The following code calculates a unit vector from coordinates (x, y) to the point (x1, y1).

vecTo.js

const degrees = radians => radians * 180 / Math.PI;
const radians = degrees => degrees * Math.PI / 180;
function vecTo(x1, y1, x2, y2) {
  const xDist = x2 - x1;
  const yDist = y2 - y1;
  let direction;
  if (xDist > 0 && yDist > 0) {
    direction = degrees(Math.atan(yDist / xDist));
  } else if (xDist > 0 && yDist < 0) {
    direction = 360 + degrees(Math.atan(yDist / xDist));
  } else {
    direction = 180 + degrees(Math.atan(yDist / xDist));
  }
  return [Math.cos(radians(direction)), Math.sin(radians(direction))];
}