Ufdjrw

I'm trying to implement a "except"-method in TypeScript.
The purpose would be something like this:

const obj = a: true, b: 1, c: 'test', d: false 
const objExcept = except(obj, 'b', 'd')

My problem is not the actual key exclusion, but the typesafe return value.
Ideally accessing objExcept.b should give me an error, cause the return value is:

objExcept: a: boolean, c: string = a: true, c: 'test' `

This is my current implementation

export function except<
 T extends Dictionary,
 K extends any[],
 R extends Pick<T, Exclude<keyof T, K[number]>>
> (value: T, ...exceptions: K): R 
 const copy: Record<string: any> = 
 for (const key in value) 
 if (!value.hasOwnProperty(key) 

 return copy as R

Many thanks in advance

You are very close you just need to get the compiler to infer literal types for K. The best way to do this is to restrict the items in K to keyof T. This will also check that property keys are part of the target object.

export function except<
 T extends Record<string, any>,
 K extends Array<keyof T>,
 R extends Pick<T, Exclude<keyof T, K[number]>>
> (value: T, ...exceptions: K): R 
 const copy: Record<string, any> = 
 for (const key in value) 
 if (!value.hasOwnProperty(key) 

 return copy as R


const obj = a: true, b: 1, c: 'test', d: false 
const objExcept = except(obj, 'b', 'd')
objExcept.a
objExcept.b //err

If you want to allow any keys not necessarily just known keys of T you can also use K extends Array<PropertyKey> this will allow any key but exclude any keys that overlap from T