meta_es6.js

// object
let obj = {
  foo: 'bar'
}
let bazStr = 'baz'
obj[bazStr] = (x, y) => {
  return x + y
}
console.log(obj.baz(2, 3)) // 5
console.log('----------')





// class
class User {
  foo () {
    return 'foo'
  }
}

['hello', 'hi', 'bye'].forEach(str => {
  User.prototype[`say_${str}`] = () => {
    return `${str}`
  }
})

let u = new User()
console.log(u.foo()) // foo
console.log(u.say_hello()) // hello
console.log(u.say_hi()) // hi
console.log(u.say_bye()) // bye
console.log('----------')





// monkey patching
class Dollar {
  constructor (value) {
    this.value = parseInt(value)
  }
  printValue () {
    return `$${this.value}`
  }
}
global['Dollar'] = Dollar

class Euro {
  constructor (value) {
    this.value = parseInt(value)
  }
  printValue () {
    return `€${this.value}`
  }
}
global['Euro'] = Euro

class Rouble {
  constructor (value) {
    this.value = parseInt(value)
  }
  printValue () {
    return `₽${this.value}`
  }
}
global['Rouble'] = Rouble

for (let someСlass of [Rouble, Euro, Dollar]) {
  String.prototype[`to_${someСlass.name.toLowerCase()}s`] = function () {
    return new global[someСlass.name](this.toString())
  }
}

console.log('11'.to_euros()) // Euro { value: 11 }
console.log('12'.to_dollars()) // Dollar { value: 12 }
console.log('13'.to_roubles()) // Rouble { value: 13 }

console.log('15'.to_euros().printValue()) // €15
console.log('16'.to_dollars().printValue()) // $16
console.log('17'.to_roubles().printValue()) // ₽17
console.log('----------')