Apply JavaScript loops (for, while, do-while)

21 Programming for the web: JavaScript Loops

🔁 The “for” Loop – A Countdown Race

Think of a for loop like a countdown race. You set a starting point, a finish line, and a step you take each time. The loop keeps going until you reach the finish line.

1. Initialize a counter: let i = 0;
2. Set a condition: i < 10; – keep going while i is less than 10.
3. Increment: i++; – add 1 to i each time.
4. Execute the code block inside the braces.

Example: Print numbers 0–9 to the console.

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

🔄 The “while” Loop – A Waiting Game

Imagine you’re waiting for a bus. You keep checking the clock until the bus arrives. That’s a while loop: it keeps running as long as a condition is true.

1. Set up a condition: let count = 0;
2. Check the condition: while (count < 5)
3. Execute the block and update the counter inside the loop.

Example: Count to 5 and stop.

let count = 0;
while (count < 5) {
    console.log(count);
    count++;
}

🌀 The “do‑while” Loop – A Promise Keeper

A do‑while loop is like a promise: you promise to do something at least once, then keep doing it while the condition is true.

1. Execute the block first.
2. Check the condition at the end.
3. Repeat if the condition remains true.

Example: Ask a user for a password until they enter the correct one.

let password = "";
do {
    password = prompt("Enter password:");
} while (password !== "openSesame");

📊 Loop Syntax Comparison

🧩 Choosing the Right Loop

• for – When you know how many times you’ll loop (e.g., iterate over an array).
• while – When the loop depends on a condition that could be false before the first run.
• do‑while – When you need to run the loop body at least once (e.g., form validation).

💡 Quick Quiz

1️⃣ Which loop guarantees at least one execution?
2️⃣ Write a for loop that prints the even numbers from 2 to 20.
3️⃣ When would you prefer a while loop over a for loop?