TypeScript - How to add a method outside the class definition

Question

typescript, how to add a method outside the class definition

I try to add it on prototype, but error

B.ts

export class B{
    name: string = 'sam.sha'
}

//Error:(21, 13) TS2339: Property 'say' does not exist on type 'B'.
B.prototype.say = function(){
    console.log('define method in prototype')
}

Answer 1

It complains because you did not define that B has the method say.
You can:

class B {
    name: string = 'sam.sha'
    say: () => void;
}

B.prototype.say = function(){
    console.log('define method in prototype')
}

Or:

class B {
    name: string = 'sam.sha'
}

interface B {
    say(): void;
}

B.prototype.say = function(){
    console.log('define method in prototype')
}