for loops

Repeating with a for loop

• A loop runs the same block many times, so you don't repeat yourself.
• for (let i = 1; i <= 5; i++) { ... } has three parts: start i = 1, test i <= 5, step i++.

for (let i = 1; i <= 3; i++) {
  console.log(i);   // → 1, then 2, then 3
}

Build a running total

• Keep a variable outside the loop and add to it on every pass.

let total = 0;
for (let i = 1; i <= 10; i++) {
  total = total + i;
}
console.log(total);   // → 55

Watch out

• i <= 5 stops after 5; i < 5 stops before 5. Picking the wrong one is the classic "off-by-one" bug.
• Start the total at 0 before the loop, not inside it.

Common mistakes

• for (let i = 0; i < n; i++) runs n times.
• Declare the counter with let, not without.

Now you try

• Use a for loop.
• Press Check answer to test it.