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.

Note: This method is available in Web Workers.


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

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


The name of the field whose data is contained in value.
The field's value. This can be a USVString or Blob (including subclasses such as File).
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.

Note: If you specify a Blob as the data to append to the FormData object, the filename that will be reported to the server in the "Content-Disposition" header used to vary from browser to browser.




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[]', myFileInput1.files[0], 'chris1.jpg');
formData.append('userpic[]', myFileInput2.files[0], 'chris2.jpg');

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


Specification Status Comment
The definition of 'append()' in that specification.
Living Standard Initial definition

Browser compatibility

Feature Chrome Firefox (Gecko) Internet Explorer Opera Safari
Basic support 7 4.0 (2.0)[1] 10 12 5
append with filename (Yes) 22.0 (22.0) ? ? ?
Available in web workers (Yes) 39.0 (39.0) ? ? ?
Feature Android Chrome for Android Firefox Mobile (Gecko) Firefox OS (Gecko) IE Mobile Opera Mobile Safari Mobile
Basic support 3.0[2] ? 4.0 (2.0)[1] 1.0.1 ?


append with filename ? ? 22.0 (22.0) 1.2 ? ? ?
Available in web workers ? ? 39.0 (39.0) ? ? ? ?

[1] Prior to Gecko 7.0 (Firefox 7.0 / Thunderbird 7.0 / SeaMonkey 2.4), if you specified a Blob as the data to append to the object, the filename reported in the "Content-Disposition" HTTP header was an empty string; this resulted in errors being reported by some servers. Starting in Gecko 7.0 the filename "blob" is sent.

[2] XHR in Android 4.0 sends empty content for FormData with blob.

See also

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 Last updated by: Dorward,