Typescript - Use generics and parameters values to set a type

Question

I'd like a way to have an interface with a single generic, in which a property is keyof T and the other is the proper T[keyof T] passed on the first property.

The following code almost meets this requirement, correctly typing the first property (field):

interface RandomType {
    foo: string;
    bar: number;
    baz: boolean;
}

type Typer<T = Record<string, any>> = {
    field: keyof T,
    value: T[any]
}

const param: Typer<RandomType> = {
    field: 'baz',
    value: 'foo'
}

However, I need to type value based on field's value

in the previous example, it should trigger a type error because RandomType['baz'] is boolean

jcalz · Accepted Answer · 2020-10-06 14:40:55Z

You want Typer<T> to be a union of field/value pairs for each field in T. You can do this a number of ways. For example:

type Typer<T> = { [K in keyof T]: {
    field: K,
    value: T[K]
} }[keyof T];

Here we are mapping over the properties in T to get a new object whose values are the types you want, and then immediately looking up all its values.

You can verify that Typer<RandomType> is what you're looking for:

type TyperRandomType = Typer<RandomType>
/* type TyperRandomType = {
    field: "foo";
    value: string;
} | {
    field: "bar";
    value: number;
} | {
    field: "baz";
    value: boolean;
} */

and then your example gives the error you expect:

const param: Typer<RandomType> = {
    field: 'baz',
    value: 'foo'
}; // error!
// Types of property 'value' are incompatible.
// Type 'string' is not assignable to type 'boolean'

Playground link to code

brunnerh · Accepted Answer · 2020-10-06 14:18:00Z

0

This would do what you want, but right now you have to specify the key twice:

type Typer<T, K extends keyof T> = {
    field: K,
    value: T[K]
}

const param: Typer<RandomType, 'baz'> = {
    field: 'baz',
    value: 'foo' // Type 'string' is not assignable to type 'boolean'. ts(2322)
}

I think there is an open issue for TS to make it possible to only specify some generics and infer the rest, so that Typer<RandomType> would work.

answered Oct 6, 2020 at 14:18

brunnerh

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1 Comment

brunnerh Over a year ago

Probably the most relevant issue on the matter: github.com/microsoft/TypeScript/issues/26242

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