Sum and Average

Now that you can go through an array, let's look at what loops are used for most often. The first pattern adds everything up. Say you have the prices of the things in your shopping cart and want the total:

const prices = [120, 45, 300, 85];
let total = 0;
for (const price of prices) {
    total += price;
}
console.log("Total:", total);

Total: 550

The variable total collects the sum. It starts at 0, because before you add anything, the sum is zero. In each iteration, total += price adds one price: after the first iteration, total is 120, then 165, 465 and finally 550.

Where each line goes matters:

• let total = 0; goes before the loop, so that it is created only once. If you created it inside the loop, it would start from 0 again in every iteration, and after the loop it wouldn't exist at all: ReferenceError: total is not defined.
• It must start at 0. With just let total;, it would start as undefined, and undefined plus a number gives NaN (“Not a Number”).
• console.log() goes after the loop, when the sum is complete. Inside the loop, it would print every partial sum: 120, 165, 465, 550.

The average is the sum divided by the number of values, and the number of values is the length of the array:

const average = total / prices.length;
console.log("Average:", average);

Average: 137.5

Use prices.length rather than the number 4. When you add another price to the array, the average will still be right.