Typescript issue with dynamic types of argument

Question

I'm trying to make getter and setter in Typescript and facing issues with dynamic types of arguments.

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

const person: PersonType = {
  name: '',
  age: 0
};

const setPersonKey = ({ key, value }: { key: keyof PersonType; value: PersonType[keyof PersonType] }) => {
  person[key] = value; // ERROR -> Type 'string | number' is not assignable to type 'never'. Type 'string' is not assignable to type 'never'.
}


const getPersonKey = ({ key }: { key: keyof PersonType; }): PersonType[keyof PersonType] => {
  return person[key];
}

I have 3 issues with the above code

person[key] throws an error in setPersonKey function
value type is not dynamic based on the key passed in setPersonKey function
return type of getPersonKey function is not dynamic based on key value

Would really appreciate a reference doc and an explanation of the solution.

Typescript playground for testing: https://www.typescriptlang.org/play

Have tried that, but the error does not change. I will update the question to remove this confusion. — Raj Rohit Yadav
– Raj Rohit Yadav, Commented Jul 26, 2022 at 14:03
typescript-typings is a tag very specifically regarding a type library system and indicates clearly in all-caps "DO NOT USE IT AS A SYNONYM OF TYPES". — crashmstr
– crashmstr, Commented Jul 26, 2022 at 14:23

Nikos Paraskevopoulos · Accepted Answer · 2022-07-26 14:10:54Z

1

The problem is with the argument type - { key: keyof PersonType; value: PersonType[keyof PersonType] } is an object with a key property, that is a key of PersonType(OK), and a value property that is of the same type as any value of PersonType, not necessarily the same as key.

Try this to make the compiler understand that key and value refer to the same proprty of PersonType::

const setPersonKey = <K extends keyof PersonType>({ key, value }: { key: K; value: PersonType[K] }) => {
  person[key] = value;
}

answered Jul 26, 2022 at 14:10

Nikos Paraskevopoulos

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1 Comment

Raj Rohit Yadav Over a year ago

Thank you for your quick help. Working sample here replit.com/@RajRohit/AutopickType#index.ts

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