Is it possible for a TypeScript interface to inherit from a Function?

Question

Is it possible for a TypeScript interface to inherit from a Function?

While studying Angular, I found this intriguing declaration:

interface IMessagesOperation extends Function {
(messages: Message[]): Message[];
}

I'm curious about what this means, especially why Function is capitalized as F instead of being lowercase like function. How would one go about implementing the concrete class in this scenario? This is unfamiliar territory for me as I'm transitioning from the Java world.

angular typescript

Answer 1

Answer №1

Perhaps there is a specific idiomatic rationale behind it that I am not aware of. Essentially, the code is defining an IMessagesOperation type, which is a function that operates on an array of Message objects and returns another array of Message objects, like so:

type IMessagesOperation = (messages: Message[]) => Message[];

The reason for not structuring it this way could be related to how interfaces can be modified by additional declarations or the resolution of conflicts in type aliases using & and extends in TypeScript. It is possible to augment an interface by adding more declarations elsewhere in the code, as demonstrated below:

interface IMessagesOperation {
    (messages: string): Message[];
    //         ^^^^^^−−−−−− note the change
}

This allows for function overloads where different types of inputs are accepted, making the code more flexible. On the other hand, such flexibility is not available when using a type alias like

type IMessagesOperation = (messages: Message[]) => Message[];

.

why Function has capital "F" instead of just being function

The uppercase "F" in Function refers to JavaScript's Function constructor. In a type context, it signifies the type of object created by the constructor (i.e., a function).

How do I implement the concrete class in this case.

Since it is a function and not a class, you would implement it as follows:

const m1: IMessagesOperation = function(messages: Message[]): Message[] {
    return messages;
};

Typescript playground example of the above

Answer 2

Perhaps there is a specific idiomatic rationale behind it that I am not aware of. Essentially, the code is defining an IMessagesOperation type, which is a function that operates on an array of Message objects and returns another array of Message objects, like so:

type IMessagesOperation = (messages: Message[]) => Message[];

The reason for not structuring it this way could be related to how interfaces can be modified by additional declarations or the resolution of conflicts in type aliases using & and extends in TypeScript. It is possible to augment an interface by adding more declarations elsewhere in the code, as demonstrated below:

interface IMessagesOperation {
    (messages: string): Message[];
    //         ^^^^^^−−−−−− note the change
}

This allows for function overloads where different types of inputs are accepted, making the code more flexible. On the other hand, such flexibility is not available when using a type alias like

type IMessagesOperation = (messages: Message[]) => Message[];

.

why Function has capital "F" instead of just being function

The uppercase "F" in Function refers to JavaScript's Function constructor. In a type context, it signifies the type of object created by the constructor (i.e., a function).

How do I implement the concrete class in this case.

Since it is a function and not a class, you would implement it as follows:

const m1: IMessagesOperation = function(messages: Message[]): Message[] {
    return messages;
};

Typescript playground example of the above

Is it possible for a TypeScript interface to inherit from a Function?

Answer №1

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