Javascript/jQuery Append to FormData() returns 'undefined'

Question

I'm trying to do a simple file upload using jQuery. I have a file input in my HTML like so:

<form id="PhotoUploadForm" action="/some/correct/url" method="post" enctype="multipart/form-data">
            <input type="file" id="uploadPhoto" accept="image">
</form>

I also have some JavaScript / jQuery bound to the change event of the input, like so:

   $('#uploadPhoto').on("change", function (event) {

        var fileData = this.files[0];
        var data = new FormData();
        data.append('image',fileData);
        data.append('content','What could go wrong');
        console.log(data.image, data.content); // this outputs 'undefined undefined' in the console

        $.ajax ({
            url: "/some/correct/url",
            type: 'POST',
            data: data,
            processData: false,
            beforeSend: function() {
               console.log('about to send');
            },
            success: function ( response ) {                   
                console.log( 'now do something!' )
            },
            error: function ( response ) {
                console.log("response failed");
            }
        });
    });

I noticed I was getting a 500 error! Most likely the data is incorrect, I know the URL is good. So I tried to output the data to the console and I noticed that my data appends return 'undefined'

console.log(data.image, data.content); // this outputs 'undefined undefined' in the console

when I console.log(data) I get the following:

FormData {append: function}__proto__: FormData

Am I doing something wrong here? Why are data.image & data.content undefined? When I output the this.files[0] I get the following:

File {webkitRelativePath: "", lastModified: 1412680079000, lastModifiedDate: Tue Oct 07 2014 13:07:59 GMT+0200 (CEST), name: "2575-Web.jpg", type: "image/jpeg"…}lastModified: 1412680079000lastModifiedDate: Tue Oct 07 2014 13:07:59 GMT+0200 (CEST)name: "2575-Web.jpg"size: 138178type: "image/jpeg"webkitRelativePath: ""__proto__: File

So the issue isn't with the image. Am I correct?

i think 2nd line should be var fileData = $(this).files[0]; — Pratik Joshi
– Pratik Joshi, Commented Oct 13, 2014 at 13:22
Can you inspect what is the content type of the request in the Network tab of the web inspector? — STT
– STT, Commented Oct 13, 2014 at 13:25
@jQueryAngryBird nope, it is good "as is". $(this).files[0] will cause error. — Regent
– Regent, Commented Oct 13, 2014 at 13:26
set contentType: false in the ajax request so that jQuery will set it properly — Arun P Johny
– Arun P Johny, Commented Oct 13, 2014 at 13:27

Marco Bonelli · Accepted Answer · 2014-10-13 13:43:49Z

Issue

You are misunderstanding the FormData object. A FormData object has only an .append() method, which doesn't append properties to it, but only stores the data provided. Hence, obviously, the .image and .content properties will be undefined.

If you want to create a object with the .image and .content properties you should instead create a regular object and then send it.

Workaround

To achieve what you want you have some options:

Using FormData:
- Call its constructor passing the <form> element as an argument, and it will do all the work for you.
- OR: create the object and then use the .append() method.
Use a regular object and send it.
Use a regular object, convert it to JSON and send it using contentType: 'application/json'.

What option would you choose? It only depends on the back-end. If you are developing the back-end make sure you are retrieving the data from the POST request in the correct way, otherwise check the site and see what kind of data you'll need to send.

Option 1

Create the FormData object:

Method 1, let the constructor work for you:

var form = document.getElementById("PhotoUploadForm"),
    myData = new FormData(form);

Method 2, do it by yourself:

var fileData = this.files[0],
    myData = new FormData();

myData.append('image', fileData);

Then, later in your code, you can send it:

$.ajax({
    url: "/some/correct/url",
    type: 'POST',
    data: myData, // here it is
    ...
});

Option 2

Create a regular object and send it:

var myData = {
        image: this.files[0]
    };

$.ajax({
    url: "/some/correct/url",
    type: 'POST',
    data: myData, // here it is
    ...
});

Option 3

var myData = {
        image: this.files[0]
    };

myData = JSON.stringify(myData); // convert to JSON

$.ajax({
    url: "/some/correct/url",
    type: 'POST',
    contentType: 'application/json',
    data: myData, // here it is
    ...
});

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