The set() method of the FormData interface sets a new value for an existing key inside a FormData object, or adds the key/value if it does not already exist.

The difference between set() and FormData.append is that if the specified key does already exist, set() will overwrite all existing values with the new one, whereas FormData.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.set(name, value);
formData.set(name, value, filename);


The name of the field whose data is contained in value.
The field's value. In the two parameter version this is a USVString, or if it is not, it is converted to a string. In the three parameter version this can be a Blob, File, or a USVString. 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".

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 set key/value pairs on this using FormData.set:

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


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

Browser compatibility

Feature Chrome Firefox (Gecko) Internet Explorer Opera Safari
Basic support 50.0 39.0 (39.0) No support No support No support
Available in web workers 50.0 39.0 (39.0) No support No support No support
Feature Android Android Webview Firefox Mobile (Gecko) Firefox OS (Gecko) IE Mobile Opera Mobile Safari Mobile Chrome for Android
Basic support No support 50.0 (Yes) (Yes) No support

No support

No support 50.0
Available in web workers No support 50.0 (Yes) (Yes) No support No support No support 50.0

See also


© 2016 Mozilla Contributors
Licensed under the Creative Commons Attribution-ShareAlike License v2.5 or later.

API FormData Method Reference set XHR XMLHttpRequest