Implement all interface members

Question

I have a TypeScript interface. The problem is it has about 40 members. When I use it and I implement only chosen members I get an error that there are some missing. How to ignore it? Do I have to implement them all? This issue prevents me from casting one type into another.

E.g.

interface A {
   // 40 members
}

class B implements A {
   // only 5 members implemented
}

// somewhere in the code
var myVar1: A = something;
var myVar2: B = <B> myVar1; // here an error (can't convert because B has missing some properties and methods:/)

Answer 1

in typescript you can mark items as optional:

interface Person {
    name: string;
    address?: string;
}

name is required and address is optional for implementation

Answer 2

If you promise that you implement an interface, you have to implement it all.

One solution would be to have a base class that implements the 40 properties if you only want to deal with 5 properties in B.

interface A {
   propA: string;
   propB: string;
}

class C implements A {
    public propA = "";
    public propB = "";
}

class B extends C {
   public propB = "Example";
}

var myVar1: A;
var myVar2: B = <B> myVar1;