Using the Typescript type 'never' for object fields: a guide to implementing it

Question

Using the Typescript type 'never' for object fields: a guide to implementing it

I'm attempting to make this specific example function similar to this one:

interface Foo {
    a: number;
    b: string;
    c: boolean;
}

type Explode<T> = keyof T extends infer K
    ? K extends unknown
    ? { [I in keyof T]: I extends K ? T[I] : never }
    : never
    : never;


type Test = Explode<Foo>;

const test: Test = {a: 1};

This results in the error:

Type '{ a: number; }' is not assignable to type '{ a: number; b: never; c: never; } | { a: never; b: string; c: never; } | { a: never; b: never; c: boolean; }'.
  Type '{ a: number; }' is missing the following properties from type '{ a: number; b: never; c: never; }': b, c

Is there a way to create an object of type Test without encountering the error? I am looking for a type that can accommodate either the field a, b, or c (or be an empty object {}).

typescript

Answer 1

Answer №1

I stumbled upon the solution. In order to resolve the issue, I had to adjust the fields of Foo to be optional in the following way:

UPDATED to incorporate the recommendation by @apokryfos

interface Foo {
    a: number;
    b: string;
    c: boolean;
}

Now I am able to:

interface Foo {
    a: number;
    b: string;
    c: boolean;
}

type AtMostOneOf<T> = keyof T extends infer K
    ? K extends unknown
    ? { [I in keyof T]+?: I extends K ? T[I] : never }
    : never
    : never;


type Test = AtMostOneOf<Foo>;

const test: Test = {a: 1}; // permitted
const test1: Test = {b: 'asd'}; // permitted
const test2: Test = {a: 1, b: 'asd'}; // not permitted
const test3: Test = {} // permitted

Playground

Answer 2

I stumbled upon the solution. In order to resolve the issue, I had to adjust the fields of Foo to be optional in the following way:

UPDATED to incorporate the recommendation by @apokryfos

interface Foo {
    a: number;
    b: string;
    c: boolean;
}

Now I am able to:

interface Foo {
    a: number;
    b: string;
    c: boolean;
}

type AtMostOneOf<T> = keyof T extends infer K
    ? K extends unknown
    ? { [I in keyof T]+?: I extends K ? T[I] : never }
    : never
    : never;


type Test = AtMostOneOf<Foo>;

const test: Test = {a: 1}; // permitted
const test1: Test = {b: 'asd'}; // permitted
const test2: Test = {a: 1, b: 'asd'}; // not permitted
const test3: Test = {} // permitted

Playground

Answer 3

Answer №2

To create the Explode type, you can do the following:


type Explode<T> = {
    [K in keyof T]: {
        [key in K]: T[K]
    }
}[keyof T]

After defining the Explode type, the type Test will be structured like this:

type Test = {
    a: number;
} | {
    b: string;
} | {
    c: boolean;
}

Check out this TS Playground for more information.

Answer 4

To create the Explode type, you can do the following:


type Explode<T> = {
    [K in keyof T]: {
        [key in K]: T[K]
    }
}[keyof T]

After defining the Explode type, the type Test will be structured like this:

type Test = {
    a: number;
} | {
    b: string;
} | {
    c: boolean;
}

Check out this TS Playground for more information.

Using the Typescript type 'never' for object fields: a guide to implementing it

Answer №1

Answer №2

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