Exclude any objects in a union type that contain keys other than a specified key

Question

Exclude any objects in a union type that contain keys other than a specified key

Consider a type such as

type Test = { name: "a" } | { name: "b" } | { name: "c", other: 1 } | { name: "d", other: 2 }

Is there a way to selectively remove types from the union that have keys other than "name". For example, in this scenario, the expected result would be { name: "a" } | { name: "b" }.

If interested, you can experiment further on TypeScript Playground by visiting this link: Link to TypeScript Playground

typescript

Answer 1

Answer №1

In situations like this, it is common to utilize a distributive conditional type to break down the union input type into its individual members and then selectively keep or discard each member before combining them back into another union. Here's one approach:

type Filter<T> = T extends unknown ?
    Exclude<keyof T, "name"> extends never ? T : never
    : never

The expression T extends unknown ? ⋯ : never distributes over unions in the generic type parameter T. The filter used here is

Exclude<keyof T, "name"> extends never ? T : never

. The utility type Exclude<X, Y> eliminates union members of X that match Y, so Exclude<keyof T, "name"> represents the keys of

T</code that are not named "name." If this set is empty (which means the impossible <code>never

type), we retain T; otherwise, we discard it.

Let's put it to the test:

type Test = { name: "a" } | { name: "b" } |
{ name: "c", other: 1 } | { name: "d", other: 2 }

type FilteredTest = Filter<Test>;
/* type FilteredTest = { name: "a"; } | { name: "b"; } */

let x: FilteredTest = { name: "c", other: 1 } // Error
let y: FilteredTest = { name: "a" } // Fine

Everything seems to be working as expected!

Try out the code yourself in the playground

Answer 2

In situations like this, it is common to utilize a distributive conditional type to break down the union input type into its individual members and then selectively keep or discard each member before combining them back into another union. Here's one approach:

type Filter<T> = T extends unknown ?
    Exclude<keyof T, "name"> extends never ? T : never
    : never

The expression T extends unknown ? ⋯ : never distributes over unions in the generic type parameter T. The filter used here is

Exclude<keyof T, "name"> extends never ? T : never

. The utility type Exclude<X, Y> eliminates union members of X that match Y, so Exclude<keyof T, "name"> represents the keys of

T</code that are not named "name." If this set is empty (which means the impossible <code>never

type), we retain T; otherwise, we discard it.

Let's put it to the test:

type Test = { name: "a" } | { name: "b" } |
{ name: "c", other: 1 } | { name: "d", other: 2 }

type FilteredTest = Filter<Test>;
/* type FilteredTest = { name: "a"; } | { name: "b"; } */

let x: FilteredTest = { name: "c", other: 1 } // Error
let y: FilteredTest = { name: "a" } // Fine

Everything seems to be working as expected!

Try out the code yourself in the playground

Exclude any objects in a union type that contain keys other than a specified key

Answer №1

Similar questions

The parameter 'string | JwtPayload' cannot be assigned to the parameter 'string'

Guide on how to navigate within an app by clicking on a link from an email

Struggling with setting up eslint in my typescript project

The assignment of Type Observable<Observable<any[]>> to Observable<any[]> is not valid

Refresh a reactive form in Angular Material

How can Angular JS handle multiple validators being triggered at once?

The error message "Property <property> is not recognized on the type 'jQueryStatic<HTMLElement>'" is indicating an issue with accessing a specific property within the TypeScript codebase that utilizes Angular CLI, NPM,

Converting date format from d-mmm-yyyy to yyyy-mm-d using Angular 2

What is the best way to capture the inputs' values and store them accurately in my object within localStorage?

determine function output based on input type

A guide on transitioning from using require imports to implementing ES6 imports with the concept of currying

The error message "ER_BAD_FIELD_ERROR: The column 'table.column' is not recognized in the 'on clause'" is displayed

Progressive series of observable conditions

Updating the JWT token in Angular 6 and making a new request with the updated token

The issue arises when the desired image size is not reflected correctly on the background after changing

I am having trouble with Visual Studio Code and ESLint not recognizing spelling errors such as "style.backgroundCOlor". What steps can I take to resolve this issue?

``There seems to be an issue with retrieving and displaying data in Ionic2 when using nav

What is the best way to transform a Map<string, boolean> into an array of objects with key-value pairs {key: string, value: boolean

Sending a CSS class to an Angular library

Having difficulty in accessing the node modules