全局變量在事件監聽器中變得未定義 - javascript

Question

我有一個函數可以創建一個新的圖像併爲它提供一個事件監聽器，它需要訪問一個全局變量。全局變量（_currentSection）被定義，直到它到達事件偵聽器的聲明，然後變爲未定義。下面是函數：

function relMouseCoords(event) { 
    var totalOffsetX = 0; 
    var totalOffsetY = 0; 
    var canvasX = 0; 
    var canvasY = 0; 
    var currentElement = this; 

    do { 
     totalOffsetX += currentElement.offsetLeft - currentElement.scrollLeft; 
     totalOffsetY += currentElement.offsetTop - currentElement.scrollTop; 
    } while (currentElement = currentElement.offsetParent) 

    canvasX = event.pageX - totalOffsetX; 
    canvasY = event.pageY - totalOffsetY; 

    //alert('x: ' + canvasX + '   y: ' + canvasY); 

    for (var i = 0; i < _currentSection.allHotSpots.length; i++) { 
     if (canvasX > _currentSection.allHotSpots[i].topLeft[0] && 
      canvasX < _currentSection.allHotSpots[i].bottomRight[0] && 
      canvasY > _currentSection.allHotSpots[i].topLeft[1] && 
      canvasY < _currentSection.allHotSpots[i].bottomRight[1]) { 
      //alert('x: ' + canvasX + '   y: ' + canvasY); 
      if (_currentSection.allHotSpots[i].isPicZoom === "false") { 
       appendHotSpot(_currentSection.allHotSpots[i], _currentSection.thePicture, _context, true); 
      } else { 
       picZoomCode = _currentSection.allHotSpots[i].myPicZoom; 
       var img = new Image(); 
       img.onload = function() { 
        _context.drawImage(img, 0, 0, _canvas.width, _canvas.height); 
       } 
       img.src = "data:image/jpeg;base64," + _currentSection.allHotSpots[i].picture; 

       img.addEventListener('load', function() { 
        if (_currentSection.allHotSpots[i].allHotSpots) { 
         for (var j = 0; j < _currentSection.allHotSpots[i].allHotSpots.length; j++) { 
          appendHotSpot(_currentSection.allHotSpots[i].allHotSpots[j], _currentSection.allHotSpots[i].thePicture, _context, false) 
         } 
        } 
       }, false); 
      } 
     } 
    } 
} 
HTMLCanvasElement.prototype.relMouseCoords = relMouseCoords; 

左右的時間內img.addEventListener我需要訪問_currentSection，但它只是剛剛進去img.addEventListener不確定的。我如何保持它的定義？

Answer 1

的東西在這個崗位類似項目8是怎麼回事，我懷疑：

What is the scope of variables in JavaScript?

Answer 2

試着把變量放在var currentElement = this;