let lineWidth; 

​

function setup() {

  createCanvas(400, 400);

  lineWidth = random(10, 20)

  background(130); // if you put background in function draw, it wont overlay like it is now

}

​

function draw() {

​

   squareSize = random(10, 250)// (minimum, maximum), random value between min and max. If you put this in 'function draw' it will form a never ending loop. putting it in 'function setup' means it will only change the proportions of the shape whenever you click the play button

  //WITH ALPHA VALUE (TRANSPARENCY)

  rectMode(CENTER);

  strokeWeight (lineWidth) // lineWidth is randomly generated number between 1 and 20

  stroke (0, 0, 255, 20) // (R, G, B)

  fill (0, 255, 0, 20)

  square(200, 200, squareSize) // squareSize is randomly generated number between 10 and 100

  //WITHOUT ALPHA VALUE (OPAQUE)

//   rectMode(CENTER);

//   strokeWeight (lineWidth) // lineWidth is randomly generated number between 1 and 20

  // stroke (0, 0, 255) // (R, G, B)

  // fill (0, 255, 0)

//   square(200, 200, squareSize) // squareSize is randomly generated number between 10 and 100

}