compute angle on unit circle (alternative to built in Math.atan2)

atan2-alternative.js

const angle = (x, y) => {
  let result = Math.atan(y / x);
  if (x < 0) {
    result += Math.PI;
  } else if (y < 0) {
    result += 2 * Math.PI;
  }
  return result;
};