p5_example.js

var x = 100;
var y = 100;

function setup() {
  createCanvas(200,200);
}

function draw() {
  background(0);
  fill(255);
  ellipse(x,y,50,50);
}

p5_instance.js

var s = function( sketch ) {

  var x = 100; 
  var y = 100;

  sketch.setup = function() {
    sketch.createCanvas(200, 200);
  };

  sketch.draw = function() {
    sketch.background(0);
    sketch.fill(255);
    sketch.rect(x,y,50,50);
  };
};

var myp5 = new p5(s);