Is it possible to save textinput (locally) from a form to a textfile, and then open that document to use it later on?
Just using HTML, javascript and jQuery. No data
You can use localStorage to save the data for later use, but you can not save to a file using JavaScript (in the browser).
localStorage
To be comprehensive: You can not store something into a file using JavaScript in the Browser, but using HTML5, you can read files.