🔁 Lesson 6: Loops

Introduction

Adopting the DRY (Don't Repeat Yourself) principle is crucial for maintainable software. Manually writing identical instructions for collection items—such as products in an inventory or users in a list—is inefficient and prone to error.

Loops automate repetitive tasks, enabling efficient data processing and collection iteration. This lesson introduces various loop types, helping you select the appropriate approach for different requirements.

The for Loop

The for loop is the most common loop. It's ideal when you know how many times you want to repeat something.

Anatomy of a for Loop

//       initialization;  condition;    update
for (let i = 0;          i < 5;       i++) {
    console.log(`Iteration ${i}`);
}
// Output: 0, 1, 2, 3, 4  (runs 5 times)
                

Looping Through Arrays by Index

const fruits = ["apple", "banana", "cherry", "date"];

for (let i = 0; i < fruits.length; i++) {
    console.log(`${i + 1}. ${fruits[i]}`);
}
// "1. apple"
// "2. banana"
// "3. cherry"
// "4. date"
                

Counting Backwards

// Countdown from 5 to 1
for (let i = 5; i >= 1; i--) {
    console.log(i);
}
console.log("Liftoff! 🚀");
                

Skipping Values

// Count by 2s
for (let i = 0; i <= 10; i += 2) {
    console.log(i);  // 0, 2, 4, 6, 8, 10
}
                

while & do...while

Use while when you don't know in advance how many iterations you need — the loop keeps running as long as the condition is true.

while Loop

let attempts = 0;
let password = "";

while (password !== "secret123") {
    attempts++;
    // Simulating user input
    password = attempts === 3 ? "secret123" : "wrong";
    console.log(`Attempt ${attempts}: ${password === "secret123" ? "✅ Correct!" : "❌ Wrong"}`);
}
console.log(`Logged in after ${attempts} attempts.`);
// Attempt 1: ❌ Wrong
// Attempt 2: ❌ Wrong
// Attempt 3: ✅ Correct!
// Logged in after 3 attempts.
                

do...while Loop

The do...while loop guarantees the body runs at least once because it checks the condition after each iteration.

let number;

do {
    number = Math.floor(Math.random() * 10) + 1;
    console.log(`Rolled: ${number}`);
} while (number !== 7);

console.log("You rolled a 7! 🎲");
// The loop always runs at least once, then keeps
// going until you roll a 7
                

for...of — Iterating Arrays

The for...of loop is the cleanest way to iterate over arrays (and other iterables like strings). It gives you the value directly — no index needed.

const colors = ["red", "green", "blue", "purple"];

for (const color of colors) {
    console.log(color);
}
// "red"
// "green"
// "blue"
// "purple"
                

Comparing for vs. for...of

const scores = [95, 87, 72, 100, 68];

// Traditional for — you manage the index yourself
for (let i = 0; i < scores.length; i++) {
    console.log(`Score ${i + 1}: ${scores[i]}`);
}

// for...of — cleaner when you don't need the index
for (const score of scores) {
    console.log(`Score: ${score}`);
}
                

Iterating Over Strings

const word = "Hello";

for (const char of word) {
    console.log(char);
}
// "H", "e", "l", "l", "o"
                

When You Need Both Index and Value

const tasks = ["Buy groceries", "Walk the dog", "Write code"];

// Use entries() to get both index and value
for (const [index, task] of tasks.entries()) {
    console.log(`${index + 1}. ${task}`);
}
// "1. Buy groceries"
// "2. Walk the dog"
// "3. Write code"
                

for...in — Iterating Object Keys

The for...in loop iterates over the keys (property names) of an object. It's designed for objects, not arrays.

const user = {
    name: "Ray",
    role: "Developer",
    level: "Senior",
    city: "Henderson"
};

for (const key in user) {
    console.log(`${key}: ${user[key]}`);
}
// "name: Ray"
// "role: Developer"
// "level: Senior"
// "city: Henderson"
                

const colors = ["red", "green", "blue"];

// ❌ for...in on arrays — keys are strings!
for (const key in colors) {
    console.log(typeof key);  // "string" — not a number!
}

// ✅ for...of on arrays — gives you values
for (const color of colors) {
    console.log(color);  // "red", "green", "blue"
}
                    

break & continue

These keywords give you finer control over loop execution.

break — Exit Early

break immediately stops the loop and moves to the code after it.

// Find the first number greater than 50
const numbers = [12, 35, 8, 67, 42, 91, 23];

for (const num of numbers) {
    if (num > 50) {
        console.log(`Found it: ${num}`);
        break;  // Stop searching — we found what we need
    }
}
// "Found it: 67"
// (doesn't check 42, 91, or 23)
                

continue — Skip and Move On

continue skips the rest of the current iteration and jumps to the next one.

// Print only even numbers
for (let i = 1; i <= 10; i++) {
    if (i % 2 !== 0) {
        continue;  // Skip odd numbers
    }
    console.log(i);
}
// 2, 4, 6, 8, 10
                

Real-World Example: Processing Valid Data

const inputs = ["hello", "", "world", null, "JavaScript", undefined, ""];

for (const input of inputs) {
    if (!input) {
        continue;  // Skip empty strings, null, undefined
    }
    console.log(`Processing: ${input.toUpperCase()}`);
}
// "Processing: HELLO"
// "Processing: WORLD"
// "Processing: JAVASCRIPT"
                

Common Pitfalls

1. Infinite Loops

If the condition never becomes false, the loop runs forever and freezes your browser.

// ❌ INFINITE LOOP — i never changes!
// for (let i = 0; i < 10; ) {
//     console.log(i);  // Prints 0 forever
// }

// ❌ INFINITE LOOP — condition always true!
// let count = 0;
// while (count < 5) {
//     console.log(count);
//     // Forgot count++!
// }

// ✅ Always make sure the loop can exit
let count = 0;
while (count < 5) {
    console.log(count);
    count++;  // This makes the condition eventually false
}
                

2. Off-by-One Errors

Starting or ending at the wrong number — the most classic loop bug.

const items = ["a", "b", "c"];  // indices 0, 1, 2

// ❌ Off-by-one: uses <= instead of <
// for (let i = 0; i <= items.length; i++) {
//     console.log(items[i]);  // Last iteration: items[3] → undefined!
// }

// ✅ Correct: < length (not <=)
for (let i = 0; i < items.length; i++) {
    console.log(items[i]);  // "a", "b", "c"
}
                

3. Modifying an Array While Looping

// ❌ Dangerous — removing items shifts indices!
const nums = [1, 2, 3, 4, 5];
// for (let i = 0; i < nums.length; i++) {
//     if (nums[i] % 2 === 0) {
//         nums.splice(i, 1);  // Removes the element, but shifts everything
//     }
// }

// ✅ Safer — loop backwards or use filter()
const odds = nums.filter(n => n % 2 !== 0);
console.log(odds);  // [1, 3, 5]
                

Hands-on Exercise

// TODO: Write a function that takes a rating (1–5)
// and returns a string of stars
// Example: starRating(3) → "★★★☆☆"
// Example: starRating(5) → "★★★★★"

// Hint: Use a for loop to build the string
// Use "★" for filled stars and "☆" for empty stars
                    

// TODO: Given this array of products, find and log
// only the ones that contain the search term

// const products = [
//     "Wireless Mouse",
//     "USB Keyboard",
//     "Wireless Headphones",
//     "Monitor Stand",
//     "Wireless Charger"
// ];
// const searchTerm = "wireless";

// Expected output:
// "Found: Wireless Mouse"
// "Found: Wireless Headphones"
// "Found: Wireless Charger"
// "3 results found."

// Hint: Use .toLowerCase() on both the product name
// and search term for case-insensitive matching
                    

// Part 1: Star Rating Display
function starRating(rating) {
    let stars = "";
    for (let i = 1; i <= 5; i++) {
        stars += i <= rating ? "★" : "☆";
    }
    return stars;
}

console.log(starRating(1));  // "★☆☆☆☆"
console.log(starRating(3));  // "★★★☆☆"
console.log(starRating(5));  // "★★★★★"

// Part 2: Simple Search Filter
const products = [
    "Wireless Mouse",
    "USB Keyboard",
    "Wireless Headphones",
    "Monitor Stand",
    "Wireless Charger"
];
const searchTerm = "wireless";
let matchCount = 0;

for (const product of products) {
    if (product.toLowerCase().includes(searchTerm.toLowerCase())) {
        console.log(`Found: ${product}`);
        matchCount++;
    }
}

console.log(`${matchCount} result${matchCount === 1 ? "" : "s"} found.`);
                        

for (let i = 0; i < 3; i++) {
    console.log(i);
}

Summary

📚 Additional Resources

• MDN — Loops and Iteration
• javascript.info — Loops: while and for
• MDN — for...of
• MDN — for...in

🚀 What's Next?

You can now make decisions and repeat actions. Next, you'll learn to package reusable logic into functions — the building blocks that keep your code organized, DRY, and powerful.