При нажатие двух раз на знак равно в текстовом поле появляться Пустота.Помогите решить эту проблему!

var xmlhttp = new XMLHttpRequest();
var first_num = null;
var second_num = null;
var sign = null;
var start = true;
var tohca_num = null;
var nol_num = null;
var rav_num = null;

function operations(sign) {
  start = true;

  if (sign != "=")
    window.sign = sign;

  if (first_num == null) {
    first_num = document.main.click.value;
    document.main.click.value = press(num);

  } else if (second_num == null) {
    second_num = document.main.click.value;

    Send();
  }

  first_num = null;
  second_num = null;
  sign = null;
  tohca_num = null;
}

function nol(click) {
  if (start) {
    nol_num = press(num);
    start = false;
  }

  document.main.click.value = document.main.click.value + click;
}

function press(num) {
  if (start) {
    document.main.click.value = "";
    start = false;
  }
  document.main.click.value = document.main.click.value + num;
}

function clean() {
  start = true;

  if (document.main.click.value == "") {
    document.main.click.value = "0";
  }

  document.main.click.value = "0";

  second_num = null;
  first_num = null;
  tohca_num = null;
  sign = null;
}

function point() {
  tohca_num = document.main.click.value;

  if (start) {
    tohca_num = "0.";
    start = false;

  } else if (tohca_num.indexOf(".") == -1) {
    tohca_num += (".");
  }

  document.main.click.value = tohca_num;
}

function del() {
  var i = document.main.click.value;

  document.main.click.value = i.substring(0, i.length - 1);

  if (document.main.click.value == "") {
    document.main.click.value = "0";
  }
}