positive decorator

Person.ts

import { ui } from 'tabris';

function positive(prototype: object, property: string) {
  const sym = Symbol();
  Object.defineProperty(prototype, property, {
    enumerable: true,
    set(value: number) {
      if (value < 0) {
        throw new Error('Positive number expected');
      }
      this[sym] = value;
    },
    get() {
      return this[sym];
    }
  });
}

class Person {
  @positive public age: number;
}

let person = new Person();

try {
    person.age = 23; // OK 
    person.age = -23; // Error: Positive number expected
} catch(ex) {
    console.error(ex);
} finally {
    ui.contentView.append(<textView>{'Age:' + person.age}</textView>);
}