Description
🔎 Search Terms
array intersection generic
array intersection binding
intersection of array types
intersection of array types is array of intersection of element types
🕗 Version & Regression Information
- This fails in all versions I tried in Playground: 3.3.3333, 5.8.3, 5.9.0-dev.20250620, and others
⏯ Playground Link
💻 Code
type X = string | number;
type A = Array<X>;
type StringA = A & Array<X & string>;
export function f(a: StringA) {
const x = a[0]; // string | undefined
if (!x) return 0;
return x.length;
}
export function g(a: StringA) {
const x = a.find((_x) => true);
if (!x) return 0;
return x.length; // error
}🙁 Actual behavior
The type of x in the last line of g is X, and thus a type error is reported.
🙂 Expected behavior
The type of x in the last line of g should be string, just like in f, and there should be no type errors.
Additional information about the issue
Why would you ever do that?
Stripe object types have many properties which can either be string or some other type, depending on whether that property was listed in expand when pulling the object.
I am trying to constrain some of those types to either option. That is: knowing exactly what I listed in expand, return a type which precisely conveys what some of those properties will be. This saves me a lot of type checking / constraining elsewhere. For example:
export type ScheduleWithoutPrices = Stripe.SubscriptionSchedule & {
phases: Array<
Stripe.SubscriptionSchedule.Phase & {
items: Array<
Stripe.SubscriptionSchedule.Phase.Item & {
price: string;
}
>;
}
>;
};
export type SubscriptionWithTestClockAndSchedule = Stripe.Subscription & {
test_clock?: Stripe.TestHelpers.TestClock;
} & {
schedule?: ScheduleWithoutPrices;
};
There may be better solutions to this, possibly using a generic type for the expands option, but I don't want to redefine the whole library, just an ad-hoc fix for the occasion.