Allow conditionally setting optional properties in a mapped type

Search Terms

conditional optional mapped type

Suggestion

Allow us to conditionally apply ? to a property in a mapped type

Use Cases

I have a function:

function myFunction(one: string, two: number, three: number){}

but I want to map the params to

function myFunction(one: string, two?: number, three?: number){}

Examples

Suggested approach:

type MapParams<T> = {
    [P in keyof T]: T[P] extends string ? T[P] : | ?T[P]
}

(or something similar)

This is the closest I could come:

type MapParams<T> = {
    [P in keyof T]: T[P] extends string ? T[P] : | T[P] | undefined
}

but that produces

function myFunction(one: string, two: number | undefined, three: number | undefined){}

myFunction("", undefined, undefined)

Playground

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.

El issue no proporciona archivos de implementación ni tests; comienza con el Playground enlazado y los ejemplos de mapped types. La finalización implica admitir una forma de hacer opcionales condicionalmente determinadas mapped properties, distinguiendo las properties omitidas de las properties establecidas explícitamente en undefined, con tests del compilador que cubran la forma de función mostrada.