Body of a functional program

atm-program.function.js

const withdraw = (balance, amount) => balance - amount;

const ATM = (state = {balance: 100, amount: 10}, event) => {
  switch (event.type) {
    case 'WITHDRAW':
      return {
        ...state,
        balance: withdraw(state.balance, state.amount),
      };
    case 'WITHDRAWAL_AMOUNT':
      return {
        ...state,
        amount: parseInt(event.amount),
      };
    default:
      return state;
  }
};