valueObjectExample.ts

class Adult {
  age: AdultAge;

  constructor(age: AdultAge) {
    this.age = age;
  }
}

/* value object */
class AdultAge {
  /* readonly, so we can't set it and go around validation in constructor */
  readonly value: number;

  constructor(value: number) {
    /* validation */
    if (value < 18) {
      throw new Error(`invalid age of: ${value}`);
    }

    this.value = value;
  }

  equals = (otherAdultAge: AdultAge) => {
    return this.value === otherAdultAge.value;
  };
}

/* 
const adultAge = new AdultAge(17);  // Throws an error, because it must be >= 18
const adult = new Adult(19);        // typescript error, must pass AdultAge object
*/

const adultAge = new AdultAge(18); // Works
const adult = new Adult(adultAge);

const adultAge2 = new AdultAge(19);
const adult2 = new Adult(adultAge2);

console.log(adultAge2.value);
console.log(adultAge.equals(adultAge2)); // false
console.log(adultAge2.equals(new AdultAge(19))); // true