Enforce object to contain all keys of an enum and still have type inference for it's values

user_1936629 0
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I have an object and I want to enforce it to contain all keys of an Enum, and I also want the type of it’s values to be inferred. So if I do this:

enum RequiredKeys {
    A = 'a',
    B = 'b'
}
const objectThatShouldContainAllRequiredKeys = {
    [RequiredKeys.A]: (id: string) => {}
};
// Argument of type '123' is not assignable to parameter of type 'string'
// Which is great, that's exactly what I want. 
objectThatShouldContainAllRequiredKeys[RequiredKeys.A](123);

But now, I tried enforcing the object keys and every solution I try breaks the type inference. For example:

enum RequiredKeys {
    A = 'a',
    B = 'b'
}
// Property 'b' is missing in type '{ a: (id: string) => void; }' but required in type 'Record<RequiredKeys, Function>'.
// Which is great, that's exactly what I want
const objectThatShouldContainAllRequiredKeys: Record<RequiredKeys, Function> = {
    [RequiredKeys.A]: (id: string) => {}
};
// No error here, which is less great...
objectThatShouldContainAllRequiredKeys[RequiredKeys.A](123);

Any idea how I can enjoy both worlds? Have the object enforce all keys from the enum and infer the object values? Thanks!!

typescript enums
2 Answers

You can create identity function with type parameter constrained to have required keys, so typescript will validate the passed object keys and will infer its values' types:

const createWithRequiredKeys = <T extends Record<RequiredKeys, unknown>>(obj: T) => obj;

const withRequiredKeys = createWithRequiredKeys({
    [RequiredKeys.A]: (id: string) => {},
    [RequiredKeys.B]: 'foo',
}); 

// withRequiredKeys is { a: (id: string) => void; b: string; }

withRequiredKeys[RequiredKeys.A](123); // 'number' is not assignable to parameter of type 'string'

Playground

user_11130020

Fast solution: (SECOND UPDATE)

enum RequiredKeys {
  A = 'a',
  B = 'b'
}

type Mapped = {
  [RequiredKeys.A]: (id: string) => any,
  [RequiredKeys.B]: (id: number) => any
}

// Property 'b' is missing in type '{ a: (id: string) => void; }' but required in type 'Record<RequiredKeys, Function>'.
// Which is great, that's exactly what I want
const objectThatShouldContainAllRequiredKeys: Mapped = {
  [RequiredKeys.A]: (id: string) => { },
  [RequiredKeys.B]: (id: number) => { }
}
// No error here, which is less great...
const result = objectThatShouldContainAllRequiredKeys[RequiredKeys.A]('ok'); // ok
const result2 = objectThatShouldContainAllRequiredKeys[RequiredKeys.B](1); // ok

const result3 = objectThatShouldContainAllRequiredKeys[RequiredKeys.B]('1'); // error
const result4 = objectThatShouldContainAllRequiredKeys[RequiredKeys.A](2); // error

If you want more generic solution, you can take a look on this answer

Please keep in mind, that in case of more generic solution, you should create a type map for enum and function types

user_84952540
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