Typescript - Preserve concrete object shape-CodePudding

Is it possible in TypeScript for a function to accept any generic object, but preserve the concrete object shape in the type information?

I'm trying to create an interface like this:

interface Object {
    state(): Object
}

Object.prototype.state = function () { ... }

in a way that the output of state() preserves the concrete object shape in the type information. But if I use the code above and do:

let obj = {somekey: 1}.state()

obj wil have a type of Object, instead of {somekey: number}.

This can easily be done with Arrays, because they are generalised on their members. Doing:

interface Array<T> {
    state(): Array<T>
}

Array.prototype.state = function () { ... }

let arr = [1].state()

will preserve arr's type as number[].

How can I do the same with Objects, please?

CodePudding user response：

I'm afraid you cannot do this with Object.prototype, but you can surely write a function like this:

function identity<T>(input: T): T { return input; }

it will preserve all type information it can gain when you call it, for example:

// x has type { somekey: number }
const x = identity({somekey: 1});

Try it in the typescript playground

CodePudding user response：

Have you tried using a this parameter?

interface Object {
    state<T>(this: T): T
}

Object.prototype.state = function () { return this }

let obj = { somekey: 1 }.state()
//  ^? { somekey: number }

Playground