foo.js

function setup() { 
  createCanvas(400, 400);
  
} 
var pilop = 38;
var pilned = 40;
var pilhøjre = 39;
var pilvenstre = 37;
var x = 2;
var y = 2;
var xret = 0
var yret = 0
var sxs = [];
var sys = [];
function draw() { 

  fill (39,209,42);
  //rect(x,y,20,20);
 	//background(200);
  rect(x*20,y*20,20,20);

  frameRate(5);
  

if (xret!=0 || yret!=0) {
  sxs.push(x);
	sys.push(y);
  console.log(sxs);  
    console.log(sys);
}
  
var i=0;
while(i<sxs.length) {
  i+=1;
  fill (39,209,42);
  //rect(x,y,20,20);
 	//background(200);
  rect(sxs[i]*20,sys[i]*20,20,20);
}
  
  if (xret==1)
  {
    x+=1;
    yret=0;
  }
  if (yret==1)
  {
    y+=1;
    xret=0;
  }
    if (yret==-1)
  {
    y+=-1;
    xret=0;
  }
   if (xret==-1)
  {
    x+=-1;
    yret=0;
  }


}

function keyPressed() {
    if(keyCode == UP_ARROW) {
 			yret=-1;
    	xret=0;
    }
      if(keyCode == DOWN_ARROW) {
 			yret=1;
    	xret=0;
    }
      if(keyCode == LEFT_ARROW) {
 			xret=-1;
    	yret=0;
    }
      if(keyCode == RIGHT_ARROW) {
 			xret=1;
    	yret=0;
    }
}