[tannhaeuser]

Not only is expressjs de-facto, it actually is a community design from before Node.js existed, with earlier implementations (connect, jackjs), including on non-Node.js SSJS-platforms; cf. [1]. Moreover, an expressjs/JSGI middleware can plug-in and directly run off the Node.js core http API, without the additional expressjs routing, etc.

[1]: https://wiki.commonjs.org/wiki/JSGI/Level0/A/Draft2

[jcpst]

It’s an approach I really enjoy. Back in the day express referred to itself as a sinatra-style library (ruby).

It also seems like the modern .NET Core Http request pipeline and minimal API are inspired by express.

[mardifoufs]

What happened to common.js?

[tannhaeuser]

Nothing really, it completed its mission of a portable environment across SSJS, and saw quite some implementation effort. The commonjs module loading convention (but not necessarily the particular core modules such as for fs, http, etc.) became a standard for bundling in browsers even, before ECMA standardized ES modules, and their dynamic nature make them being used on Node.js until today, such as for expressjs. But around 2010, Node.js became the leading, then the only relevant SSJS platform.