Adapted from Adele Goldberg’s book, Smalltalk-80: The Language and its Implementation.

client.mjs

import FinancialHistory from './FinancialHistory.mjs'

const personalBudget = new FinancialHistory()

console.log(personalBudget.cashOnHand())

personalBudget.receive(2200, 'salary')

console.log(personalBudget.cashOnHand())

personalBudget.spend(200, 'books')

console.log(personalBudget.cashOnHand())

FinancialHistory.mjs

export default class FinancialHistory {
  #cashOnHand
  #incomes = new Map()
  #expenditures = new Map()
  
  constructor(initialBalance = 0) {
    this.#cashOnHand = initialBalance
  }

  cashOnHand() {
    return this.#cashOnHand
  }

  receive(amount, source) {
    this.#incomes.set(source, this.totalReceivedFrom(source) + amount)
    this.#cashOnHand += amount
  }

  spend(amount, reason) {
    this.#expenditures.set(reason, this.totalSpentFor(reason) + amount)
    this.#cashOnHand -= amount
  }

  totalReceivedFrom(source) {
    if (this.#incomes.has(source)) {
      return this.#incomes.get(source)
    }
    return 0
  }

  totalSpentFor(reason) {
    if (this.#expenditures.has(reason)) {
      return this.#expenditures.get(reason)
    }
    return 0
  }
}