> Honestly that's less confusing than what express.js does to the middleware function: `app.get("/", (req, res) => { ... })` and `app.get("/", (req, res, next) => { ... })` behave differently because it tries to infer the presence of `next` by probing `Function.prototype.length`.
This feels like a completely random swipe at an unrelated feature of a JavaScript framework, and I'm not even sure that it's an accurate swipe.
The entire point of Function.length (slight nit: Function.prototype.length is different and is always zero) is to check the arity of the function [0]. There's no "tries to": if your middleware function accepts three arguments then it will have a length of 3.
Aside from that, I've also done a bunch of digging and can't find any evidence that they're doing [1]. Do you have a source for the claim that this is what they're doing?
Because we were talking about HTTP server frameworks, it seemed not that problematic to mention one of the most surprising things I've seen in this space. Not necessarily JS bashing, but sorry for that anyway.
I'm surprised to see that it's now gone too! The exact commit is [1], which happened before Express.js 4.7, and you can search for the variable name `arity` in any previous versions to see what I was talking. It seems that my memory was slightly off as well, my bad. The correct description would be that older versions of Express.js used to distinguish "error" callbacks from normal router callbacks by their arities, so `(req, res)` and `(req, res, next)` would have been thankfully okay, while any extra argument added by an accident will effectively disable that callback without any indication. It was a very good reason for me to be surprised and annoyed at that time.
Actually, it still uses ≤ 3 vs. = 4 arguments to distinguish between request callbacks and error callbacks. Check out the added lines to lib/router/layer.js in the commit you mention, or the equivalent functions in the current router v2.0.0 package [0].
> The entire point of Function.length (slight nit: Function.prototype.length is different and is always zero)
If you want to go that route, it's not Function.length, either—which is different and is always 1 (barring potential future spec changes that change the arity of the Function global).
This feels like a completely random swipe at an unrelated feature of a JavaScript framework, and I'm not even sure that it's an accurate swipe.
The entire point of Function.length (slight nit: Function.prototype.length is different and is always zero) is to check the arity of the function [0]. There's no "tries to": if your middleware function accepts three arguments then it will have a length of 3.
Aside from that, I've also done a bunch of digging and can't find any evidence that they're doing [1]. Do you have a source for the claim that this is what they're doing?
[0] https://developer.mozilla.org/en-US/docs/Web/JavaScript/Refe...
[1] https://github.com/search?q=repo%3Aexpressjs%2Fexpress%20%22...