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.

The issue names no files, tests, or entry points. Start by examining how TypeScript represents and evaluates utility types, then determine the intended behavior of a Merge type for overlapping properties, optional properties, and incompatible types. Done means the utility is specified, implemented, and covered by type-checking tests for the examples and edge cases.