We implemented isomorphic fetching (i.e. fetching with the browser's implementation on the client and node-fetch on the server)

Universal = code that can run in any JS runtime (browser and/or node). Isomorphic = application that runs the same universal code in multiple runtimes to avoid code duplication.

React is an isomorphic/universal framework. That means that there is a virtual representation of the UI component tree, and that is separate from the actual rendering that it outputs in the browser.

zxcvbn works identically on the server

Running the same code in the browser and on the server in order to avoid code duplication is a very different problem. It is simply a matter of good development practices to avoid code duplication. This however is not limited to isomorphic applications. A utility library such as Lodash is “universal”, but has nothing to do with isomorphism. Sharing code between environments does not give you an isomorphic application. What we’re referring to with Universal JavaScript is simply the fact that it is JavaScript code which is environment agnostic. It can run anywhere. In fact most JavaScript code will run fine on any JavaScript platform.

Isomorphism itself is not purely a JavaScript thing. You can also build an isomorphic application in Dart or even use two different languages/stacks to render the same result. In fact there are many “isomorphic” applications which render a full HTML document on the server using PHP, Java or Ruby and use JavaScript and AJAX to create a rich user experience in the browser. This was the way things were done before the rise of Single Page Applications. The only real reason isomorphism is tied to JavaScript is because it’s the only language widely supported in the browser, and so at least part of the whole thing will use JavaScript. There’s no such thing as Isomorphic JavaScript, only isomorphic applications.

Maybe we should distinguish between: The technique of asssembling pages on either client or server. Here, “isomorphic” and “full stack” (as proposed by Rodrigo Medeiros) seem good choices. JavaScript that runs in all (or most) JavaScript environments, especially browsers and Node.js. Here, “universal” seems a good choice.