Edit in JSFiddle

<canvas id="knight" width="300" height="300"></canvas>

              
let canvas, ctx, width, height;

let Game = function(){

  canvas = document.getElementById('knight');
  ctx = canvas.getContext('2d');
  ctx.fillStyle = 'red';

  canvas = width = window.innerWidth;
  canvas = height = window.innerHeight;

  var gameSize = {

  }
	this.knight();
  this.loadGame();
}

const bg = new Image();
bg.src = "https://placehold.it/300";

Game.prototype.startGame = function(){
  this.loadGame();
}


Game.prototype.loadGame = function(){
  
  this.update();
  this.draw();

  requestAnimationFrame(()=>{
 		this.loadGame();
  });
}


Game.prototype.knight = function(game, gameSize,position){
  this.game = game;
  this.position = {x : 150, y : 150};
  this.size = {x : 200, y : 230};
  this.current = 0;
  //this.knight = [];
}

Game.prototype.update = function(){
  
}

let i = 0;

Game.prototype.draw = function(){
  ctx.drawImage(bg, 0, 0);
  ctx.fillRect(this.position.x++, this.position.y, 150, 150);
}

new Game("ctx");
window.onload = function(){
  
}