Another way is to do interface, so compiler will know that property exists.
interface IFirst{
first:number;
}
let object = {} as IFirst;
Object.defineProperty(object, 'first', {
value: 37,
writable: false,
enumerable: true,
configurable: true
});
console.log('first property: ' + object.first);
Take a look at this question How to customize properties in TypeScript