var a = document.getElementById("myCanvas");
var b = a.getContext("2d");

var w = b.canvas.width;
var h = b.canvas.height;

drawBranch(w/2, h, 200, 0);

function drawBranch(x, y, length, direction) {
  var x2 = x + length * direction;
  var y2 = y - length;
  drawLine(x, y, x2, y2);
  if (length > 20) {
    drawBranch(x2, y2, length / 2, -0.8);
    drawBranch(x2, y2, length / 2, 0.8);
  }
}

function drawLine(x1, y1, x2, y2) {
  b.beginPath();
  b.strokeStyle = "#1e8214";
  b.moveTo(x1, y1);
  b.lineTo(x2, y2);
  b.stroke();
}