typescript extension getter

1interface A {
2    testGet: SomeType|null;
3}
4
5Object.defineProperty(A.prototype, "testGet", {
6    get (this: A) {
7        if(this.something) { 
8          return this.something;
9        }
10        return null;
11    },
12    enumerable: false,
13    configurable: true
14});