HTML5 File API: get File object within FileReader callback

问题

With the new File API in Javascript you can read files in Javascript to create dataURLs to show clientside pictures clientside. I'm wondering if you can reach the File object within the FileReader's onload callback. I will illustrate this with an example:

var div = document.createElement('div');
div.ondrop = function(e) {
  e.preventDefault();
  e.stopPropagation();
  var files = e.dataTransfer.files;
  for ( var i=0; i<files.length; i++ ) {
    var file = files[i]; // this is the file I want!!
    var filereader = new FileReader();
    filereader.onload = function(e) {
      this; // the FileReader object
      e.target; // the same FileReader object
      this.result; // the dataURL, something like data:image/jpeg;base64,.....
      var img = document.createElement('img');
      img.src = this.result;
      img.title = file.fileName; // This won't be working
      document.appendChild(img);
    }
  }
  return false;
}

What I could do - what I do right now - is wrap the contents of the for loop in a function and execute it to create a new scope and keep a file in that scope like so:

  for ( var i=0; i<files.length; i++ ) {
    var _file = files[i]; // this is the file I want!!
    (function(file) {
      // do FileReader stuff here
    })(_file);
  }

I was just wondering... Maybe I'm missing something. Is there a way to get the File object from within the onload function of the FileReader? Both this and e.target are the FileReader object and not the File. Is there something in this or e that is the File?? I can't find it :(

Thanks a bunch.

PS. A fiddle: http://jsfiddle.net/rudiedirkx/ZWMRd/1/

回答1:

I already found a way. Maybe not better than the scope wrapper, but I think it's neater:

for ( var i=0; i<files.length; i++ ) {
    var file = files[i]; // this is the file I want!!
    var filereader = new FileReader();
    filereader.file = file;
    filereader.onload = function(e) {
        var file = this.file; // there it is!
        // do stuff
    }
}

There is now a much easier (apparently faster) way (sync!) to get a file's data URL:

img.src = URL.createObjectURL(file);

Demo on http://jsfiddle.net/rudiedirkx/ZWMRd/8/show/ of both methods, and the original problem illustrated (drag multiple images and check the title tooltips).

回答2:

I don't think this.file is still supported. When I try to run the answer code, this.file is undefined whereas if I run the code from the question I get the expected results. I think you have to use the closure (at least this is how they do it on html5rocks (Example)).

来源：https://stackoverflow.com/questions/4404361/html5-file-api-get-file-object-within-filereader-callback

标签

javascript

file

html5

filereader