Ryfujk

Title says it all - why doesn't Object.keys(x) in TypeScript return the type Array<keyof typeof x>? That's what Object.keys does, so it seems like an obvious oversight on the part of the TypeScript definition file authors to not make the return type simply be keyof T.

Should I log a bug on their GitHub repo, or just go ahead and send a PR to fix it for them?

The current return type (string[]) is intentional. Why?

Consider some type like this:

interface Point {

    x: number;

    y: number;

}

You write some code like this:

function fn(k: keyof Point) {

    if (k === "x") {

        console.log("X axis");

    } else if (k === "y") {

        console.log("Y axis");

    } else {

        throw new Error("This is impossible");

    }

}

Let's ask a question:

In a well-typed program, can a legal call to fn hit the error case?

The desired answer is, of course, "No". But what does this have to do with Object.keys?

Now consider this other code:

interface NamedPoint extends Point {

    name: string;

}



const origin: NamedPoint = { name: "origin", x: 0, y: 0 };

Note that according to TypeScript's type system, all NamedPoints are valid Points.

Now let's write a little more code:

function doSomething(pt: Point) {

    for (const k of Object.keys(pt)) {

        // A valid call iff Object.keys(pt) returns (keyof Point)[]

        fn(k);

    }

}

// Throws an exception

doSomething(origin);

Our well-typed program just threw an exception!

Something went wrong here!
By returning keyof T from Object.keys, we've violated the assumption that keyof T forms an exhaustive list, because having a reference to an object doesn't mean that the type of the reference isn't a supertype of the type of the value.

Basically, (at least) one of the following four things can't be true:

1. 
   keyof T is an exhaustive list of the keys of T
   


2. A type with additional properties is always a subtype of its base type


3. It is legal to alias a subtype value by a supertype reference


4. 
   Object.keys returns keyof T
   

Throwing away point 1 makes keyof nearly useless, because it implies that keyof Point might be some value that isn't "x" or "y".

Throwing away point 2 completely destroys TypeScript's type system. Not an option.

Throwing away point 3 also completely destroys TypeScript's type system.

Throwing away point 4 is fine and makes you, the programmer, think about whether or not the object you're dealing with is possibly an alias for a subtype of the thing you think you have.

The "missing feature" to make this legal but not contradictory is Exact Types, which would allow you to declare a new kind of type that wasn't subject to point #2. If this feature existed, it would presumably be possible to make Object.keys return keyof T only for Ts which were declared as exact.