Singleton Design Pattern

• Singletons are objects that can only have a single instance, with a single point of access
• The module system in nodejs offers a rudimentary implementation of a singleton
• In modules, single instance of class is created and cached

Example

// CashRegister.js

let cash = 0

const CashRegister = {
  credit(amount) {
    cash = cash + amount
    return cash
  },
  debit(amount) {
    if (amount <= cash) {
      cash = cash - amount
      return true
    } else {
      return false
    }
  },
  total() {
    return cash
  },
}

module.exports = CashRegister

// App.js

const cashRegister = require('./CashRegister')
const cashRegister2 = require('./CashRegister')

cashRegister.credit(10)
cashRegister2.credit(20)
cashRegister.debit(5)
console.log(cashRegister.total()) // answer is 25 as both object are using same instance