calculator.js

module.exports.initialState = { display: '0', initial: true }

module.exports.nextState = (calculatorState, character) => {
  if (isDigit(character)) {
    return addDigit(calculatorState, character)
  } else if (isOperator(character)) {
    return addOperator(calculatorState, character)
  } else if (isEqualSign(character)) {
    return compute(calculatorState)
  } else {
    return calculatorState
  }
}

//....