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 delete() method of the FormData interface deletes a key and its value(s) from a FormData object.
The FormData() constructor creates a new FormData object.
The get() method of the FormData interface returns the first value associated with a given key from within a FormData object. If you expect multiple values and want all of them, use the getAll() method instead.
The getAll() method of the FormData interface returns all the values associated with a given key from within a FormData object.
The has() method of the FormData interface returns a boolean stating whether a FormData object contains a certain key.
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 FormData object lets you compile a set of key/value pairs to send using XMLHttpRequest. It is primarily intended for use in sending form data, but can be used independently from forms in order to transmit keyed data. The transmitted data is in the same format that the form's submit() method would use to send the data if the form's encoding type were set to multipart/form-data.
XMLHttpRequest is an API that provides client functionality for transferring data between a client and a server. It provides an easy way to retrieve data from a URL without having to do a full page refresh. This enables a Web page to update just a part of the page without disrupting what the user is doing.  XMLHttpRequest is used heavily in AJAX programming.
An EventHandler that is called whenever the readyState attribute changes. The callback is called from the user interface thread. The XMLHttpRequest.onreadystatechange property contains the event handler to be called when the readystatechange event is fired, that is every time the readyState property of the XMLHttpRequest changes. The callback is called from the user interface thread.
The XMLHttpRequestEventTarget.onprogress is the function called periodically with information when an XMLHttpRequest before success completely .
The XMLHttpRequest.abort() method aborts the request if it has already been sent. When a request is aborted, its readyState is set to 0 (UNSENT), but the readystatechange event is not fired.