What methods are available to restrict the values of properties to specific keys within the current type?

Question

What methods are available to restrict the values of properties to specific keys within the current type?

I am looking to declare an interface in typescript that is extensible through an indexer, allowing for the dynamic association of additional functions. However, I also want sub properties within this interface that can refer back to those indexed functions by name. Do you think this is achievable with the capabilities of the TS compiler? I haven't been able to stump it yet, but I'm curious if anyone has any ideas.

interface CustomInterface {
  someProp: {
    [x: string]: keyof this; // Attempting to refer to indexed keys here
  };
  [x: string]: Function; // Clients can provide their own functions here
}

// Example of how to use it
let myCustomInterface: CustomInterface = {
   bar: {
      validFuncName: "someFunc", // This should work
      invalidFuncName: "thisIsNotAFunction" // This should trigger an error
   },
   someFunc: () => {}
}

typescript typescript-types

Answer 1

Answer №1

After reviewing @kelly's response, a more simplified solution is presented below.

function customHelper<T extends Record<string, any>>(obj: { 
   [K in keyof T]: K extends "bar" 
     ? Record<string, Exclude<keyof T, "bar">> 
     : Function 
}){}

customHelper({
   bar: {
      validFuncName: "someFooFunc", // valid
      validFuncName2: "someOtherFunc", // valid
      invalidFuncName: "thisAintNoFunc" // Error
   },
   someFooFunc: () => {},
   someOtherFunc: () => {}
})

Playground

Answer 2

After reviewing @kelly's response, a more simplified solution is presented below.

function customHelper<T extends Record<string, any>>(obj: { 
   [K in keyof T]: K extends "bar" 
     ? Record<string, Exclude<keyof T, "bar">> 
     : Function 
}){}

customHelper({
   bar: {
      validFuncName: "someFooFunc", // valid
      validFuncName2: "someOtherFunc", // valid
      invalidFuncName: "thisAintNoFunc" // Error
   },
   someFooFunc: () => {},
   someOtherFunc: () => {}
})

Playground

Answer 3

Answer №2

This is a modified version of Tobias S.'s response (with inspiration from Kelly's original code) that upholds the integrity of the object type passed to the support function.

type ExtractFunctions<T> = {
    [K in keyof T as T[K] extends Function ? K : never]: T[K]
}

function adjuster<
    T extends { 
        [K in keyof T]: K extends "bar"
            ? Record<string, keyof ExtractFunctions<T>>
            : T[K]
    }
>(obj: T): T {
    return obj;
}

adjuster({
   bar: {
      validFuncName: "someFooFunc", // acceptable
      validFuncName2: "someOtherFunc", // okay
      invalidFuncName: "thisAintNoFunc" // Issue
   },
   someFooFunc: () => {},
   someOtherFunc: () => {}
});

Answer 4

This is a modified version of Tobias S.'s response (with inspiration from Kelly's original code) that upholds the integrity of the object type passed to the support function.

type ExtractFunctions<T> = {
    [K in keyof T as T[K] extends Function ? K : never]: T[K]
}

function adjuster<
    T extends { 
        [K in keyof T]: K extends "bar"
            ? Record<string, keyof ExtractFunctions<T>>
            : T[K]
    }
>(obj: T): T {
    return obj;
}

adjuster({
   bar: {
      validFuncName: "someFooFunc", // acceptable
      validFuncName2: "someOtherFunc", // okay
      invalidFuncName: "thisAintNoFunc" // Issue
   },
   someFooFunc: () => {},
   someOtherFunc: () => {}
});

What methods are available to restrict the values of properties to specific keys within the current type?

Answer №1

Answer №2

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