Using FormData Objects

This article is in need of a technical review.

The FormData object lets you compile a set of key/value pairs to send using XMLHttpRequest. Its primarily intended for use in sending form data, but can be used independently from forms in order to transmit keyed data. The transmitted data is in the same format that the form's submit() method would use to send the data if the form's encoding type were set to "multipart/form-data".

Creating a FormData object from scratch

You can build a FormData object yourself, instantiating it then appending fields to it by calling its append() method, like this:

var formData = new FormData();

formData.append("username", "Groucho");
formData.append("accountnum", 123456); // number 123456 is immediately converted to string "123456"

// HTML file input user's choice...
formData.append("userfile", fileInputElement.files[0]);

// JavaScript file-like object...
var content = '<a id="a"><b id="b">hey!</b></a>'; // the body of the new file...
var blob = new Blob([content], { type: "text/xml"});

formData.append("webmasterfile", blob);

var request = new XMLHttpRequest();
request.open("POST", "http://foo.com/submitform.php");
request.send(formData);
Note: The fields "userfile" and "webmasterfile" contain both a file. The number assigned to the field "accountnum" is immediately converted into a string by the FormData.append() method (the field's value can be a Blob, File, or a string: if the value is neither a Blob nor a File, the value is converted to a string).

This example builds a FormData instance containing values for fields named "username", "accountnum", "userfile" and "webmasterfile", then uses the XMLHttpRequest method send() to send the form's data. The field "webmasterfile" is a Blob. A Blob object represents a file-like object of immutable, raw data. Blobs represent data that isn't necessarily in a JavaScript-native format. The File interface is based on Blob, inheriting blob functionality and expanding it to support files on the user's system. In order to build a Blob you can invoke the Blob constructor.

Retrieving a FormData object from an HTML form

To construct a FormData object that contains the data from an existing <form>, specify that form element when creating the FormData object:

var formData = new FormData(someFormElement);

For example:

var formElement = document.getElementById("myFormElement");
var request = new XMLHttpRequest();
request.open("POST", "submitform.php");
request.send(new FormData(formElement));

You can also append additional data to the FormData object between retrieving it from a form and sending it, like this:

var formElement = document.getElementById("myFormElement");
formData = new FormData(formElement);
formData.append("serialnumber", serialNumber++);
request.send(formData);

This lets you augment the form's data before sending it along, to include additional information that's not necessarily user editable on the form.

Sending files using a FormData object

You can also send files using FormData. Simply include an <input> element of type "file":

<form enctype="multipart/form-data" method="post" name="fileinfo">
  <label>Your email address:</label>
  <input type="email" autocomplete="on" autofocus name="userid" placeholder="email" required size="32" maxlength="64" /><br />
  <label>Custom file label:</label>
  <input type="text" name="filelabel" size="12" maxlength="32" /><br />
  <label>File to stash:</label>
  <input type="file" name="file" required />
  <input type="submit" value="Stash the file!" />
</form>
<div id="output"></div>

Then you can send it using code like the following:

var form = document.forms.namedItem("fileinfo");
form.addEventListener('submit', function(ev) {

  var
    oOutput = document.getElementById("output"),
    oData = new FormData(document.forms.namedItem("fileinfo"));

  oData.append("CustomField", "This is some extra data");

  var oReq = new XMLHttpRequest();
  oReq.open("POST", "stash.php", true);
  oReq.onload = function(oEvent) {
    if (oReq.status == 200) {
      oOutput.innerHTML = "Uploaded!";
    } else {
      oOutput.innerHTML = "Error " + oReq.status + " occurred uploading your file.<br \/>";
    }
  };

  oReq.send(oData);
  ev.preventDefault();
}, false);

Note: If you pass in a reference to the form the method specified in the form will be used over the method specified in the open() call.

Note: This example is directing the output to a Perl CGI script running on the server, and handles HTTP errors, although not prettily.

You can also append a File or Blob directly to the FormData object, like this:

data.append("myfile", myBlob, "filename.txt");

When using the append method it is possible to use the third optional parameter to pass a filename inside the Content-Disposition header; that is send to the server. When no filename is specified (or the parameter isn't supported), the name "blob" is used.

You can also use FormData with jQuery if you set the right options:

var fd = new FormData(document.getElementById("fileinfo"));
fd.append("CustomField", "This is some extra data");
$.ajax({
  url: "stash.php",
  type: "POST",
  data: fd,
  processData: false,  // tell jQuery not to process the data
  contentType: false   // tell jQuery not to set contentType
});

Submitting forms and uploading files via AJAX without FormData objects

If you want to know how to serialize and submit via AJAX a form without using FormData objects, please read this paragraph.

See also

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Last updated by: geoffstokes,