Calculator demo

calculator.js

var buttons = document.getElementsByTagName('button');
var display = document.getElementById('display');
var currentValue = 0;
var operand = 0;
var operator = '=';
var previousClicked = '=';
var cleared = true;

/**
 * Given a button value, preform the correct operation and update the display
 */
var pressButton = function () {
  
  buttonValue = this.value;

  if (buttonValue.match(/\d/)) {
    pressNumber(buttonValue);
  } else if (buttonValue === '=') {
    pressEquals();
  } else if (buttonValue === 'C') {
    pressClear(buttonValue);
  } else {
    pressOperator(buttonValue);
  }

  console.log('current operator: ', operator)
  console.log('current opeand:   ', operand)
  console.log('current value:    ', currentValue)
  previousClicked = buttonValue;

  return currentValue;
}

/**
 * Reset the calculator
 */
var pressClear = function () {
  resetOperand();
  currentValue = 0;
  operator = '=';
  updateDisplay(currentValue);
}

/**
 * Press =
 * Simply calls the last action with previous values
 */
var pressEquals = function () {
  currentValue = calculate(currentValue, operand, operator);
  updateDisplay(currentValue);
}

/**
 * Press + - * /
 *
 * Sets the new operation
 *
 * If the last button clicked was not =, calculate a new value
 *
 * Reset the operand
 */
var pressOperator = function (buttonValue) {
  operator = buttonValue;

  if (previousClicked !== '=') {
    currentValue = calculate(currentValue, operand, operator);
  }
  updateDisplay(currentValue);

  resetOperand();
}

/**
 * Press a number
 * 
 * Stores the current state of numbers pressed
 */
var pressNumber = function (buttonValue) {
  operand = operand.toString() + buttonValue;
  operand = parseInt(operand);

  if (previousClicked === '=') {
    operator = '=';
    operand = parseInt(buttonValue);
  }

  updateDisplay(operand);
}

/**
 * Reset the operand
 */
var resetOperand = function () {
  operand = 0;
}

/**
 * Given the current value, an operand, and operator, calculate the value
 */
var calculate = function (currentValue, operand, operator) {
  switch (operator) {
    case '+':
    return currentValue + operand;
    case '-':
    return currentValue - operand;
    case '*':
    return currentValue * operand;
    case '/':
    return currentValue / operand;
    case '=': 
    return currentValue;
  }
}

var updateDisplay = function (displayValue) {
  display.value = displayValue
}

for (var i = 0; i < buttons.length; ++i) {
  buttons[i].addEventListener('click', pressButton);
}