vectorAngle

Calculates 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 arccosine 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