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 set() and append() is that if the specified key already exists, 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.

name

The name of the field whose data is contained in value.

value

The field's value. This can be a string 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 string), 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.

None (undefined).

formData.append("username", "Chris");

When the value is a Blob (or a File), you can specify its name with the filename parameter:

formData.append("userpic", myFileInput.files[0], "chris.jpg");

As with regular form data, you can append multiple values with the same name:

formData.append("userpic", myFileInput.files[0], "chris1.jpg");
formData.append("userpic", myFileInput.files[1], "chris2.jpg");

If the value is not a string or a Blob, append() will convert it to a string automatically:

formData.append("name", true);
formData.append("name", 72);
formData.getAll("name"); // ["true", "72"]

• Using FormData objects
• <form>