Learn what loops are and how to use the for loop to repeat actions in JavaScript.

For Loop

Imagine you want to print numbers 1 to 100. You could write console.log(1), console.log(2) ... all the way to 100. That's 100 lines of the same thing. Nobody does that.

This is exactly the problem loops solve.

A loop is a way to repeat a block of code multiple times without writing it multiple times. You define what to repeat and how many times — JavaScript handles the rest.

for (let i = 1; i <= 100; i++) {
  console.log(i);
}

One block. Runs 100 times. That's the power of a loop.

The `for` Loop

The for loop is the most commonly used loop in JavaScript. You use it when you know exactly how many times you want to repeat something.

for (initialization; condition; update) {
  // code to repeat
}

It has three parts inside the parentheses, separated by semicolons:

Part	What it does	Example
Initialization	Runs once at the start — sets up a counter	`let i = 0`
Condition	Checked before every iteration — if false, loop stops	`i < 5`
Update	Runs after every iteration — moves the counter forward	`i++`

How It Executes — Step by Step

Let's trace exactly what happens with this loop:

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

Step 1 — Initialize:  i = 0
Step 2 — Check:       0 < 5 → true  → run block → prints 0 → i becomes 1
Step 3 — Check:       1 < 5 → true  → run block → prints 1 → i becomes 2
Step 4 — Check:       2 < 5 → true  → run block → prints 2 → i becomes 3
Step 5 — Check:       3 < 5 → true  → run block → prints 3 → i becomes 4
Step 6 — Check:       4 < 5 → true  → run block → prints 4 → i becomes 5
Step 7 — Check:       5 < 5 → false → loop exits

Output: 0 1 2 3 4

The initialization runs once. The condition and update run every iteration.

Basic Examples

Counting up

for (let i = 1; i <= 5; i++) {
  console.log(i);
}
// 1
// 2
// 3
// 4
// 5

Counting down

for (let i = 5; i >= 1; i--) {
  console.log(i);
}
// 5
// 4
// 3
// 2
// 1

Counting in steps

for (let i = 0; i <= 20; i += 5) {
  console.log(i);
}
// 0
// 5
// 10
// 15
// 20

You are not limited to i++. The update can be anything — i--, i += 2, i *= 2.

Looping Through an Array

This is where for loops become genuinely useful. Arrays store lists of data — loops let you go through every item.

const fruits = ["apple", "mango", "banana", "grape"];

for (let i = 0; i < fruits.length; i++) {
  console.log(fruits[i]);
}
// apple
// mango
// banana
// grape

fruits.length is 4 — so the loop runs for i = 0, 1, 2, 3 — which are exactly the valid indexes of the array.

Arrays in JavaScript are zero-indexed — the first item is at index 0, not 1. That's why we start with let i = 0 and use i < array.length instead of i <= array.length.

A real example — calculating a total:

const prices = [120, 450, 89, 300, 55];
let total = 0;

for (let i = 0; i < prices.length; i++) {
  total += prices[i];
}

console.log(`Total: ${total}`); // Total: 1014

Nested `for` Loops

You can put a for loop inside another for loop. The inner loop completes fully for every single iteration of the outer loop.

for (let i = 1; i <= 3; i++) {
  for (let j = 1; j <= 3; j++) {
    console.log(`${i} x ${j} = ${i * j}`);
  }
}
// 1 x 1 = 1
// 1 x 2 = 2
// 1 x 3 = 3
// 2 x 1 = 2
// 2 x 2 = 4
// 2 x 3 = 6
// 3 x 1 = 3
// 3 x 2 = 6
// 3 x 3 = 9

For every value of i, j runs from 1 to 3. This is commonly used for grids, tables, and multi-dimensional data.

A Real Example — Finding the Highest Score

const scores = [72, 95, 88, 61, 99, 74];
let highest = 0;

for (let i = 0; i < scores.length; i++) {
  if (scores[i] > highest) {
    highest = scores[i];
  }
}

console.log(`Highest score: ${highest}`); // Highest score: 99

Loop through every score, keep updating highest whenever you find a bigger one. Simple, readable, effective.

Common Mistakes

Off-by-one error

const letters = ["a", "b", "c"];

// ❌ Wrong — i <= letters.length goes one index too far
for (let i = 0; i <= letters.length; i++) {
  console.log(letters[i]); // last iteration prints undefined
}

// ✅ Correct — i < letters.length
for (let i = 0; i < letters.length; i++) {
  console.log(letters[i]);
}

Infinite loop — forgetting the update

// ❌ Never do this — i never increases, condition is always true
for (let i = 0; i < 5;) {
  console.log(i); // runs forever, crashes the browser
}

Always make sure your update moves the counter toward making the condition false.

Summary

A loop repeats a block of code so you don't have to write it multiple times
The for loop is best when you know exactly how many times to repeat
It has three parts — initialization, condition, and update
Initialization runs once, condition is checked every iteration, update runs after every iteration
When the condition becomes false, the loop exits
Use i < array.length to loop through arrays — arrays start at index 0
Always make sure the loop has an exit condition — otherwise it runs forever

For Loop

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