let y1 = 50;

let x2 = 350;

let y2 = 350;

let x3 = 300;

let y3 = 101;

let x4 = 350;

let y4 = 350;

let t, u, px, py;

​

function setup() {

  createCanvas(400, 400);

}

​

function draw() {

  background(0);

  let x4 = mouseX;

  let y4 = mouseY;

  t = ((x1-x3)*(y3-y4)-(y1-y3)*(x3-x4))/((x1-x2)*(y3-y4)-(y1-y2)*(x3-x4));

  u = -((x1-x2)*(y1-y3)-(y1-y2)*(x1-x3))/((x1-x2)*(y3-y4)-(y1-y2)*(x3-x4)); 

  px = x1+t*(x2-x1);

  py = y1+t*(y2-y1);

  stroke(255);

  strokeWeight(3);

  line(x1, y1, x2, y2);

  line(x3, y3, x4, y4);

  stroke(255, 0, 0);

  strokeWeight(10);

  if ((0 <= t && t <= 1) && (0 <= u && u <= 1)) {

    point(px, py);

  }

}