vectorAngle

Returns the angle (theta) between two vectors.

Use Array.prototype.reduce(), Math.pow() and Math.sqrt() to calculate the magnitude of each vector and the scalar product of the two vectors. Use Math.acos() to calculate the arccos and get the theta value.

const vectorAngle = (x, y) => {
  let mX = Math.sqrt(x.reduce((acc, n) => acc + Math.pow(n, 2), 0));
  let mY = Math.sqrt(y.reduce((acc, n) => acc + Math.pow(n, 2), 0));
  return Math.acos(x.reduce((acc, n, i) => acc + n * y[i], 0) / (mX * mY));
};

vectorAngle([3, 4], [4, 3]); // 0.283794109208328