错误：不是函数（java脚本）

Question

我有以下代码部分：

	Canvas.prototype.pixelOnMouseOver =	function pixelOnMouseOver(callback){
  var canvas = this._canvas;
  var w = this._canvasSize.width, h=this._canvasSize.height;
  var data = this._canvas.getImageData(0,0,w,h).data;
  canvas.addEventListener('mousemove',function(e){
    var idx = (e.offsetY*w + e.offsetX)*4;
    var parts = Array.prototype.slice.call(data,idx,idx+4);
    callback.apply(canvas,parts);
  },false);

和

	var wasOver;
antCanvas.pixelOnMouseOver(function(r,g,b,a){
  var isOver = a > 10; // arbitrary threshold
  if (isOver != wasOver){
    wasOver = isOver;
  }
  
});

现在我得到了canvas.addEventListener不是函数的错误。我该如何解决这个问题？ Whar是什么原因？

问候

编辑1：这是HTML代码：

<div class="fill pad">
	<div style="border:1px solid #ccc" id="aco-canvas"></div>
    <input type="file" id="fileInput">
    <button id="save_personal_points">Save</button>
    <div class="hr vpad"></div>
    <div>
        <table>
            <tr>
                <td colspan="2"><b>Debug Info</b></td>
            </tr>
            <tr style="display:none;" class="aco-info">
                <td>Interation: </td><td id="iteration-info"></td>
            </tr>
            <tr style="display:none;" class="aco-info">
                <td>Best Distance: </td><td id="best-distance"></td>
            </tr>
			 <tr style="display:none;" class="position">
                <td>Position Info: </td><td id="position"></td>
            </tr>
            <tr id="aco-buttons">
                <td colspan="2"><button id="start-search-btn">Start</button> <button id="clear-graph">Clear</button></td>
            </tr>
        </table>
        

我只发布了我认为有趣的部分。因为它只是没有被这么多代码所禁止....