CSE 331: JavaScript in the page
The guestbook's page now carries a program, written in JavaScript, which every visitor's browser runs. The server no longer builds the page: it sends the page and the program as files, and answers the program's requests with data. So the guestbook is two programs in two languages, one on the server and one in the browser, and what travels between them is text, JSON.
The browser keeps the page as a tree, the DOM, and the program
reads and changes that tree, so the page can change with no
request sent. The program asks the server for the messages with
fetch, which returns a promise, a stand-in for the response
that will arrive, and await waits for it. Waiting is not
running: the browser runs a loop, as the server does, waiting for
an event (a click, a response arriving) and running its
callback, and an await cuts a function in two, the part before
running now and the part after running later, as a callback, when
the promise's result arrives. So the browser does one thing at a
time and is never stuck on the network, and the order the events
come in is part of each run.
The request the program sends is a call across the network: the program in the browser is the client, the server is the implementation, and the specification between them is written in prose, in the server's comment, with nothing but that comment to keep the two sides in step.