Add a Merge utility type

Search Terms

merge types

Suggestion

It would be great if you could merge two types. The closest I can get right now is a union:

type Test1 = { id: number, code: string }
type Test2 = { id: string, code: number }
type Test3 = Test1 | Test2

// This is not allowed, because x doesn't match Test1 OR Test2
export const x: Test3 = {
  id: "bob",
  code: "bob"
}

Basically I want to be able to create the following type by merging Test1 and Test2:

type Test3 = { id: number | string, code: number | string }

Perhaps there could be a utility to merge the types:

type Test3 = Merge<Test1, Test2>

Use Cases

React-Native has a few style types (TextStyle, ImageStyle, ViewStyle) and they are very useful for autocomplete and type checking in my code. The TextStyle has a property fontSize: number. I want to be able to redefine the TextStyle type to allow a string value for all of the properties (so that I can use my own variable system). Something like fontSize: "$fontSizeMD"

Examples

type EImageStyle = ImageStyle | Partial<Record<keyof ImageStyle, string>>
type ETextStyle = TextStyle | Partial<Record<keyof TextStyle, string>>
type EViewStyle = ViewStyle | Partial<Record<keyof ViewStyle, string>>

type EComponentStyle = EImageStyle | ETextStyle | EViewStyle
type EComponentStyles = { [P in keyof any]: EComponentStyle }

With the merge it would look like:

type EImageStyle = Merge<ImageStyle, Partial<Record<keyof ImageStyle, string>>>
type ETextStyle = Merge<TextStyle, Partial<Record<keyof TextStyle, string>>>
type EViewStyle = Merge<ViewStyle, Partial<Record<keyof ViewStyle, string>>>

type EComponentStyle = EImageStyle | ETextStyle | EViewStyle
type EComponentStyles = { [P in keyof any]: EComponentStyle }

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.

Issue にはファイル、テスト、エントリーポイントが記載されていません。まず、TypeScript がユーティリティ型をどのように表現し評価するかを調べ、次に、重複するプロパティ、オプショナルなプロパティ、互換性のない型に対する Merge 型の意図された動作を決定してください。ユーティリティが仕様化・実装され、例とエッジケースについて型チェックテストでカバーされていれば完了です。