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.
Syntax
There are two versions of this method: a two and a three parameter version:
formData.append(name, value); formData.append(name, value, filename);
Parameters
name- The name of the field whose data is contained in
value. value- The field's value. This can be a
USVStringorBlob(including subclasses such asFile). filenameOptional- The filename reported to the server (a
USVString), when aBloborFileis passed as the second parameter. The default filename forBlobobjects is "blob". The default filename forFileobjects 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.
Returns
Void.
Example
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.
Specifications
| Specification | Status | Comment |
|---|---|---|
| XMLHttpRequest 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 | ? |
12 |
? |
| 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.