Loops

What you'll learn

Count with for
Repeat with while as long as something holds
Add up results in a loop

The for loop

The classic for loop has three parts in its brackets – start, condition, step:

for (let i = 1; i <= 5; i++) {
  console.log("Round", i);
}

let i = 1 – the counter starts at 1
i <= 5 – carry on as long as this holds
i++ – increase by 1 after every round (short for i = i + 1)

Python comparison: for i in range(1, 6): in Python is exactly this loop.

Adding up

let total = 0;
for (let i = 1; i <= 3; i++) {
  total += i;          // short for total = total + i
  console.log("i =", i, "total =", total);
}

The while loop

while repeats as long as a condition is true – handy when you don’t know beforehand how often:

let money = 100;
let years = 0;
while (money < 200) {
  money = money * 1.1;
  years++;
}
console.log(`After ${years} years you have £${Math.round(money)}.`);

Careful: if the condition never becomes false, the loop runs forever. You can then stop it with the Stop button.

Quick check

What does this program print?

let text = "";
for (let i = 3; i > 0; i--) {
  text += i;
}
console.log(text);

Your task

Your turn

Write the function sumUpTo(n) that adds up all numbers from 1 to n and returns the result – sumUpTo(4) gives 1 + 2 + 3 + 4 = 10. Use a loop for it.

function sumUpTo(n) {
  let total = 0;
  // Add up here with a loop
  return total;
}

console.log(sumUpTo(4));

Something to think about
A for loop from 1 to n: for (let i = 1; i <= n; i++) { total += i; }

Gap code – fill in the ___

                        function sumUpTo(n) {
  let total = 0;
  for (___) {
    total += ___;
  }
  return total;
}

console.log(sumUpTo(4));

Solve it without hints to earn a ⭐ on your progress page.

Show solution

Try it yourself first – mistakes teach you the most. Here is one possible solution:

function sumUpTo(n) {
  let total = 0;
  for (let i = 1; i <= n; i++) {
    total += i;
  }
  return total;
}

console.log(sumUpTo(4));