CODEWITHSUNDEEP
typescripttypeskey
Anders
Anders

Reputation: 8577

TypeScript function that returns value for string literal of property name?

Consider this TypeScript function that returns the value of an objects property:

export function getProperty<T>(object: T, key: string): any {
    return (object as any)[key]
}

It is not very nice to use, since it always returns an any:

interface Animal {
    name: string,
    hungry: boolean,
}

const myHippo: Animal = {
    name: "Hippo",
    hungry: true,
}

const name = getProperty(myHippo, "name") // Returns any, but I want a string.
const size = getProperty(myHippo, "size") // Returns any, but I want a compiler error.

I would like it to return a value with the correct type, e.g. string for "name", and cause a compiler error if the string literal provided in key is not a property of object. (I do not need to support non literal strings, those can be compiler errors too.)

Is this even possible with TypeScript? I suspect not, but I am not sure.

Something that would be useful as a step in the right direction would be an utility type Keys<T> that represent the union of all string literals that are property names in T. But that doesn't seem to exist?

Upvotes: 2

Views: 1076

Answers (1)

MacD
MacD

Reputation: 426

export function getProperty<T, K extends keyof T>(object: T, key: K): T[K] {
  return object[key];
}

This gives you the compiler failure you want and the return type matches the property type

Playground Link

Upvotes: 3

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