Is it possible to enforce a value to be within a list using TypeScript with enums? Can this be achieved with TypeScript type definitions?
enum Friend {
JOHN,
SALLY,
PAUL,
}
type MyFriends = {
friends: Friend[],
bestFriend: <> //How can I ensure that this value is part of the specified list of friends?
}
enum Friend {
JOHN,
SALLY,
PAUL,
}
// implementing at the type level
// defining a type constructor
type MyCircle<C extends Friend[]> = {
friends: C,
bestFriend: C[number]
}
// creating a specific instance:
type JohnCircle = MyCircle<[Friend.SALLY,Friend.PAUL]>
const johnCircle: JohnCircle = {
friends: [Friend.SALLY,Friend.PAUL],
bestFriend: Friend.SALLY // must be a member of friends array
}
const errorJohnCircle: JohnCircle = {
friends: [Friend.SALLY,Friend.PAUL],
bestFriend: Friend.JOHN // should result in an error!
}
// solution at the value level with a value constructor
// we can also define a function for creating values of this type
const makeCircle = <X extends Friend[], BF extends X[number]>(friends: X, bestFriend: BF): MyCircle<X> => ({
friends,
bestFriend
})
// properly created value of MyCircle
const johnCircle2 = makeCircle([Friend.PAUL, Friend.JOHN], Friend.JOHN);
const errorJohnCircle2 = makeCircle([Friend.PAUL, Friend.JOHN], Friend.SALLY); // should result in an error
In summary:
MyCircle is a generic type that takes elements from the Friend enum, specifying which ones will be included in the friends propertybestFriend: C[number] - ensures that the second field can only be a value present in the friends array, preventing the use of Enum members not provided in the initial listI've been struggling to set up push notifications in my Ionic3 app for the past couple of days, and no matter what I try, it doesn't seem to work due to the current versions I'm using. Here are my current versions: rxjs: 5.5.11 Angular: 5 ...
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