javascript - JavaScript 无法从函数的数组参数中获取值

Question

我正在尝试根据它们的高度在列中订购一些图像，并且我有以下代码。问题是即使 imgHeights.length 工作正常， imgHeights[i] （在 redrawThumbs 函数中）不会返回值。我究竟做错了什么？谢谢 =)

function redrawThumbs(imgHeights){
  // some irrelevant code here

  // initialise an array that will hold all the column's heights
  var colHeights = new Array(cols);
  for(a=0; a <= colHeights.length - 1; ++a){
    colHeights[a] = 0;
  }

  // take each image's height and add it to the shortest column
  for(i=0; i <= imgHeights.length - 1; ++i){
    var shortestCol = 0;

    for(c=0; c <= colHeights.length - 2; ++c){
      if(colHeights[c+1] < colHeights[c]){
        shortestCol = colHeights[c+1];
      }
    }

    alert("imgHeights[" + i + "] " + imgHeights[i]);

    colHeights[shortestCol] += imgHeights[i];
  }
}

// make an array of image heights
var imgHeights = new Array(totalThumbs);    
for(i=1; i <= totalThumbs; ++i){
  var img = new Image();
  img.onload = function(){
  imgHeights[i-1] = this.height;
}
img.src = i + ".jpg";

// call function that orders the images
redrawThumbs(imgHeights);

Answer 1

问题之一是i变量是通过引用捕获的，而不是通过值捕获的，因为所有的回调img.onload都获得了相同的值i。考虑将onload分配更改为，

var loadedCount = 0;
img.onload = (function (i) { 
    return function () { 
        imgHeights[i - 1] = this.height;
        if (++loadedCount == totalThumbs) allLoaded();
    };
})(i);

function allLoaded() {
    // called after all the images have been loaded.
}

i这会在自动执行函数执行时捕获值的副本，并allLoaded()在加载所有图像后调用。