[TypeScript] Type '{}' is not assignable to type '<Type>'. Property '<property>' is missing in type '{}'.

[acro5piano]

Hi, Thank you for the really great framework. I love zeit/micro which brings me a lot of productivity.

Environment

package.json

    "@types/micro": "^7.3.1",
    "typescript": "^3.0.1"

tsconfig.json

{
  "compilerOptions": {
    "module": "commonjs",
    "esModuleInterop": true,
    "target": "es6",
    "noImplicitAny": true,
    "moduleResolution": "node",
    "sourceMap": true,
    "outDir": "dist",
    "baseUrl": ".",
    "lib": ["es2015"],
    "strict": true,
    "noUnusedLocals": true,
    "baseUrl": "./",
    "typeRoots": [
      "node_modules/@types"
    ],
    "paths": {
      "*": [ "./src/*" ]
    }
  },
  "include": [
    "./src/**/*",
    "./src/**/__tests__/*.test.ts"
  ]
}

Problem

I use Micro with TypeScript. When I decode request body using json, I got this error:

Type '{}' is not assignable to type 'Transaction'.  Property 'sender' is missing in type '{}'.

the code:

index.ts

import { json } from 'micro'
import { IncomingMessage } from 'http'

interface Transaction {
  sender: string
}

const newTransaction = async (req: IncomingMessage) => {
  const transaction: Transaction = await json(req)
}

Possible solution

This may be because json returns Promise<object>.

https://github.com/DefinitelyTyped/DefinitelyTyped/blob/master/types/micro/index.d.ts#L26

If I update the definition using Generic Type like the following code, the error disappeared.

@types/micro/index.d.ts

export function json<T extends {} = {}>(req: IncomingMessage, info?: { limit?: string, encoding?: string }): Promise<T>;

index.ts

  const transaction: Transaction = await json<Transaction>(req)

I tried send a PR to DefinitelyTyped, but I faced the following dtslint rule, no-unnecessary-generics.

https://github.com/Microsoft/dtslint/blob/master/docs/no-unnecessary-generics.md

Currently I created my d.ts file, so not a serious problem. However, I think it would be great if someone else can use generic type.

How do you think?