TypedPropertyDescriptorMap<T>

Search Terms

TypedPropertyDescriptor
TypedPropertyDescriptorMap
Typed Property Descriptor Map

Suggestion

Include TypedPropertyDescriptorMap<T>, the typed version of PropertyDescriptorMap. The following type should do the trick:

type TypedPropertyDescriptorMap<T> = {
  [P in keyof T]: TypedPropertyDescriptor<T[P]>
}

This would require a change to the Object.defineProperties type definition since it only accepts a PropertyDescriptorMap type, an indexed type. Passing in an object with a type of TypedPropertyDescriptorMap<T> results in the error: Index signature is missing in type 'TypedPropertyDescriptorMap<T>'. There's a related issue on this topic (https://github.com/Microsoft/TypeScript/issues/15300).

Use Cases

It's a headache to have to define a type like the following when I already have defined the final object type:

// Final object looks like
interface MyProps {
  prop1: string
  prop2: (params: Record<string, string>) => Promise<void>
}

// My custom TypedPropertyDescriptor map
interface MyPropDescriptors {
  prop1: TypedPropertyDescriptor<MyProps['prop1']>
  prop2: TypedPropertyDescriptor<MyProps['prop2']>
  ...
}

Examples

// Final object looks like
interface MyProps {
  prop1: string
  prop2: (params: Record<string, string>) => Promise<void>
}

// My custom TypedPropertyDescriptor map
type MyPropDescriptors = TypedPropertyDescriptorMap<MyProps>

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.

Start by locating the declaration for Object.defineProperties and the existing TypedPropertyDescriptor and PropertyDescriptorMap types. Review related issue #15300, then verify the proposed mapped type and its acceptance by Object.defineProperties with a focused type-checking test; done means typed descriptor maps work without the index-signature error.