Help converting JavaScript code to ECMAScript 3

Help converting JavaScript code to ECMAScript 3 to solve the angle 𝑎⋅𝑏|𝑎||𝑏|=cos𝜃 where  X0 Y0 . X0 Y20

// What is the angle?
// 𝑎⋅𝑏|𝑎||𝑏|=cos𝜃,  X0 Y0 . X0 Y20

function cosineTheta(a, b) {
  let dotProduct = 0;
  let aMagnitude = 0;
  let bMagnitude = 0;

  // Calculate dot product
  for (let i = 0; i < a.length; i++) {
    dotProduct += a[i] * b[i];
  }

  // Calculate magnitudes
  for (let i = 0; i < a.length; i++) {
    aMagnitude += a[i] * a[i];
    bMagnitude += b[i] * b[i];
  }

  aMagnitude = Math.sqrt(aMagnitude);
  bMagnitude = Math.sqrt(bMagnitude);

  // Calculate cosine
  return dotProduct / (aMagnitude * bMagnitude);
}

// Example usage needs to solve for X0 Y0 . X0 Y20 in 2D plane
// where the resulting angle between the vectors a and b is 90°
const vectorA = [1, 2, 3];
const vectorB = [4, 5, 6];

const cosTheta = cosineTheta(vectorA, vectorB);
console.log(cosTheta);