Using property name in typescript declaration

Question

Is there a way to get/use the actual property name in an Typescript type definition.

type Mapped = {
  [P:string]: P;
}

Where Mapped["foo"] is of type "foo" and Mapped["bar"] is of type "bar", not string.

I tried different variations on [P:string], [P in keyof any], typeof P, readonly and as const or type Mapped<Props extends keyof any> = { [P in Props]: P } and everything I came up with along these lines, but P is at best typed as string, never as the actual property name.

What do I need this for?

To build upon. Typing Proxies that dynamically generate utility functions using the accessed property name.

Something like

type GetProperty = {
  [P in (keyof any)]: <T>(obj:T) => P extends keyof T ? T[P] : void;
}

My issue is, that since the generated/returned values are dynamic, I don't have a type T to keyof T the names from.

Could you please provide an example of expected bahaviour?

var obj:Mapped = new Proxy(...);
// where the properties of `obj` should be typed as
var foo:"foo" = obj.foo;
var bar:"bar" = obj.bar;

And obj.asdf1234 should be of type "asdf1234"

@captain-yossarian does this explain what I'm trying to achieve? — Thomas
– Thomas, Commented Feb 6, 2021 at 15:18

jcalz · Accepted Answer · 2021-02-07 04:01:39Z

2

This cannot be done in TypeScript. There is an open feature request at microsoft/TypeScript#22509 which asks for this functionality, and while you should feel free to go there and give it a 👍 or describe your use case if you think it's particularly compelling, I'd say that there's a low chance it'll happen. From this comment:

It is simply not possible to model that with our current type system features (because nothing allows you to capture the literal type corresponding to the property name in a property access).

So I wouldn't hold my breath if I were you.

My suggestion is to give up on dynamic property keys and instead try a single method takes the desired key as an argument:

const obj = {
  get<P extends PropertyKey>(p: P) {
    return <T,>(t: T): P extends keyof T ? T[P] : void =>
      (t as any)[p]

  }
}
  
const foo = {
  a: 1,
  b: "two",
  c: true
}

console.log(obj.get("a")(foo).toFixed(2)); // 1.00
console.log(obj.get("b")(foo).toUpperCase()); // TWO
obj.get("z")(foo) // void

Or, at that point, you might as well just use a function instead of a method:

function getter<P extends PropertyKey>(p: P) {
  return <T,>(t: T): P extends keyof T ? T[P] : void =>
    (t as any)[p]

}

console.log(getter("a")(foo).toFixed(2)); // 1.00
console.log(getter("b")(foo).toUpperCase()); // TWO
getter("z")(foo) // void

The idea is that instead of obj.a(foo) you have either obj.get("a")(foo) or just getter("a")(foo). It has the same functionality but it's packaged a little differently. It might not be the exact form you want, but at least it has the advantage that TypeScript can represent what it's doing!

Playground link to code

answered Feb 7, 2021 at 4:01

jcalz

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1 Comment

Thomas Over a year ago

Thank you for the detailed answer. I am aware that I can use a function for this. I'm currently playing around with Proxies, and whether such constructs would be usable. Something like array.map(property.name) feels neat (at first glance) and I want to explore these realms. I hoped that there was a way to properly type constructs like that.

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