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.
append(name, value)
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.
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();
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');