Refer to unknown at advance object key in TypeScript gereric

Question

First look, below function addPropertyIfValueIsNotUndefined works:

function addPropertyIfValueIsNotUndefined<Value>(key: string, value: Value): { key?: Value; } {
    return isNotUndefined(value) ? { [key]: value } : {};
}

function isNotUndefined<TargetValue>(targetValue: TargetValue | undefined): targetValue is TargetValue {
  return typeof targetValue !== "undefined";
}


type Test = {
    alpha: string;
    bravo?: string; 
}

const test1: Test = { 
    alpha: "ALPHA1" ,
    // bravo: "BRAVO1"
};

const test2: Test = {
    alpha: "ALPHA2",
    ...addPropertyIfValueIsNotUndefined("bravo", test1.bravo)
};

console.log(test2)

// Neither TypeScript compiler nor JavaScript VM errors until here

But what if try to add some property that does not declare on type Test? I am expecting that TypeScript emit the error TS2322 Type { gibberish: string; bravo: string | undefined; alpha: string; }' is not assignable to type 'Test', but TypeScript will not emit error or warning!

const test2: Type = {
    alpha: "ALPHA2",
    ...addPropertyIfValueIsNotUndefined("gibberish", test1.bravo)
};

Seems like I understood why. But how to annotate the returning value of addPropertyIfValueIsNotUndefined? We don't know the key at advance but need refer to it.

Fiddle

What exatcly you're trying to achieve? Are you trying to copy some property from one object to another? Is it correct that first parameter is a key that should be added to object and the second parameter is value that should be copied? What kind of type check do you expect? — Николай Гольцев, Nov 03 '20 at 07:11
@НиколайГольцев, I'm sorry about unclear question. "What exatcly you're trying to achieve?" - I want TypeScript emit the error `TS2322 Type { gibberish: string; bravo: string | undefined; alpha: string; }' is not assignable to type 'Test'`. — Takeshi Tokugawa YD, Nov 03 '20 at 07:32
@TakesiTokugawaYD I got your question now. Sadly it's a known case in typescript. You can check this so [question](https://stackoverflow.com/questions/59318739/is-there-an-option-to-make-spreading-an-object-strict) for further info. — Eldar, Nov 03 '20 at 08:01
@Eldar, thank you for reading my comments. So the verdict of this question is "No simple way to reach desired effect in TypeScript <=4.1"? — Takeshi Tokugawa YD, Nov 04 '20 at 02:12
Yes, and it's a feature of typescript rather than a lack of a feature. And it seems it will stay as-is for a while. — Eldar, Nov 04 '20 at 06:58

score 0 · Answer 1 · answered Nov 03 '20 at 08:54

Here's a slightly modified approach which I think accomplishes your goal. Instead of having a separate function to create the {key: value} object, we pass the existing object into the function.

function addPropertyIfValueIsNotUndefined<T extends {}, K extends keyof T>(
   object: T, key: K, value: Required<T>[K]
): T & Required<Pick<T, K>> {
    return propertyIsNotUndefined(object, key) ? object : { ...object, [key]: value } as T & Required<Pick<T, K>>
}

Note that we have to use as to return the correct type because of how typescript treats dynamic keys and because it is theoretically possibly for the generic K to be wider than just the single key, in which case our return type could be incorrect.

The boolean checker now looks at the property on the object:

function propertyIsNotUndefined<T extends {}, K extends keyof T>(object: T, key: K): object is T & Required<Pick<T, K>> {
  return object[key] !== undefined;
}

The usage looks like this:

const test2 = addPropertyIfValueIsNotUndefined(test1, "bravo", "value") // ok

const test3 = addPropertyIfValueIsNotUndefined(test1, "bravo", undefined) 
// error: Argument of type 'undefined' is not assignable to parameter of type 'string'

const test4 = addPropertyIfValueIsNotUndefined(test1, "wrongKey", "value") 
// error: Argument of type '"wrongKey"' is not assignable to parameter of type '"bravo" | "alpha"'

Typescript Playground Link

Refer to unknown at advance object key in TypeScript gereric

1 Answers1