Is there a `valueof` similar to `keyof` in TypeScript?

There is another way to extract the union type of the object:

  const myObj = {
    a: 1,
    b: 'some_string'
  } as const;

  type Values = typeof myObj[keyof typeof myObj];

Result union type for Values is 1 | "some_string"

It's possible thanks to the const assertions (as const part) introduced in TS 3.4.

If anyone still looks for implementation of valueof for any purposes, this is a one I came up with:

type valueof<T> = T[keyof T]

Usage:

type actions = {
  a: {
    type: 'Reset'
    data: number
  }
  b: {
    type: 'Apply'
    data: string
  }
}
type actionValues = valueof<actions>

Works as expected :) Returns an Union of all possible types

With the function below you can limit the value to be the one for that particular key.

function setAttribute<T extends Object, U extends keyof T>(obj: T, key: U, value: T[U]) {
    obj[key] = value;
}

Example

interface Pet {
     name: string;
     age: number;
}

const dog: Pet = { name: 'firulais', age: 8 };

setAttribute(dog, 'name', 'peluche')     <-- Works
setAttribute(dog, 'name', 100)           <-- Error (number is not string)
setAttribute(dog, 'age', 2)              <-- Works
setAttribute(dog, 'lastname', '')        <-- Error (lastname is not a property)

Try this:

type ValueOf<T> = T extends any[] ? T[number] : T[keyof T]

It works on an array or a plain object.

// type TEST1 = boolean | 42 | "heyhey"
type TEST1 = ValueOf<{ foo: 42, sort: 'heyhey', bool: boolean }>
// type TEST2 = 1 | 4 | 9 | "zzz..."
type TEST2 = ValueOf<[1, 4, 9, 'zzz...']>

You can made a Generic for your self to get the types of values, BUT, please consider the declaration of object should be declared as const, like:

export const APP_ENTITIES = {
  person: 'PERSON',
  page: 'PAGE',
} as const; <--- this `as const` I meant

Then the below generic will work properly:

export type ValueOf<T> = T[keyof T];

Now use it like below:

const entity: ValueOf<typeof APP_ENTITIES> = 'P...'; // ... means typing

   // it refers 'PAGE' and 'PERSON' to you

Update

Sometimes I haven't good intellisence by using T[keyof T], So by searching and asking LLMs I found the following code better than before:

export type ValueOf<T> = `${T}`;

Thanks the existing answers which solve the problem perfectly. Just wanted to add up a lib has included this utility type, if you prefer to import this common one.

https://github.com/piotrwitek/utility-types#valuestypet

import { ValuesType } from 'utility-types';

type Props = { name: string; age: number; visible: boolean };
// Expect: string | number | boolean
type PropsValues = ValuesType<Props>;

with type-fest lib, you can do that with ValueOf like that:

import type { ValueOf } from 'type-fest';

export const PATH_NAMES = {
  home: '/',
  users: '/users',
  login: '/login',
  signup: '/signup',
};

interface IMenu {
  id: ValueOf<typeof PATH_NAMES>;
  label: string;
  onClick: () => void;
  icon: ReactNode;
}

  const menus: IMenu[] = [
    {
      id: PATH_NAMES.home,
      label: t('common:home'),
      onClick: () => dispatch(showHome()),
      icon: <GroupIcon />,
    },
    {
      id: PATH_NAMES.users,
      label: t('user:users'),
      onClick: () => dispatch(showUsers()),
      icon: <GroupIcon />,
    },
  ];

You could use help of generics to define T that is a key of JWT and value to be of type JWT[T]

function onChange<T extends keyof JWT>(key: T, value: JWT[T]);

the only problem here is in the implementation that following obj[key] = value + ' (assigned)'; will not work because it will try to assign string to string & Date. The fix here is to change index from key to token so compiler knows that the target variable type is string.

Another way to fix the issue is to use Type Guard

// IF we have such a guard defined
function isId(input: string): input is 'id' {
  if(input === 'id') {
    return true;
  }

  return false;
}

// THEN we could do an assignment in "if" block
// instead of switch and compiler knows obj[key] 
// expects string value
if(isId(key)) {
  obj[key] = value + ' (assigned)';
}

I realize this is slightly off topic, That said every time I've looked for a solution to this. I get sent to this post. So for those of you looking for String Literal Type generator, here you go.

This will create a string Literal list from an object type.

export type StringLiteralList<T, K extends keyof T> = T[keyof Pick<T, K>];

type DogNameType = { name: "Bob", breed: "Boxer" } | { name: "Pepper", breed: "Spaniel" } | { name: "Polly", breed: "Spaniel" };

export type DogNames = StringLiteralList<DogNameType, "name">;

// type DogNames = "Bob" | "Pepper" | "Polly";

Building on @Dima's answer

const REGISTRY = 
{
  one: MyClassOne,
  two: MyClassTwo
};

function select(value: keyof typeof REGISTRY): typeof REGISTRY[keyof typeof REGISTRY]
{
  return REGISTRY[value];
}

const myNewthing = new select('one');

One-liner:

type ValueTypesOfPropFromMyCoolType = MyCoolType[keyof MyCoolType];

Example on a generic method:

declare function doStuff<V extends MyCoolType[keyof MyCoolType]>(propertyName: keyof MyCoolType, value: V) => void;