aoc3.js

const data = [
  [151, 671, 11, 15],
  [887, 913, 28, 22],
] // etc

// get size of square
const longest = data.reduce((acc, arr) => {
  const x = arr[0] + arr[2];
  const y = arr[1] + arr[3];
  const longerCoord = x > y ? x : y;
  return acc > longerCoord ? acc : longerCoord;
}, 0)

// pt 1

const square = new Array(longest + 1).fill(new Array(longest + 1).fill(0))

const applyCoordsToSquare = (square, coords) => {
  return square.map((row, i) => {
    if (coords[1] <= i && i < (coords[1] + coords[3])) {
      return row.map((point, i) => {
        if (coords[0] <= i && (i < coords[0] + coords[2])) {
          return point + 1;
        }
        return point;
      })
    }
    return row
  })
}

const filledSquare = data.reduce(applyCoordsToSquare, square);

const totalOverlapping = filledSquare.reduce((acc, row) => acc + row.reduce((acc, point) => point > 1 ? acc + 1 : acc, 0), 0)