adder.spec.ts

describe('adder', () => {
  it('should build', () => {
    expect(new Adder()).toBeTruthy();
  });

  it('adds up the two values', () => {
    const adder = new Adder();
    adder.a = 1;
    adder.b = 1;
    expect(adder.value).toEqual(2);
  });
});

adder.tsx

export class Adder {
  @Prop() a: number;
  @Prop() b: number;

  @Method() get value(): number {
    return this.a + this.b;
  }

  render(): JSX.Element {
    return this.value;
  }
}