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. 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.

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');

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

formData.set('name', 72);
formData.get('name'); // "72"


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

Browser compatibility

Update compatibility data on GitHub
ChromeEdgeFirefoxInternet ExplorerOperaSafariAndroid webviewChrome for AndroidFirefox for AndroidOpera for AndroidSafari on iOSSamsung Internet
setChrome Full support 50Edge Full support 18Firefox Full support 39IE No support NoOpera Full support 37Safari Full support 11WebView Android Full support 50Chrome Android Full support 50Firefox Android Full support 39Opera Android Full support 37Safari iOS Full support 11Samsung Internet Android Full support 5.0


Full support  
Full support
No support  
No support

See also