add "has_property" and "has_value" type guards for objects

[raythurnevoid]

Is your feature request related to a real problem or use-case?

Type guards for objects are done using the in operator. However it doesn't provide autocompletion and doesn't show an error if the tested property is not part of the object.

interface A {
	a?: number;
}

interface B {
	b: number;
}

const obj = { a: 1 } as A | B;

if ("invalid" in obj) {
	console.log(obj.invalid); // invalid is typed as "unknown"
}

Describe a solution including usage in code example

I propose to add two functions, I provide the implementation I'm using in personal projects.

type KeysOf<T> = T extends T ? keyof T : never;
type ExtractByKey<T, K extends KeysOf<T>> = Extract<T, { [P in K]?: any }>;
export function hasProperty<T extends object, K extends KeysOf<T>>(
	obj: T,
	key: K,
): obj is ExtractByKey<T, K> {
	return key in obj;
}

if (hasProperty(obj, "a") { // autocompletion is provided
	console.log(obj.a); // `a` is `number | undefined` because it's optional
}

if (hasProperty(obj, "invalid") { // type error here
	console.log(obj.invalid);
}

type ExtractByKeyAndAssertNonUndefined<T, K extends KeysOf<T>> =
	& ExtractByKey<T, K>
	& { [P in K]: Exclude<T[P], undefined> };
export function hasValue<T extends object, K extends KeysOf<T>>(
	obj: T,
	key: K,
): obj is ExtractByKeyAndAssertNonUndefined<T, K> {
	return key in obj && obj[key] !== undefined && obj[key] !== null;
}

if (hasValue(obj, "a") {
	console.log(obj.a); // `a` is `number`
}

Who does this impact? Who is this for?

TypeScript users

[piotrwitek]

Hey @raythurnevoid,
Thanks for proposal.

• hasProperty looks good I will accept PR.
• hasValue implementation is good, but the name needs refinement as it's not obvious it's reffering to the object property defined value. Please propose some alternatives.
  Thanks