Can't make square on canvas move, and now the

Question

I am trying to make an already drawn square move with the WASD keys.

I wasn't sure how to do this, so I looked up some code, and after about 2 hours came up with my own non-working code. It wasn't working, but at least it was drawing my square... Or was.

Now it isn't, and I have no clue why, here's my JavaScript:

    function initCanvas(){
    var ctx = document.getElementById('my_canvas').getContext('2d');
    ctx.addEventListener("keydown", move, true);

    function move(event){
            //W
            if(event.keyCode == 87){
                    y = y + 20;
            }
            //A
            else if(event.keyCode == 65){
                    x = x - 20;
            }

            //S
            else if(event.keyCode == 83){
                    y = y + 20;
            }

            //D
            else if(event.keyCode == 68){
                    x = x + 20;
            }  
    }

    var x = 0;
    var y = 0;

    ctx.fillStyle = "green";
    ctx.fillRect(x + 20, y + 20, 20, 20);

    }

 window.addEventListener('load', function(event){
    initCanvas();
 });

And HTML/CSS (entire page): http://pastebin.com/wjXv5tdK It probably has to do with the Event Listener because it seems to work without it.

TL;DR

So I basically want a square to be drawn on the canvas, and have the user control it using the WASD keys.

Answer 1

Listen for keyboard events on the document, not the context.

document.addEventListener("keydown",move,false);

Here's some annotated code to get your started again:

// create canvas related variables
var canvas=document.getElementById("canvas");
var ctx=canvas.getContext("2d");
var cw=canvas.width;
var ch=canvas.height;

// set canvas to be a tab stop (necessary to give it focus)
canvas.setAttribute('tabindex','0');

// set focus to the canvas
canvas.focus();

// create an x & y indicating where to draw the rect
var x=150;
var y=150;

// draw the rect for the first time
draw();

// listen for keydown events on the document
// the canvas does not trigger key events
document.addEventListener("keydown",handleKeydown,false);

// handle key events
function handleKeydown(e){

  // if the canvas isn't focused,
  // let some other element handle this key event
  if(e.target.id!=='canvas'){return;}

  // change x,y based on which key was down
  switch(e.keyCode){
    case 87: x+=20; break;  // W
    case 65: x-=20; break;  // A
    case 83: y+=20; break;  // S
    case 68: y-=20; break;  // D
  } 

  // redraw the canvas
  draw();
}

// clear the canvas and redraw the rect in its new x,y position
function draw(){
  ctx.clearRect(0,0,cw,ch);
  ctx.fillRect(x,y,20,20);
}

body{ background-color: ivory; padding:10px; }
#canvas{border:1px solid red;}

<h4>Click in the canvas to have it respond to keys</h4>
<canvas id="canvas" width=300 height=300></canvas>

Answer 2

The reason the square doesn't draw anymore is you are trying to attach an event listener to a canvas context, and you can only attach listeners to the DOM object (the canvas). So if you change the statements to (for example):

    var canvas = document.getElementById('my_canvas');
    canvas.addEventListener("keydown", move, true);

And leave the ctx statements as they are the canvas will draw again. Unless you really need a canvas you might be better off using an svg img.