for (let i = 2; i < 9; i = i + 2) {
  console.log('hello world');
}

✅ Dry Run

✅ Output:

hello world
hello world
hello world
hello world

✅ Only prints for even numbers: 2, 4, 6, 8

⚠️ Note: Stops before i = 9 (since i < 9)

🔁 Example 4: Decrementing Loop — i = i - 1

for (let i = 5; i > 0; i = i - 1) {
  console.log('Hello world');
}

✅ Dry Run

✅ Output:

Hello world
Hello world
Hello world
Hello world
Hello world

✅ 5 times — counts down from 5 to 1

🟡 i-- and i = i - 1 are equivalent — both decrement by 1

❌ Example 5: Infinite Loop (Mistake!)

for (i = 0; i > 0; i++) {
  console.log('hello world');
}

❌ Why is this wrong?

🚨 This loop doesn't execute — condition fails from the start.

🚨 If you meant for (i=0; i<=5; i++) — this is a different infinite loop.

✅ Correct Version (Infinite Loop — Misused)

// ❌ This will cause infinite loop (if condition never fails)
for (let i = 0; i > 0; i++) {
  console.log('hello world');
}

❌ Condition i > 0 fails at start (i=0) → loop never runs

✅ To cause infinite loop:

for (let i = 0; true; i++) {
  console.log('This will never stop!');
}

⚠️ Never use for loops with true condition without break or return

✅ Example 6: Function Inside Loop

function greet() {
  console.log("Hello, Sagar!");
}

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

✅ Dry Run

✅ Output:

Hello, Sagar!
Hello, Sagar!
Hello, Sagar!
Hello, Sagar!
Hello, Sagar!

🎯 You can pass logic, conditions, or math inside loops.

🔄 Example 7: While Loop (Alternative)

let i = 0;

while (i < 5) {
  console.log('Hello World');
  i++;
}

✅ Dry Run

✅ Output:

Hello World
Hello World
Hello World
Hello World
Hello World

✅ while loop is useful when you don't know the number of iterations in advance.

📦 Example 8: for Loop with Array

let arr = [2, 2, 3, 5, 6, 7, 8, 9, 10, 11];

console.log(arr.length); // 10
console.log(arr[0]);     // 2
console.log(arr[1]);     // 2
console.log(arr[2]);     // 3
console.log(arr[3]);     // 5
console.log(arr[4]);     // 6
console.log(arr[5]);     // 7
console.log(arr[6]);     // 8
console.log(arr[7]);     // 9
console.log(arr[8]);     // 10
console.log(arr[9]);     // 11

✅ Use Loop to Access All Elements

console.log('-------------------------');
for (let i = 0; i < arr.length; i++) {
  console.log(arr[i]);
}

✅ Dry Run

✅ Output:

2
2
3
5
6
7
8
9
10
11

🚀 Alternative to manually writing indices — much cleaner and scalable!

🔍 Filter: Print Only Even Numbers

console.log('------------Even Numbers--------------');
for (let i = 0; i < arr.length; i++) {
  if (arr[i] % 2 == 0) {
    console.log(arr[i]);
  }
}

✅ Output:

2
2
6
8
10

🎯 Even numbers: divisible by 2 → remainder 0

🔍 Filter: Print Only Odd Numbers

console.log('------------Odd Numbers--------------');
for (let i = 0; i < arr.length; i++) {
  if (arr[i] % 2 == 1) {
    console.log(arr[i]);
  }
}

✅ Output:

3
5
7
9
11

🎯 Odd numbers: remainder 1 when divided by 2

📌 Summary Table

🚨 Common Mistakes & Fixes

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📚 Final Tips

• Use let for loop variables — avoids hoisting issues

• Always test your loops with dry runs

• Use i < arr.length for arrays

• Filter data with if conditions inside loops

• Avoid infinite loops — always check the condition

📌 Ready for coding interviews? Practice these loops with arrays, conditions, and functions. This guide will help you understand, debug, and explain for loops confidently.

💡 Next Step: Learn about for...of loops, forEach(), and map() for more advanced array iteration.

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