Proposal: InvariantOf

[younho9]

Inspired by InvariantOf.

It could be useful to make object invariant. It is similar opaque type without brand or tag.

import {Opaque} from './opaque';

export type InvariantOf<Type> = Opaque<Type, keyof Type>;

I published this utility type as package, but I wish to add it to this project.

Here are some test cases.

I'm willing to do PR. 😃

[younho9]

Update: use function signature to transform type into a invariant type

declare const tag: unique symbol;

declare type InvariantProp<Type> = (arg: Type) => Type;

declare type InvariantTagged<Type> = {
  readonly [tag]: InvariantProp<Type>;
};

/**
 * Transfrom type to Invariant type
 */
export declare type InvariantOf<Base> = Base & InvariantSignature<Base>;

[sindresorhus]

What's the practical use-case? I'm not sure when I would need this.

[younho9]

In structural typing, Object can have more properties at run-time, so Object.keys returning (keyof T)[] is quite dangerous.

microsoft/TypeScript#12253 (comment)

If use invariantOf, because it requires exactly the same type, typescript warn at the type level when the structure of the object changes.

TypeScript Playground

This has a tricky side, but there are rare use cases to accurately distinguish the type according to the structure.

[sindresorhus]

This is accepted.

For anyone submitting a pull request for this, it should come with a clear description, use-cases, and a practical example.