Default error handler, next(), and ESLint's no unused var rule

[expressjs]

Default error handler, next(), and ESLint's no unused var rule

[zckrs]

Since 2015 the rule no-unused-vars have news options.

You can use http://eslint.org/docs/rules/no-unused-vars#argsignorepattern

Example in package.json

"eslintConfig": {
    "rules": {
      "no-unused-vars": ["error", { "argsIgnorePattern": "next" }]
    }
  }

[vvo]

You can also use:

// eslint-disable-next-line no-unused-vars

To be extra specific

[dougwilson]

Yes, the biggest issue is that Express error middleware is generally not compatible with that rule. You can probably add some kind of ESLint comment to disable that rule just for the error handling function.

The reason it's there, but unused, is because in Express, an error middlware is differentiated from a normal middleware by the function taking 4 arguments rather than 3 arguments (i.e. function (req, res, next) vs function (err, req, res, next)).

You are also getting an error when you manually call next() because in Express, you can only call next() if you have not responded. The res.render statement responds to the client, to the call to next() will cause an error, since you cannot do both.

It boils down to that rule not being compatible with Express error middlewares is all. I have never user ESLint (only JSHint), so I don't know how to tell you what you need to change in your ESLint rules.

[nvzard]

None of the above-mentioned solutions are working for something like:

    this.express.use((err: Error, req: Request, res: Response, next: NextFunction) => {
      handleError(err, res);
    });

error 'next' is defined but never used @typescript-eslint/no-unused-vars

Had to use:

    /* eslint-disable */
    this.express.use((err: Error, req: Request, res: Response, next: NextFunction) => {
      handleError(err, res);
    });
    /* eslint-enable */

Weird thing is, I am not getting error for unused req but only for next argument.

[adrianosouzacostacr]

My Function

requestHandler(_req: Request, res: Response, _next: Next): Promise<void> {
    return new UserController().list(res);
  }

You can use:

"rules": {
    "@typescript-eslint/no-unused-vars": ["warn", { "argsIgnorePattern": "^_" }]
  }

Result is zero warnings on lint

This pattern is shown in https://eslint.org/docs/rules/no-unused-vars

[gghez]

@zckrs it is a little bit unsafe, custom error handler appears once in your code, so you may prefer using // eslint-disable-line to disable eslint locally.

[dougwilson]

It looks like here is the issue on ESLint: eslint/eslint#1494 seems they say just don't use that rule.

[kevinSuttle]

Thanks so much, @dougwilson. I have relaxed it to a warning for now.

[rafis]

Is it possible to use no-unused-vars in JSHint?

This is nearest answer I have found.