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.

The issue provides no implementation files or tests; begin with the linked Playground and the mapped-type examples. Completion means supporting a way to conditionally make selected mapped properties optional, distinguishing omitted properties from properties explicitly set to undefined, with compiler tests covering the shown function shape.