class Calculator {

  constructor (value = 0) {
    this.value = value
  }

  add (value = 0) {
    this.value += value
    return this
  }

  multiply (value = 0) {
    this.value *= value
    return this
  }

}

const value = new Calculator(7)

value
  .add(7)
  .add(10)
  .multiply(2)

console.log(value.value)