draw_arrow_vector.js

function draw_arrow(context, x, y, angle, magnitude) {
  const head_length = 10;
  const dx = Math.cos(angle) * magnitude;
  const dy = Math.sin(angle) * magnitude;
  const end_x = x + dx;
  const end_y = y + dy;
  context.beginPath();
  context.moveTo(x, y);
  context.lineTo(end_x, end_y);
  context.lineTo(end_x - head_length * Math.cos(angle - Math.PI / 6), end_y - head_length * Math.sin(angle - Math.PI / 6));
  context.moveTo(end_x, end_y);
  context.lineTo(end_x - head_length * Math.cos(angle + Math.PI / 6), end_y - head_length * Math.sin(angle + Math.PI / 6));
  context.stroke();
  context.closePath();
}