var w = window.innerWidth;

var h = window.innerHeight;  

​

function setup() {

  canvas=createCanvas(w, h);

}

​

function draw() {

  background(255,255,128);

  // A rectangle

  fill(200, 200, 200);

  noStroke();

  rect(20, 20, w-40, h-40);

  // uses global variables for width and height

  }

​

window.onresize = function() {

  // assigns new values for width and height variables

  w = window.innerWidth;

  h = window.innerHeight;  

  canvas.size(w,h);

}