TypeScript Wrapper Function Inferring Types

user_14101494 0
Viewed 81

I use a little utility/wrapper function for functions related to database operations. It should take a specific function fn with the type Promise<PromiseReturnType<GENERIC>>, then do some stuff (here: checking for connection) before executing it:

// File `A.ts`
type PromiseReturnType<T = undefined> = {success: boolean, response: T } 

const DatabaseOperation = <Fn extends (...args: any) => Promise<PromiseReturnType<Data>>, Data>(fn: (...args: any) => Promise<PromiseReturnType<Data>>) => {

    return async function (...args: Parameters<Fn>): Promise<PromiseReturnType<Data>> {
        if (!isConnected) return Promise.reject({ success: false, response: new Error("Not connected to the Database!") })

        return await fn(...args)
    }

}

Then I use it like:

// File `B.ts`
const dbOperation = async (myArg: number): Promise<PromiseReturnType<string> => {

    return Promise.resolve({ success: true, response: `hello world ${myArg}` })

}


export default DatabaseOperation<typeof dbOperation, string>(dbOperation)

It works fine except the return type can not be inferred. It will be Promise<any> except Promise<PromiseReturnType<string>>.

Using that newly wrapped function will look like this:

import dbOperation from "B.ts"

(function () {
   await dbOperation() // Type: dbOperation(myArg: number): Promise<any>
})()

Any suggestions on how to solve this or implement this better in general?

Appreciate your help, thanks.

typescript-generics typescript strong-typing type-constraints javascript
1 Answers

You were almost there:



declare var isConnected: boolean;

type PromiseReturnType<T = undefined> = { success: boolean, response: T }

const DatabaseOperation = <Data, Fn extends (...args: any[]) => Promise<PromiseReturnType<Data>>,>(fn: (...args: any[]) => Promise<PromiseReturnType<Data>>) => {

    return async function (...args: Parameters<Fn>): Promise<PromiseReturnType<Data>> {
        if (!isConnected) return Promise.reject({ success: false, response: new Error("Not connected to the Database!") })

        return await fn(...args)
    }

}

const dbOperation = (myArg: number): Promise<PromiseReturnType<string>> =>
    Promise.resolve({ success: true, response: `hello world ${myArg}` })



const fn = DatabaseOperation(dbOperation)

const result = fn(42) // Promise<PromiseReturnType<string>>

Playground

You should have to made ...args an array any[] instead of just any

user_84952540
Related