Before you start developing an express-based NW.js(node-webkit) application, you should see About Node.js server side script in nw.js, as it provides some good ways to replace those frameworks.

But if you already have an express-based webapp, and you want NW.js to use it as desktop app, you might encounter problems.

First, express provide a web server and doesn't have an index.html file, so what should you do with the package.json main property?

The solution is that you can create a index.html file, and use the following code to run the system:

<script type="text/javascript" src="app.js"></script>

In NW.js you can use node modules in html files, for more details on this, see: Using Node modules

Second, express 3.x/4.x use the ./bin/www folder to start a web server, so if you run the app.js it will fail to start. If you want run a node app directly, not through a html file, you can use the following way.

We can setup node-main in package.json. The node-main property is a command which will be call when NW.js app start, for more details see: Node main.
And for express, we should also setup the main property to http://localhost:3000. If we setup a filename to that, NW.js will open it with the file protocol, so you will see the source code. If you setup a url with http protocol, NW.js will open it just like a browser.

"node-main": "./bin/www",
"main": "http://localhost:3000"

Put the above code in your package.json and run it. It seems OK but there is a problem (maybe a bug), in that NW.js shows a blank page. You must refresh the page, and then the content will show.
There is a way to solve this problem: you can create a html file and write JavaScript code location.href="http://localhost:3000/" in it, then setup the main as your-dir-html.html.

If the application that you're working with doesn't contain a www binary, try something like this:

"node-main": "./app.js",
"main": "http://localhost:3000"

This worked with the mean-stack-relational application (https://github.com/jpotts18/mean-stack-relational) which doesn't have a www binary.