Comment by oneshtein

  /* Produce a quaternion from an axis-angle representation.*/
  function quat_from_axis(theta: number, x: number, y: number, z: number) {    
    const halfcos = Math.cos(theta * 0.5);    
    const halfsin = Math.sin(theta * 0.5);    

    return new Quaternion(
        halfcos,
        axis[0] * halfsin,
        axis[1] * halfsin,
        axis[2] * halfsin,
    );    
  }

Function is vibe coded.