Unused variable in arrow functions

[dcousens]

Example

let xs = [1, 2, 3]
let ys = xs.map((x) => xs + 1)

Expected unused variable "x", instead got nothing. Intentional?

[yoshuawuyts]

This doesn't happen for normal functions either - hasn't for a while now iirc

[LinusU]

I think it's because express forces you to have four arguments to be able to be an error handler, thus we cannot enforce unused variables when they are function parameters.

Relevant: expressjs/express#2896

This is a duplicate of: #419

[yoshuawuyts]

I think it's because express forces you to have four arguments to be able to be an error handler

Oh wow, that's not a good pattern hey - had no idea they did that 😰

edit: looks like maintainers agree and they want to change it; that would unblock reintroducing this behavior for us I reckon

[dcousens]

Ah true... closing as duplicate of #419

[feross]

Yeah, happy to see about reintroducing this rule once there's a solution for all the express users out there (including myself!).

[LinusU]

Since we are now allowing _ as an unused variable, I think that we should add this rule now since you can do the following:

app.use(function (err, _, __, ___) {

})

[dcousens]

@feross thoughts?

[LinusU]

ping @feross 😀