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


Answers

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';
   }
   else
   {
      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.readAsDataURL(res); 
         reader.onloadend = function() { done(reader.result.split('base64,')[1]); }
      }
   }
   xhr.open(method, url);
   xhr.setRequestHeader('Content-type', 'application/x-www-form-urlencoded');
   xhr.send('fname=Henry&lname=Ford');
   xhr.responseType = 'blob';
   xhr.send(postEnc);
}
Question

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?




Asynchronously load images with jQuery

No need for ajax. You can create a new image element, set its source attribute and place it somewhere in the document once it has finished loading:

var img = $("<img />").attr('src', 'http://somedomain.com/image.jpg')
    .on('load', function() {
        if (!this.complete || typeof this.naturalWidth == "undefined" || this.naturalWidth == 0) {
            alert('broken image!');
        } else {
            $("#something").append(img);
        }
    });



Don't give up so quickly, because this can be done (in modern browsers) using parts of the FileAPI:

Edit 2017-09-28: Updated to use File constructor when available so it works in Safari >= 10.1.

Edit 2015-10-16: jQuery ajax is not able to handle binary responses properly (can't set responseType), so it's better to use a plain XMLHttpRequest call.

var xhr = new XMLHttpRequest();
xhr.open('POST', url, true);
xhr.responseType = 'arraybuffer';
xhr.onload = function () {
    if (this.status === 200) {
        var filename = "";
        var disposition = xhr.getResponseHeader('Content-Disposition');
        if (disposition && disposition.indexOf('attachment') !== -1) {
            var filenameRegex = /filename[^;=\n]*=((['"]).*?\2|[^;\n]*)/;
            var matches = filenameRegex.exec(disposition);
            if (matches != null && matches[1]) filename = matches[1].replace(/['"]/g, '');
        }
        var type = xhr.getResponseHeader('Content-Type');

        var blob = typeof File === 'function'
            ? new File([this.response], filename, { type: type })
            : new Blob([this.response], { type: type });
        if (typeof window.navigator.msSaveBlob !== 'undefined') {
            // IE workaround for "HTML7007: One or more blob URLs were revoked by closing the blob for which they were created. These URLs will no longer resolve as the data backing the URL has been freed."
            window.navigator.msSaveBlob(blob, filename);
        } else {
            var URL = window.URL || window.webkitURL;
            var downloadUrl = URL.createObjectURL(blob);

            if (filename) {
                // use HTML5 a[download] attribute to specify filename
                var a = document.createElement("a");
                // safari doesn't support this yet
                if (typeof a.download === 'undefined') {
                    window.location = downloadUrl;
                } else {
                    a.href = downloadUrl;
                    a.download = filename;
                    document.body.appendChild(a);
                    a.click();
                }
            } else {
                window.location = downloadUrl;
            }

            setTimeout(function () { URL.revokeObjectURL(downloadUrl); }, 100); // cleanup
        }
    }
};
xhr.setRequestHeader('Content-type', 'application/x-www-form-urlencoded');
xhr.send($.param(params));

Here is the old version using jQuery.ajax. It might mangle binary data when the response is converted to a string of some charset.

$.ajax({
    type: "POST",
    url: url,
    data: params,
    success: function(response, status, xhr) {
        // check for a filename
        var filename = "";
        var disposition = xhr.getResponseHeader('Content-Disposition');
        if (disposition && disposition.indexOf('attachment') !== -1) {
            var filenameRegex = /filename[^;=\n]*=((['"]).*?\2|[^;\n]*)/;
            var matches = filenameRegex.exec(disposition);
            if (matches != null && matches[1]) filename = matches[1].replace(/['"]/g, '');
        }

        var type = xhr.getResponseHeader('Content-Type');
        var blob = new Blob([response], { type: type });

        if (typeof window.navigator.msSaveBlob !== 'undefined') {
            // IE workaround for "HTML7007: One or more blob URLs were revoked by closing the blob for which they were created. These URLs will no longer resolve as the data backing the URL has been freed."
            window.navigator.msSaveBlob(blob, filename);
        } else {
            var URL = window.URL || window.webkitURL;
            var downloadUrl = URL.createObjectURL(blob);

            if (filename) {
                // use HTML5 a[download] attribute to specify filename
                var a = document.createElement("a");
                // safari doesn't support this yet
                if (typeof a.download === 'undefined') {
                    window.location = downloadUrl;
                } else {
                    a.href = downloadUrl;
                    a.download = filename;
                    document.body.appendChild(a);
                    a.click();
                }
            } else {
                window.location = downloadUrl;
            }

            setTimeout(function () { URL.revokeObjectURL(downloadUrl); }, 100); // cleanup
        }
    }
});






$(<img />).attr('src','http://somedomain.com/image.jpg');

Should be better than ajax because if its a gallery and you are looping through a list of pics, if the image is already in cache, it wont send another request to server. It will request in the case of jQuery/ajax and return a HTTP 304 (Not modified) and then use original image from cache if its already there. The above method reduces an empty request to server after the first loop of images in the gallery.