A simple BankAccount implementation that uses composability and FP concepts in ES2017

BankAccount.js

// Factory function
const createBankAccount = (balance = 0, type = 'CHECKING') => ({balance, type})

// Helper
const checkIfNegative = x => {
  if (x < 0)
    throw Error('Value can not be negative')
  return x
}

const checkIfEnoughFunds = (ba, amt) => {
  if (amt > ba.balance)
    throw Error('Not enough funds')
  return amt
}

const deposit = (bankAccount) => (amount) => bankAccount.balance += checkIfNegative(amount)

const withdraw = (bankAccount) => (amount) => bankAccount.balance -= checkIfEnoughFunds(bankAccount, checkIfNegative(amount))

module.exports = {
  createBankAccount,
  deposit,
  withdraw,
}