createCanvas(400, 400);

  background(0);

  var x1 = 100;

  var y1 = 100;

  var x2 = 200;

  var y2 = 200;

  var x3 = 200;

  var y3 = 100;

  var x4 = 100;

  var y4 = 300;

  var den = (y4 - y3) * (x2 - x1) - (x4 - x3) * (y2 - y1);

  var ua = (x4 - x3) * (y1 - y3) - (y4 - y3) * (x1 - x3);

  var ub = (x2 - x1) * (y1 - y3) - (y2 - y1) * (x1 - x3);

  ua = ua / den;

  ub = ub / den;

  var candidate = createVector(x1 + ua * (x2 - x1), y1 + ub * (y2 - y1));

  stroke(255);

  line(x1, y1, x2, y2);

  line(x3, y3, x4, y4);

  ellipse(candidate.x, candidate.y, 8, 8);

}