Calculate the angle between two vectors in JavaScript

Given two vectors, you can calculate the angle between them using a few simple math operations. All you need to do is calculate the magnitude of each vector, the scalar product of the two vectors and use the arccosine function to get the theta value.

Simply put, you can use Math.hyptot() to calculate the magnitude of each vector, then multiply the corresponding elements of the two vectors and sum them up. Then, use Array.prototype.reduce() to calculate the scalar product of the two vectors. Finally, divide the scalar product by the product of the magnitudes of the two vectors and pass the result to Math.acos() to get the angle in radians.

const vectorAngle = (x, y) =>
  Math.acos(
    x.reduce((acc, n, i) => acc + n * y[i], 0) /
      (Math.hypot(...x) * Math.hypot(...y))
  );

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