javascript - substr() with negative value not working in IE

Question

EDIT:I've changed the title, because the issue had nothing to do with IE image.load() firing - my substr() wasn't working (see accepted answer).

---

There's a ton of posts about making sure that you define your onload handler prior to assigning img.src in order to guarantee that the onload handler is in place in case the image is loaded from cache first.

This does not appear to the be issue in my code, since that's precisely what I have done.

Note that this script works across all other browsers, but IE 8 and lower does not trigger the onload inline function.

var i = lastSlideIndex || 1;

while(imagesQueued < MAX_IMAGES){
    var preloadImage = new Image();
    preloadImage.arrayIndex = i;
    preloadImage.onload = function(eventObj){
        debug('Image ' + this.src + ' loaded.');
        slideshowImages[this.arrayIndex] = this;
        if (this.arrayIndex > lastImageIndex) lastImageIndex = this.arrayIndex;
        triggerSlideshow(this.arrayIndex);
    }

    // add leading zeros
    var leadingZeros = "0000000".substr(-(String(MAX_IMAGES).length));
    imageNumber = leadingZeros.substr(String(i).length) + i;

    debug('preloading Image #' + imageNumber);
    preloadImage.src = fullImagePrefix + imageNumber + "." + IMAGES_TLA;

    if (++i > MAX_IMAGES) i = 1;
    imagesQueued++;
}

Any other suggestions would be deeply appreciated!

Answer 1

Now that this question is retitled to be about substr() in IE 8, here's the quick fix to the negative index problem. Paste the following code from Mozilla Developer Network:

// only run when the substr() function is broken
if ('ab'.substr(-1) != 'b') {
  /**
   *  Get the substring of a string
   *  @param  {integer}  start   where to start the substring
   *  @param  {integer}  length  how many characters to return
   *  @return {string}
   */
  String.prototype.substr = function(substr) {
    return function(start, length) {
      // call the original method
      return substr.call(this,
        // did we get a negative start, calculate how much it is from the beginning of the string
        // adjust the start parameter for negative value
        start < 0 ? this.length + start : start,
        length);
    }
  }(String.prototype.substr);
};

This code detects the broken implementation of substr and replaces it with a compliant one.