问如何在document.onkeydown中使用箭头键移动对象？

EN

window.onload = function(){
var canvas = document.getElementById('myCanvas');
var context = canvas.getContext('2d');

var x = 0;
var y = 0;

context.fillStyle = "#c409a3";
context.fillRect(x, y, 100, 100);
}

document.onkeydown = function(e){
alert(e.keycode);
var rect = new Object();
rect.x = 0;
rect.y = 0;
rect.moveright = function(){rect.x++;}
rect.draw = function(){
   context.fillStyle = "#c409a3";
   context.fillRect(rect.x, rect.y, 100, 100)

}
}
rect.erease = function(){
}
rect.moveleft = function(){rect.x--;}

rect.moveup = function(){rect.y++;}

rect.movedown = function(){rect.y--;}


if (e.keyCode == "39"){
    rect.erease();
    rect.moveright();
    rect.draw();
}
if (e.keyCode == "37"){
   rect.erease();
   rect.moveleft();
   rect.draw();
}
if (e.keyCode == "38"){
   rect.erease();
   rect.moveup();
   rect.draw();
}
if (e.keyCode == "40"){
   rect.erease();
   rect.movedown();
   rect.draw();
}