Online Text Editor, how can i save a 'textarea' very large?

I need a little help with one personal project that i've working.

I need build a web application like Wattpad(is a web application to write 'books/historys' and share with others) and i'm using Nodejs and MongoDB to do this, but i have no ideia how the 'textarea'(Online Text Editor) works... can i use an API to save always the document(a very large document) ? Or i should use something like Socket.io ? I need save in every second this document ? A button thats going to save whole textarea ? i'am lost on this...

Guys my english is terrible, so, if anyone don't understand my point, i will delete this question, and thanks for everythink.