FormData.append()

The append() method of the FormData interface appends a new value onto an existing key inside a FormData object, or adds the key if it does not already exist.

The difference between FormData.set and append() is that if the specified key already exists, FormData.set will overwrite all existing values with the new one, whereas append() will append the new value onto the end of the existing set of values.

There are two versions of this method: a two and a three parameter version:

formData.append(name, value);
formData.append(name, value, filename);

name

The name of the field whose data is contained in value.

value

The field's value. This can be a USVString or Blob (including subclasses such as File). If none of these are specified the value is converted to a string.

filename Optional

The filename reported to the server (a USVString), when a Blob or File is passed as the second parameter. The default filename for Blob objects is "blob". The default filename for File objects is the file's filename.

undefined.

The following line creates an empty FormData object:

var formData = new FormData(); // Currently empty

You can add key/value pairs to this using FormData.append:

formData.append('username', 'Chris');
formData.append('userpic', myFileInput.files[0], 'chris.jpg');

As with regular form data, you can append multiple values with the same name. For example (and being compatible with PHP's naming conventions by adding [] to the name):

formData.append('userpic[]', myFileInput.files[0], 'chris1.jpg');
formData.append('userpic[]', myFileInput.files[1], 'chris2.jpg');

This technique makes it simpler to process multi-file uploads because the resultant data structure is more conducive to looping.

If the sent value is different than String or Blob it will be automatically converted to String:

formData.append('name', true);
formData.append('name', 74);
formData.append('name', 'John');

formData.getAll('name'); // ["true", "74", "John"]

• XMLHTTPRequest
• Using XMLHttpRequest
• Using FormData objects
• <form>