checkinput.js

  document.addEventListener("DOMContentLoaded", function(event) {
    new ShowButton().load();
  });

  function ShowButton() {
    this.btn = document.querySelectorAll('.class');
    this.target = document.querySelectorAll('input[type="checkbox"]');
  }

  ShowButton.prototype.load = function() {
    var self = this;
    for(var i = 0; i < self.target.length; i++){
      self.target[i].addEventListener('change', function(){
        self.event();
      }, null);
    }
  };

  ShowButton.prototype.event = function() {
    var self = this;
    var checkCount = 0;

    for(var i = 0; i < self.target.length; i++) {
      if(self.target[i].checked) {
        checkCount ++;
      }
      if(checkCount === 0) {
        self.btn[0].style.opacity = 0;
      } else {
        self.btn[0].style.opacity = 1;
      }
    }
  };