Arrays

An array is an ordered list of values. You'll reach for arrays any time you have "a list of things" — numbers, names, todo items, etc.

What you'll learn

• Create arrays with [value1, value2, ...]
• Read elements by index (arr[0]) and check the size with arr.length
• Iterate with for...of
• Transform arrays with .map(...) and .filter(...)

Instructions

Write a function sumOfSquares(nums) that takes an array of numbers and returns the sum of each number squared. For example:

• sumOfSquares([1, 2, 3]) → 14 (because 1 + 4 + 9)
• sumOfSquares([]) → 0

Key concepts

Creating and indexing arrays

const colors = ["red", "green", "blue"];
console.log(colors[0]);      // "red"
console.log(colors.length);  // 3

Indexing starts at 0, and colors[colors.length - 1] is the last item.

Adding and iterating

const numbers = [];
numbers.push(10);
numbers.push(20);

for (const n of numbers) {
  console.log(n);
}

push appends. for...of walks each element without managing a counter.

Transforming: map and filter

These return new arrays — the originals are untouched.

const doubled = [1, 2, 3].map((n) => n * 2);
// [2, 4, 6]

const positives = [-2, -1, 0, 3, 7].filter((n) => n > 0);
// [3, 7]

The (n) => expression syntax is an arrow function — a compact way to write a function inline. It's exactly like writing:

function double(n) { return n * 2; }

Reducing to a single value

When you need to collapse an array into one value (a sum, a count, a max), .reduce is the tool:

const total = [1, 2, 3, 4].reduce((acc, n) => acc + n, 0);
// 10

The second argument (0 here) is the starting accumulator.

Hints

1. Each element squared — then add them all up.
2. Map squares the numbers, then sum them with .reduce or a loop.
3. return nums.map((n) => n * n).reduce((a, b) => a + b, 0);