eslint no-misused-promise in notFoundHandler

[mindrunner]

Prerequisites

• I have written a descriptive issue title
• I have searched existing issues to ensure the bug has not already been reported

Fastify version

4.25.2

Plugin version

9.0.1

Node.js version

20.10.0

Operating system

Linux

Operating system version (i.e. 20.04, 11.3, 10)

Description

I am rate limiting 404 errors as documented here:
https://github.com/fastify/fastify-rate-limit#preventing-guessing-of-urls-through-404s

notFoundHandler expects a preValidationHookHandler, but fastify.rateLimit() is a preHandlerAsyncHookHandler

Steps to Reproduce

Use

fastify.setNotFoundHandler({
  preHandler: fastify.rateLimit()
}, function (request, reply) {
  reply.code(404).send({ hello: 'world' })
})

• Enable no-misused-promise eslint rule

Expected Behavior

No erorrs / warnings thrown

[mcollina]

Fastify normalizes the flow between promises and non-promisified code.

Essentially that rule is too strict for no specific reason. Reply has a .then() but it is not a Promise.

To solve, just add a void, like so:

fastify.setNotFoundHandler({
  preHandler: fastify.rateLimit()
}, function (request, reply) {
  void reply.code(404).send({ hello: 'world' })
})

or return it:

fastify.setNotFoundHandler({
  preHandler: fastify.rateLimit()
}, function (request, reply) {
  return reply.code(404).send({ hello: 'world' })
})

[mindrunner]

The problem here is not the handler itself, but the preHandler which does not match:

[mcollina]

Ah it might be a bug in our types. That does not return a promise. That returns a function that returns a promise.

[mcollina]

Would you like to send a PR?

[mindrunner]

Yea that was my assumption. Can have a look into this later.

[mindrunner]

I am not sure how to fix this. preHandler expects a preHandlerHookHandler, but rateLimit returns a preHandlerAsyncHookHandler

[mcollina]

Can you share a reproduction for this? So somebody can quickly work for a resolution.

[mindrunner]

See 'Steps to reproduce' in Original Post. What else would you need to reproduce?

[mcollina]

We often need a reproducible example, e.g. some code that allows someone else to recreate your problem by just copying and pasting it. If it involves more than a couple of different file, create a new repository on GitHub and add a link to that.

TL;DR Set up a tiny repository that shows the problem.

[mindrunner]

app.ts:

import Fastify from "fastify";

const main = async () => {
  const fastify = Fastify();
  await fastify.register(import("@fastify/rate-limit"), {
    max: 100,
    timeWindow: "1 minute",
  });

  fastify.setNotFoundHandler(
    {
      preHandler: fastify.rateLimit(),
    },
    function (request, reply) {
      reply.code(404).send({ hello: "world" });
    },
  );
};

main().then(() => {});

package.json

{
  "name": "rl-fastify",
  "version": "1.0.0",
  "description": "",
  "dependencies": {
    "fastify": "^4.25.2",
    "@fastify/rate-limit": "^9.1.0"
  },
  "devDependencies": {
    "@typescript-eslint/eslint-plugin": "^6.16.0",
    "@typescript-eslint/parser": "^6.16.0",
    "eslint": "^8.56.0"
  }
}

tsconfig.json

{
  "compilerOptions": {
    "target": "es2016",
    "module": "commonjs",
    "esModuleInterop": true,
    "forceConsistentCasingInFileNames": true,
    "strict": true,
    "skipLibCheck": true
  },
  "include": [
    "app.ts",
    ".eslintrc.js"
  ]
}

.eslintrc.js

module.exports = {
  parser: "@typescript-eslint/parser",
  parserOptions: {
    sourceType: "module",
    project: ["./tsconfig.json"],
  },
  env: {
    es6: true,
    node: true,
  },
  rules: {},
  extends: ["plugin:@typescript-eslint/recommended"],
};

Steps to reproduce:

• copy all 4 files to same directory
• run pnpm install
• run eslint .

Actual output:

  12:19  error  Promise-returning function provided to property where a void return was expected  @typescript-eslint/no-misused-promises

✖ 1 problem (1 error, 0 warnings)

Expected output:

<empty> (e.g. eslint does not show any errors)

[mcollina]

I think you got your file names wrong. I recommend you to upload everything to a repository.

[mindrunner]

Sorry, accidentally hit save before I was ready.

[mindrunner]

https://github.com/mindrunner/rl-fastify

[mcollina]

Running npx eslint . in that repository does not trigger any errors.

[mindrunner]

updated repo. pls try again

[mcollina]

Thanks, that was helpful.

Here is the fix: fastify/fastify#5258

[mindrunner]

Awesome, thanks for fixing!