Handler return is not type-checked.

[canassa]

When using TypeScript, the return of a handler is not being validated by the type checker.

To Reproduce

Add this file

import type {APIGatewayProxyEvent, APIGatewayProxyResult} from "aws-lambda";
import middy from "@middy/core";


export const handler = middy<APIGatewayProxyEvent, APIGatewayProxyResult>()
    .handler(async (req, context) => {
        return 123
    })

• Run the type checker.
• Observe that no error is detected.

Expected behaviour
The type checker should have throw an error since 123 does not match the APIGatewayProxyResult.

Note: I know that this can be workaround by manually adding the return type to the handler function, but that's more error prone.

Environment (please complete the following information):

• Node.js: 20
• Middy: 5.4.5

[canassa]

This seems to be a regression with Middy 5. Middy 4 is able to correctly validate the type:

error TS2345: Argument of type '(req: APIGatewayProxyEvent, context: Context) => Promise<number>' is not assignable to parameter of type 'MiddyInputHandler<APIGatewayProxyEvent, APIGatewayProxyResult, Context>'.
  Type 'Promise<number>' is not assignable to type 'void | APIGatewayProxyResult | Promise<APIGatewayProxyResult>'.
    Type 'Promise<number>' is not assignable to type 'Promise<APIGatewayProxyResult>'.
      Type 'number' is not assignable to type 'APIGatewayProxyResult'.

8     .handler(async (req, context) => {
               ~~~~~~~~~~~~~~~~~~~~~~~~~

[willfarrell]

A PR is welcome for this.

[naorpeled]

On it 🙏

Will keep you posted.