Home Given a generic object (T) and a key of that object (K), can I constrain K to only those keys with a string value?

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Given a generic object (T) and a key of that object (K), can I constrain K to only those keys with a string value?

June 9, 2023

Very basic example of what I’d like to do:

function extractStringValue<T extends object, K extends keyof T>(obj: T, key: K): string {
    return obj[key]; // error: Type 'T[K]' is not assignable to type 'string'
}

// Desired usage

const myObj = { stringKey: "hi", boolKey: false }
const stringVal = extractStringValue(myObj, "stringKey"); // all OK
const stringVal2 = extractStringValue(myObj, "boolKey"); // should raise error

>Solution :

You should define T based on K. Since you want T[K] to be a string you have to change T to extend a Record<K, string>, since we don’t need anything else this definition is enough:

function extractStringValue<T extends Record<K, string>, K extends keyof T & string>(
  obj: T,
  key: K,
): string {
  return obj[key]; 
}

Usage:

const myObj = { stringKey: 'hi', boolKey: false };
const stringVal = extractStringValue(myObj, 'stringKey'); // all OK
const stringVal2 = extractStringValue(myObj, 'boolKey'); // expected error