Typescript: error TS6133 ... "is declared but its value is never read."

[dgofman-equinix]

I am using D3 and all callback functions should receiving 3 parameters.
"ValueFn"

I defined my event listeners as

on("mouseover", (d: any, index: number, elems: SVGGElement[]): void => {
   d3.select(elems[index]);
}) 

When I compiling to JS file and running via LINT/JSHINT no errors, however by enabling in tsconfig.json
"noUnusedParameters": true

I am getting errors: "error TS6133: 'd' is declared but its value is never read."

"d" is first argument you cannot avoid it. So, why TS compiler so crazy and cannot recognize it?

[andy-ms]

Prefix the unused parameter with an underscore, as in _d.

[massimonewsuk]

https://www.typescriptlang.org/docs/handbook/release-notes/typescript-2-0.html#flag-unused-declarations-with---nounusedparameters-and---nounusedlocals

[dgofman-equinix]

Why do we need that? It's kind of strange all my express functions will contains different parameters:

route.get("/foo1", (_req: Request, _res: Response, next: NextFunction) => {
   next();
}

route.get("/foo2", (_req: Request, res: Response, next: NextFunction) => {
   res.send("HELLO WORLD!");
   next();
}

route.get("/foo3", (req: Request, res: Response, next: NextFunction) => {
   res.send(req.params.helloworld);
   next();
}

[MartinJohns]

@dgofman-equinix You enable a feature that will error on unused parameters, then complain that it errors about unused parameters? If you don't like this feature, then don't enable it. The _-prefix is a way to opt-out of this feature by being explicitly saying "I'm aware this is not used, ignore it".

[RyanCavanaugh]

It's --noUnusedParameters, not --noUnusedNonleadingParameters. Lots of options available to you and it's not a bug that we literally do the thing the flag is called.

[RyanCavanaugh]

Prior discussion at #9458