For Loops

Mission: Decision Desk

A loop can visit each value in a list, adding to a total or collecting a new order.

Loops let you repeat a block of code. An array such as [2, 4, 6] is an ordered list; you will study arrays in depth later, but here you only need to visit its values. numbers.length counts entries, numbers[i] reads one entry (starting at index 0), and result.push(value) adds an entry to a new array. To log a list on one line, result.join(",") joins its values with commas.

Traditional for loop

for (let i = 0; i < 5; i++) {
    console.log(i); // five separate lines: 0 through 4
}

Iterating an array:

const fruits = ["apple", "banana", "cherry"];
for (let i = 0; i < fruits.length; i++) {
    console.log(fruits[i]);
}

for...of — iterate values

Cleaner syntax when you only need the value, not the index:

for (const fruit of fruits) {
    console.log(fruit);
}

The fruits list here is the same list from the preceding example. If you copy this block into a new script, declare fruits first.

For the reverse task, start at arr.length - 1 (the last index), step backward with i--, and push each visited value into a new array. An empty array gives a sum of 0 because the loop runs zero times. Object-key iteration comes in the Objects module; it is not needed here.