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typescripttypescript1.8

What does interface with two functions type declarations means and how to use it?

interface MyInterface {
    (select: string): number;
    (element: number): string;
}

How do I use above interface? What is the use case?

Solution

  • It means that the function has two different signatures, or in other wise overloading.
    For example:

    interface MyInterface {
    (select: string): number;
    (element: number): string;
}

let fn: MyInterface;
fn = (obj): any => {
    // do something 
    return null;
}

let str = fn(3); // type of str is string
let num = fn("string"); // typeof num is number
let error = fn(false); // error as false is neither string or number

    (code in playground)