Lesson · 10 min

Loops

Do it again

A loop runs the same code many times. Loops save you from writing the same line again and again.

The for loop

A for loop has three parts in its parentheses, separated by semicolons:

for (let i = 1; i <= 3; i++) {
  console.log('Pass number ' + i);
}

Start: let i = 1 runs once, before the loop. It makes a counter.
Condition: i <= 3 is checked before each pass. The loop stops when it is false.
Step: i++ runs after each pass. It adds 1 to i. You can also write i += 2 to add 2.

Each run of the code in the braces is called a pass, or an iteration.

The while loop

A while loop has only a condition. You make the counter yourself, and you change it inside the loop.

let n = 3;
while (n > 0) {
  console.log(n);
  n--;
}

If the condition never becomes false, the loop never ends. Here, if you forget n--, the loop runs forever. Absolute Path stops code that runs for too long, but a loop that never ends is always a bug.

Keeping a running total

A common pattern is to make a variable before the loop and change it in each pass:

let total = 0;
for (let i = 1; i <= 3; i++) {
  total += i;   // the same as total = total + i
}
console.log(total); // 6

Your turn

Some tasks print lines, and some tasks use functions. Do them in order.

Tasks

4 to do

Task 1, not checked yet. Use a for loop to print the numbers 1 to 5, one number on each line.
Task 2, not checked yet. In sumTo, add the numbers from 1 to n with a loop, and return the sum. sumTo(4) is 1 + 2 + 3 + 4, which is 10.
Task 3, not checked yet. In countDown, use a while loop to build the text of the numbers from start down to 1, separated by commas. countDown(3) returns '3,2,1'.
Task 4, not checked yet. Print the even numbers from 2 to 10, one on each line. Change the counter by 2 in each pass.

Task 1 of 4

Use a for loop to print the numbers 1 to 5, one number on each line.

All tasks