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Using jQuery's ajax method to retrieve images as a blob

Posted by: admin November 30, 2017 Leave a comment


I recently asked another (related) question, which lead to this follow up question:
Submitting data instead of a file for an input form

Reading through the jQuery.ajax() documentation (http://api.jquery.com/jQuery.ajax/), it seems the list of accepted dataTypes doesn’t include images.

I’m trying to retrieve an image using jQuery.get (or jQuery.ajax if I have to), store this image in a Blob and upload it to another server in a POST request. Currently, it looks like due to the mismatch in datatypes, my images end up being corrupt (size in bytes mismatch, etc.).

The code to perform this is as follows (it is in coffeescript but shouldn’t be difficult to parse):

handler = (data,status) ->
  fd = new FormData
  fd.append("file", new Blob([data], { "type" : "image/png" }))
  jQuery.ajax {
    url: target_url,
    data: fd,
    processData: false,
    contentType: "multipart/form-data",
    type: "POST",
    complete: (xhr,status) ->
      console.log xhr.status
      console.log xhr.statusCode
      console.log xhr.responseText

jQuery.get(image_source_url, null, handler)

How can I retrieve this image as a blob instead?


You can’t do this with jQuery ajax, but with native XMLHttpRequest.

var xhr = new XMLHttpRequest();
xhr.onreadystatechange = function(){
    if (this.readyState == 4 && this.status == 200){
        //this.response is what you're looking for
        console.log(this.response, typeof this.response);
        var img = document.getElementById('img');
        var url = window.URL || window.webkitURL;
        img.src = url.createObjectURL(this.response);
xhr.open('GET', 'http://jsfiddle.net/img/logo.png');
xhr.responseType = 'blob';



A big thank you to @Musa and here is a neat function that converts the data to a base64 string. This may come handy to you when handling a binary file (pdf, png, jpeg, docx, …) file in a WebView that gets the binary file but you need to transfer the file’s data safely into your app.

// runs a get/post on url with post variables, where:
// url ... your url
// post ... {'key1':'value1', 'key2':'value2', ...}
//          set to null if you need a GET instead of POST req
// done ... function(t) called when request returns
function getFile(url, post, done)
   var postEnc, method;
   if (post == null)
      postEnc = '';
      method = 'GET';
      method = 'POST';
      postEnc = new FormData();
      for(var i in post)
         postEnc.append(i, post[i]);
   var xhr = new XMLHttpRequest();
   xhr.onreadystatechange = function() {
      if (this.readyState == 4 && this.status == 200)
         var res = this.response;
         var reader = new window.FileReader();
         reader.onloadend = function() { done(reader.result.split('base64,')[1]); }
   xhr.open(method, url);
   xhr.setRequestHeader('Content-type', 'application/x-www-form-urlencoded');
   xhr.responseType = 'blob';