For similar reasons, this scenario is deemed unsafe as described here: the caller selects the type parameter, which could potentially be more restrictive than the generic restriction. This means that a type Shirt defined as:
type Shirt = { size: "S" | "M" | "L", color: string }
const redShirt: Shirt = { size: "L", color: "red" };
is considered a subtype of StrKeyStrVal:
const strKeyStrVal: StrKeyStrVal = redShirt; // valid
This poses an issue since the size property is more specific than string, allowing for unintended assignments like:
setKeyVal(redShirt, "size", "XXXXXL"); // whoops
So what are your options? If you're willing to take the risk, you can use a type assertion. Otherwise, a safer alternative would be refactoring.
One refactoring method just as unsafe as an assertion is removing generics entirely:
function setKeyVal(obj: StrKeyStrVal, key: string, value: string) {
obj[key] = value; // valid
}
setKeyVal(redShirt, "size", "XXXXXL"); // yikes!
This approach is considered risky due to TypeScript's lack of soundness in non-generic supertype property writing scenarios. The conclusion being that prioritizing productivity over type safety is preferred by many TypeScript users. Perhaps that suits your needs as well.
A more secure way to refactor involves making the function sufficiently generic to only accept appropriate strings for each key:
function setKeyVal<T extends StrKeyStrVal, K extends keyof T>(obj: T, key: K, value: T[K]) {
obj[key] = value; // now safe
}
This implementation is now seen as safe, with erroneous calls like the one involving redShirt resulting in errors at the call site:
setKeyVal(redShirt, "size", "XXXXXL"); // error!
// -----------------------> ~~~~~~~~
// Argument of type '"XXXXXL"' is not assignable to parameter of type '"S" | "M" | "L"'
This method ensures the function is less prone to misuse but also makes it more difficult to use and annotate. It offers increased type safety with some trade-offs.
Your choice of approach may depend on your specific use cases and priorities. Regardless, I hope this sheds light on the situation. Best of luck!
Playground link to code