Expose MappedType in typescript.d.ts

[JoshuaKGoldberg]

🔍 Search Terms

mappedtype interface typeParameter constraintType templateType

✅ Viability Checklist

• [X] This wouldn't be a breaking change in existing TypeScript/JavaScript code
• [X] This wouldn't change the runtime behavior of existing JavaScript code
• [X] This could be implemented without emitting different JS based on the types of the expressions
• [X] This isn't a runtime feature (e.g. library functionality, non-ECMAScript syntax with JavaScript output, new syntax sugar for JS, etc.)
• [X] This isn't a request to add a new utility type: https://github.com/microsoft/TypeScript/wiki/No-New-Utility-Types
• [X] This feature would agree with the rest of our Design Goals: https://github.com/Microsoft/TypeScript/wiki/TypeScript-Design-Goals

⭐ Suggestion

Coming over from typescript-eslint, we need to look at the properties on mapped types for the @typescript-eslint/no-unnecessary-type-parameters ("Golden Rule of Generics") rule. But, MappedType is marked as @internal in TypeScript's definition:

https://github.com/microsoft/TypeScript/blob/22bbe867fd4b86e2187270f6c284e6cfd92a3fea/src/compiler/types.ts#L6584-L6594

Could we have that interface be public, please? Even if all the fields other than constraintType, templateType, and typeParameter are switched to @internal. I don't know of a more sanctioned way to access those type properties. Is there one?

📃 Motivating Example

Marking MappedType as public would provide a way for type checker API consumers to reason about mapped types.

💻 Use Cases

See https://github.com/typescript-eslint/typescript-eslint/pull/8173 & https://github.com/typescript-eslint/typescript-eslint/pull/9530: in typescript-eslint, we need to access a few properties of mapped types to check if they're uses of a type parameter. We ended up writing our own interface MappedType extends ts.ObjectType:

interface MappedType extends ts.ObjectType {
  typeParameter?: ts.Type; // #8173
  constraintType?: ts.Type; // #9530
  templateType?: ts.Type; // #9530
}

[weswigham]

FYI, those properties are lazily created by functions within the checker - they're just cache fields, which is why they're optional, and we use functions within the checker to calculate, cache, and access them. Relying on their direct presence is risky.