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// Constructor
var Calculator = function(eq){
  this.eqCtrl = document.getElementById(eq);
};

Calculator.prototype = function(){
  var add = function(x, y) {
    var val = x+y;
    this.eqCtrl.innerHTML = val;
  };
  return {add: add };
}();

var Calculator = function(eq){
  // Private member.
  var eqCtrl = document.getElementById('something'),
      doAdd = function(x, y){
              var val = x+y;
              eqCtrl.innerHtml = val;
            };
  
  return {
    // Expose public member.
    add: doAdd // can be named add also
  };
}('eqCtrl');

// Constructor
var Calculator = function(eq){
  this.eqCtrl = document.getElementById(eq);
};

Calculator.prototype = {
  add: function(x, y){
    var val = x+y;
    this.eqCtrl.innerHTML = val;
  }
};

var Calculator = function(eq){
  // Private member.
  var eqCtrl = document.getElementById('something');
  
  return {
    // Expose public member.
    add: function(x, y){
      var val = x+y;
      eqCtrl.innerHtml = val;
    }
  };
};