polygon.js

const polygonArea = (ps) => {
  const v = ps.reduce(
    (acc, p, i) => (acc += p.x * ps.at(i - 1).y - ps.at(i - 1).x * p.y),
    0
  );

  return Math.abs(v) / 2;
};

// or

const polygonArea = (ps) =>
  ps.reduce(
    (acc, p, i) => (acc += Math.abs(p.x * ps.at(i - 1).y - ps.at(i - 1).x * p.y)),
    0
  ) / 2;