let pB = {};

​

function setup() {

  createCanvas(400, 400);

  pA.x = 0;

  pA.y = 0;

  pB.x = width;

  pB.y = height;

}

​

function draw() {

  background(220);

  line(pA.x, pA.y, pB.x, pB.y);

  pA.y = lerp(0, height, (frameCount % 40) / 40);

  pB.y = lerp(height, 0, (frameCount % 40) / 40);

}