Update the line's path when a tip control point is dragged

updatePathFromTip.js

export function updatePathFromTip(path, index, position) {
  // clone points and update position
  const newPoints = [...path.points];
  newPoints[index] = {
    ...newPoints[index],
    ...position
  };
  // Get the index of the other point to update
  const otherIndex = index === 0 ? 1 : index - 1;
  // Calculate the axis that the edge leading to the tip is on
  const axis = path.points[index].y === path.points[otherIndex].y ? "y" : "x";
  // If the position has changed then update the value on the axis to move the edge with the tip
  if (position[axis] !== path.points[index][axis]) {
    newPoints[otherIndex][axis] = position[axis];
  }
  return {
    ...path,
    points: newPoints
  };
}