Expose inferred type for use in type annotations

Search Terms

infer inferred type annotation

Suggestion

When annotating the type of a variable, allow the variable to have access to the type that would've been inferred for the RHS, so that the author can build a type annation derived from that inferred value. This value could be exposed as the inferred keyword or similar.

Use Cases/Examples

// All properties in `deletable` are subject to being deleted, so we want
// the type to be a Partial of the would-have-been-inferred type.
// I don't think there's a great way to write this at the moment.
const deletable: Partial<inferred> = { a: true, b: false, c: "xyz" };

// The `mutable` property might be reassigned to, but all the other properties wont.
// So combined `Readonly` and `inferred`, with an exception for mutable.
type Legal = "initial" | "middle" | "final"
const partiallyMutable: Readonly<Omit<inferred, "mutable">> & { mutable: Legal } = { 
  x: "literally", 
  y: true,
  mutable: "initial",
  lotsOfOther: 47,
  literalPropsHere: "name"
}

// We want to verify assignability to the mapped type, to make sure that, as new 
// required keys are added, this object literal is updated. However, we also want the
// values at each key to be inferred narrowly as a literal type for use in the code that follows.
type RequiredKeys = "A" | "B" | "C"
const mustHaveAllKeys: inferred & { [K in RequiredKeys]: any } =  = {
  "A": true,
  "B": false,
  "C": "hi!"
}

// typeof mustHaveAllKeys.A should be true!

Related issues

• https://github.com/Microsoft/TypeScript/issues/24375
• https://github.com/microsoft/TypeScript/issues/17574

Checklist

My suggestion meets these guidelines:

• This wouldn't be a breaking change in existing TypeScript/JavaScript code
• This wouldn't change the runtime behavior of existing JavaScript code
• This could be implemented without emitting different JS based on the types of the expressions
• This isn't a runtime feature (e.g. library functionality, non-ECMAScript syntax with JavaScript output, etc.)
• This feature would agree with the rest of TypeScript's Design Goals.

まず関連する issue 24375 と 17574 を確認し、次に TypeScript が推論された型と型注釈をどのように扱うかを調べてください。示されている例に対する inferred の動作を定義し、要求された注釈が意図した narrow 型および mapped 型で型チェックに通ることを検証できれば完了です。