Value Object in JavaScript, or non-primitve value

MonetaryAmount.js

const valueSymbol = Symbol('value');
class MonetaryAmount { 
  constructor(value) {
    this[valueSymbol] = value; 
  } 
  add(amount) { 
    return new MonetaryAmount(amount.value + this.value); 
  }
  equal(amount) {
    return amount.value == this.value;
  }
  get value() {
    return this[valueSymbol];
  }
}

describe('monetary value object', () => {
  
  let oneEuro, twoEuro, threeEuro;
  
  beforeEach(() => {
    oneEuro = new MonetaryAmount(1);
    twoEuro = new MonetaryAmount(2);
    threeEuro = new MonetaryAmount(3);
  });
  
  it('`oneEuro` equals `new MonetaryAmount(1)`', () => {
    assert.ok(oneEuro.equal(new MonetaryAmount(1)));
  });
  
  it('1 + 2 = 3', () => {
    let result = oneEuro.add(twoEuro);
    assert.ok(result.equal(threeEuro));
  });
  
  it('`oneEuro + twoEuro` does NOT modify `oneEuro`', () => {
    oneEuro.add(twoEuro);
    assert.ok(oneEuro.equal(new MonetaryAmount(1)));
  });
  
  it('`oneEuro + twoEuro` does NOT modify `twoEuro`', () => {
    oneEuro.add(twoEuro);
    assert.ok(twoEuro.equal(new MonetaryAmount(2)));
  });
  
  it('the inner value may be readable, but not modifiable', () => {
    assert.throws(() => { oneEuro.value = 2; });
  });
  
});

Great Example! Do you think it could be usefull to add a toNumber-Method to avoid duplicating all arithmetic methods like add, sub, ... ? Or it this against the concept?