Consider the TypeScript interface provided below:
export interface Update {
type: 'STATUS_UPDATE';
}
I am interested in extending this interface by adding one more value to the type property, as shown here:
export interface HttpUpdate extends Update {
type: 'HTTP_UPDATE' | super.type;
}
This way, the type property of HttpUpdate will be either HTTP_UPDATE or STATUS_UPDATE
Do you know if it is achievable in TypeScript?
One way to incorporate generics is demonstrated in the following code snippet
type BaseOptions = 'PREFERENCE_UPDATE';
export interface Option<O extends string = BaseOptions> {
selection: O;
}
type ExtendedOptions = BaseOptions | 'CUSTOM_UPDATE'
type CustomUpdate = Option<ExtendedOptions>;
const option: Option = { selection: 'PREFERENCE_UPDATE' };
const customOption: CustomUpdate = { selection: 'CUSTOM_UPDATE'};
In essence, by utilizing generics, you are expanding the possibilities of the selection property rather than modifying the core interface directly. The inclusion of the CustomUpdate type may not be imperative here, but it aids in retaining the structure of your original code
According to the TypeScript handbook, when we utilize the extends keyword on an interface, it allows us to essentially duplicate members from other named types while also adding any new members as desired. This feature is beneficial for reducing the amount of type declaration repetition and indicating that multiple declarations of the same property might be interconnected.
This implies that any type derived by extending another type must always be assignable to the original extended type. In simpler terms, the extended type is considered a subtype or a more specific type.
In a specific example mentioned, the HttpUpdate type does not qualify as a subtype of Update due to its type property being a string union, which cannot be assigned to the single string literal in the Update type. This discrepancy becomes evident when attempting to create the type with literal values.
If you encounter a scenario where extension of an interface doesn't serve practical purpose because of certain constraints, like having only one property, one workaround would be to introduce an additional property such as a message property with a value of type string.
Another method to achieve the desired type is by creating a new type that replicates all properties except the type property using the utility function Omit<Type, Keys>, and then setting the type property accordingly with a string union.
To simplify this process, using type aliases can also yield the desired outcome, keeping the structure concise and understandable.
To streamline this pattern, a custom type utility can be constructed to merge attributes from two distinct types selectively, resulting in a unified type that encompasses common properties while maintaining their individual data types.
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