Creating an Object Type from a String Union Type in TypeScript

How can I go about implementing this?

type ActionNames = 'init' | 'reset';

type UnionToObj<U> = {/* SOLUTION NEEDED HERE */}

type Result = UnionToObj<ActionNames>;
// Expected type for Result: `{ init: any, reset: any }`

I tried to write an implementation, but it didn't work as expected. It ran into the issue of union types extending covariance:

type UnionToObj<U> = U extends string ? { [K in U]: any } : never;
type Result = UnionToObj<'init' | 'reset'>;
// The expected Result type is `{ init: any, reset: any }`
// However, I ended up with a union object: `{ init: any } | { reset: any }`
// How can I fix this problem?

Main problems that need resolution:

  1. Converting string union types to object types
  2. Covariance of unions in TypeScript's extends clause

Answer №1

Exploring the concept of a mapped type, we encounter a simple example:

type UnionToObj<U extends PropertyKey> = { [K in U]: any }

type Result = UnionToObj<ActionNames>;
/* type Result = {
    init: any;
    reset: any;
} */

By constraining U to behave like a key, we define clear boundaries. Alternatively, if you prefer using a conditional type for checking, consider this approach:

type UnionToObj<U> = [U] extends [PropertyKey] ? { [K in U]: any } : never;

An important distinction lies in preventing distributed types through conditional checks, unlike your original unintentionally distributive type. By encapsulating the checked type within one-tuple brackets, union distribution is avoided.

Explore the code further in the Playground link.

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