The FormData object lets you compile a set of key/value pairs to send using XMLHttpRequest. It is 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

Section

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 a string "123456"

// HTML file input, chosen by user
formData.append("userfile", fileInputElement.files[0]);

// JavaScript file-like object
var content = 'hey!'; // 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” both contain 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

Section

To construct a FormData object that contains the data from an existing 

var formData = new FormData(someFormElement);

For example:

var formElement = document.querySelector("form");
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.querySelector("form");
var formData = new FormData(formElement);
var request = new XMLHttpRequest();
request.open("POST", "submitform.php");
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.

Sending files using a FormData object

Section

You can also send files using FormData. Simply include an 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></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.querySelector("div"),
      oData = new FormData(form);

  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 when trying to upload your file.
";
    }
  };

  oReq.send(oData);
  ev.preventDefault();
},  
false);
 
 
Note: If you pass in a reference to the form the request method specified in the form will be used over the method specified in the open() call.

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 sent to the server. When no filename is specified (or the parameter isn’t supported), the name “blob” is used.