typescript default function inside interface

Your needs will be fulfilled by abstract classes

abstract class Department {

    constructor(public name: string) {
    }

    printName(): void {
        console.log("Department name: " + this.name);
    }

    abstract printMeeting(): void; // must be implemented in derived classes
}

class AccountingDepartment extends Department {

    constructor() {
        super("Accounting and Auditing"); // constructors in derived classes must call super()
    }

    printMeeting(): void {
        console.log("The Accounting Department meets each Monday at 10am.");
    }

    generateReports(): void {
        console.log("Generating accounting reports...");
    }
}

Credits https://www.typescriptlang.org/docs/handbook/classes.html#:~:text=Abstract%20classes%20are%20base%20classes,methods%20within%20an%20abstract%20class.

For more info https://www.typescriptlang.org/docs/handbook/classes.html#:~:text=Abstract%20classes%20are%20base%20classes,methods%20within%20an%20abstract%20class.

No. TypeScript Interfaces are not something available at runtime, that is they are completely absent in the generated JavaScript.

Perhaps you meant to use class:

class Test{
    imp(){return 123}
 }