Arrays – many values at once

What you'll learn

Create arrays and read values by index
Add values with push and go through them with for…of
Process arrays with filter and map

Creating an array

An array is a list of values in square brackets. As in Python, counting starts at 0:

const animals = ["dog", "cat", "mouse"];
console.log(animals[0]);
console.log(animals.length);
console.log(animals[animals.length - 1]);   // the last one

Adding and going through

const shopping = ["bread"];
shopping.push("milk");
shopping.push("apples");

for (const item of shopping) {
  console.log("-", item);
}

for…of goes through every element – like for item in shopping: in Python.

Good to know: you can change a const array too (with push). const only means the variable always points to the same array.

filter and map

Arrays have built-in tools. The two most important:

filter keeps only the elements for which a condition holds.
map turns every element into something new.

const numbers = [3, 8, 12, 5, 20];
const big = numbers.filter((n) => n > 6);
const doubled = numbers.map((n) => n * 2);
console.log(big);
console.log(doubled);

(n) => n > 6 is a small function without a name – more about that in the lesson on functions.

Quick check

What does this program print?

const a = [1, 2, 3, 4];
a.push(5);
console.log(a.filter((x) => x > 2).length);

Your task

Your turn

Write two functions. even(numbers) returns a new array with all even numbers. average(numbers) returns the average (total divided by count).

function even(numbers) {
}

function average(numbers) {
}

console.log(even([1, 2, 3, 4, 6]));
console.log(average([2, 4, 9]));

Something to think about
numbers.filter((n) => n % 2 === 0) keeps only the even numbers.
Which command helps?
Add up with for…of and then divide by numbers.length.

Gap code – fill in the ___

                        function even(numbers) {
  return ___;
}

function average(numbers) {
  let total = ___;
  for (___) {
    total += ___;
  }
  return ___;
}

console.log(even([1, 2, 3, 4, 6]));
console.log(average([2, 4, 9]));

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 even(numbers) {
  return numbers.filter((n) => n % 2 === 0);
}

function average(numbers) {
  let total = 0;
  for (const n of numbers) {
    total += n;
  }
  return total / numbers.length;
}

console.log(even([1, 2, 3, 4, 6]));
console.log(average([2, 4, 9]));