Let's consider the scenario with this specific data structure:
type Event = {
name: string;
dateCreated: string;
type: string;
}
Now, I am looking to expand upon this type, for instance,
type UserEvent extends Event = {
UserId: string;
}
Unfortunately, this approach is not successful. What other method could be employed?
When it comes to the keyword extends, it can only be utilized for interfaces and classes specifically.
If you are looking to define a type with additional properties, consider utilizing an intersection type:
type UserEvent = Event & {UserId: string}
UPDATE: With TypeScript 2.2, there is now the possibility of extending an object-like type within an interface, given that the type meets certain criteria. Check out this link for more information:
type Event = {
name: string;
dateCreated: string;
type: string;
}
interface UserEvent extends Event {
UserId: string;
}
Keep in mind that the UserEvent must be declared as an interface rather than a type if you intend to use the extends syntax.
Unfortunately, using extend with arbitrary types is still not supported. This means that it won't work if Event is a type parameter lacking any constraints.
You have the ability to combine types in your code:
type Animal = {
name: string;
};
type Color = {
color: string;
};
export type Pet = Animal & Color;
Now you can use this combined type like so:
const myPet: Pet = {
color: 'brown',
name: 'Buddy',
};
Here is an example of a typical extension type:
const ExtensionType = <T extends object>(obj: T): T & { someExtensionProperty: string } => {
return Object.assign(obj, {someExtensionProperty: "extension"});
};
What you are aiming for is essentially
interface Schedule {
event: string;
dateScheduled: string;
eventType: string;
}
interface PersonalSchedule extends Schedule {
UserId: string;
}
The way you structured the interfaces doesn't explicitly support inheritance, but you can achieve a similar concept using intersection types, as artem highlighted.
To achieve this, you have the option to utilize the type keyword along with interface:
type EventDetails = {
name: string;
dateCreated: string;
type: string;
}
type UserEventDetails = {
UserId: string;
} & EventDetails
An alias cannot be restricted.
It is acceptable, despite the incorrect semantics:
type CustomerProfile /* extends Profile */ = {
CustomerId: string;
}
However, this example is not allowed:
type CustomerProfile extends Profile = {
CustomerId: string;
}
To accurately convey your intention, you should use & Profile instead of extends Profile:
type CustomerProfile = { CustomerId: string } & Profile;
If you have a type that is defined with Union (meaning it can be one of multiple types) and you wish to include an additional type, you can achieve this by following these steps
interface TextInputProps = { type: string };
interface TextAreaProps = { type: string | number };
export type InputCompProps = TextInputProps | TextAreaProps
you can modify it to
interface CustomProps = { length: number };
export type InputCompProps = (TextInputProps | TextAreaProps) & CustomProps
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