Type information lost after Pick on object with [key: string]: any;

[evollu]

TypeScript Version: 2.8.3

Search Terms: Pick

Code

type X = {
    [key: string]: any;
    x: string,
    y: string
}

type Y = Pick<X, keyof X>

Expected behavior: Y to be same as X

Actual behavior: Y = {[key: string]: any;}

Playground Link:
https://www.typescriptlang.org/play/#src=type%20X%20%3D%20%7B%0D%0A%20%20%20%20%5Bkey%3A%20string%5D%3A%20any%3B%0D%0A%20%20%20%20x%3A%20string%2C%0D%0A%20%20%20%20y%3A%20string%0D%0A%7D%0D%0A%0D%0Atype%20Y%20%3D%20Pick%3CX%2C%20keyof%20X%3E
Related Issues:
My use case is to make a required field optional in type X

type X = {
    [key: string]: any;
    x: string,
    y: string
    ....another 20 params
}

type Omit<T, K> = Pick<T, Exclude<keyof T, K>>; // this works for type without index type
type Y = Omit<X, "x"> & { x?: string}
// expect: x becomes optional and rest remain the same
// actual: y is removed

[Bnaya]

Its very makes sense it will behave like that,
You can use mapped types if you want to keep type info.
See:

type YY<T> = { [K in keyof T]: T[K] };

type YYY = YY<X>; 

[evollu]

@Bnaya yes your solution works for my example. But this issue is causing a problem on Omit. I updated the description with my original use case

my other finding:

// this copies everything
type Y = {
  [P in keyof X]: X[P];
}

// this only copies index type
type Z = keyof X
type Y = {
  [P in Z]: X[P];
}

[Bnaya]

Got it.
I've created this example to make it more clear

type X = {
    [key: string]: any;
    x: string;
    y: string;
}

type X3 = {
    x: string;
    y: string;
}

type Omit<T, TR extends keyof T> = {
    [K in Exclude<keyof T, TR>]: T[K];
};

/**
     Y2 = {
        [x: string]: any;
    }
 */
type Y2 = Omit<X, "x">;

/**
    type Y3 = { y: string; }
 */
type Y3 = Omit<X3, "x">;

https://www.typescriptlang.org/play/#src=type%20X%20%3D%20%7B%0D%0A%20%20%20%20%5Bkey%3A%20string%5D%3A%20any%3B%0D%0A%20%20%20%20x%3A%20string%3B%0D%0A%20%20%20%20y%3A%20string%3B%0D%0A%7D%0D%0A%0D%0Atype%20X3%20%3D%20%7B%0D%0A%20%20%20%20x%3A%20string%3B%0D%0A%20%20%20%20y%3A%20string%3B%0D%0A%7D%0D%0A%0D%0Atype%20Omit%3CT%2C%20TR%20extends%20keyof%20T%3E%20%3D%20%7B%0D%0A%20%20%20%20%5BK%20in%20Exclude%3Ckeyof%20T%2C%20TR%3E%5D%3A%20T%5BK%5D%3B%0D%0A%7D%3B%0D%0A%0D%0A%2F**%0D%0A%20%20%20%20%20Y2%20%3D%20%7B%0D%0A%20%20%20%20%20%20%20%20%5Bx%3A%20string%5D%3A%20any%3B%0D%0A%20%20%20%20%7D%0D%0A%20*%2F%0D%0Atype%20Y2%20%3D%20Omit%3CX%2C%20%22x%22%3E%3B%0D%0A%0D%0A%2F**%0D%0A%20%20%20%20type%20Y3%20%3D%20%7B%20y%3A%20string%3B%20%7D%0D%0A%20*%2F%0D%0Atype%20Y3%20%3D%20Omit%3CX3%2C%20%22x%22%3E%3B

[mhegazy]

The type has an index signature that allows it to have any string named property.. there are no way int eh language to define exclusion from infinite ranges, e.g. string - "x".. that said, not sure what you expect from Pick rely..

[evollu]

@mhegazy I'm expecting Pick to pick both index signature and extra definition below (y: string and x: string). But now it is only picking index signature

[mhegazy]

keyof X is string| number. string subsumes all the other property names, "x" and "y".

[evollu]

is there a way for Typescript to pick "x" and "y" then? Because during type checking, the definition of "x" and "y" takes precedence

[mhegazy]

The compiler will try to keep the properties as long as it can, e.g. Partial<X> does have x and y. the issue is once you get keyof, which is a union, the string subsumes all the other property names, and you end up with only string.
As i noted earlier, it does not make much sense to use operations like Pick, Exclude and Omit on a type like that. if you want to specify the types of properties other than x/y with the index signature, i think you are looking for #17867 instead.

[evollu]

will it make sense for keyof to return like string|"x"|"y" so the information will be kept?

my use case is to make one required field optional, which is common for React component with higher order components. The only solutions I can find is to use Extract which doesn't work here.

[typescript-bot]

Automatically closing this issue for housekeeping purposes. The issue labels indicate that it is unactionable at the moment or has already been addressed.