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Beginner-Friendly Topic

Take your time - it's perfectly normal to re-read this topic 2-3 times. Try the interactive code editor below to run code yourself. Use the Q&A section to check your understanding before moving on. You've got this! 🚀

for Loop

The for loop is the most common loop in programming. It repeats a block of code a specific number of times. Use it for counting, iterating arrays, and any task that needs repetition.

15 min•By Priygop Team•Updated 2026

for Loop Structure

for (init; condition; update) { } - Three parts: initialization, condition check, update
init - Runs once before the loop starts: let i = 0
condition - Checked before each iteration. Loop stops when false: i < 10
update - Runs after each iteration: i++ (increment by 1)
Common pattern - for (let i = 0; i < array.length; i++) { ... }
Use cases - Counting, iterating arrays by index, repeating operations N times

for Loop Code

// Count 1 to 5
for (let i = 1; i <= 5; i++) {
  console.log("Count:", i);
}

// Iterate an array
const fruits = ["Apple", "Banana", "Cherry", "Date"];
for (let i = 0; i < fruits.length; i++) {
  console.log(`${i + 1}. ${fruits[i]}`);
}

// Count backwards
for (let i = 5; i >= 1; i--) {
  console.log("Countdown:", i);
}
console.log("🚀 Launch!");

// Skip by 2
for (let i = 0; i <= 10; i += 2) {
  console.log("Even:", i);  // 0, 2, 4, 6, 8, 10
}

// Sum of numbers 1-100
let sum = 0;
for (let i = 1; i <= 100; i++) {
  sum += i;
}
console.log("Sum 1-100:", sum);  // 5050

Try It Yourself: Multiplication Table

Try It Yourself: Multiplication TableJavaScript

JavaScript Editor

✓ ValidTab = 2 spaces

// Generate multiplication table
const num = 7;
console.log("=== " + num + " Times Table ===");

for (let i = 1; i <= 10; i++) {
  console.log(num + " x " + i + " = " + (num * i));
}

// Try changing num to see different tables!

Result

Click ▶ Run to see the result

Edit the code on the left, then click Run (Ctrl+Enter)

JavaScript|9 lines|228 chars|✓ Valid syntax

UTF-8

Tip

Always use let (not var or const) for the loop variable: for (let i = 0; ...). let keeps the variable scoped to the loop. Using var causes it to leak outside, and const would error on i++ since it can't be reassigned.

Diagram

Loading diagram…

map/filter return new arrays; sort/splice mutate the original

Common Mistake

Warning

Using <= instead of < with array.length causes an off-by-one error. for (let i = 0; i <= array.length; i++) accesses array[array.length] which is undefined. Always use i < array.length for zero-based arrays.

Practice Task

Note

Practice for loops: (1) Print numbers 1-20. (2) Sum all even numbers from 1-100. (3) Print a countdown from 10 to 1 followed by 'Launch!'. (4) Generate the 7 times table from 1-12.

Quick Quiz

Key Takeaways

The for loop is the most common loop in programming.
for (init; condition; update) { } - Three parts: initialization, condition check, update
init - Runs once before the loop starts: let i = 0
condition - Checked before each iteration. Loop stops when false: i < 10

Topics in This Module