es5-singleton.js

var mySingleton = (function () {
  var instance;
 
  function init() {
    function privateMethod() {
        console.log( "I am private" );
    }
    
    var privateVariable = "Im also private";
 
    return {
      publicMethod: function () {
        console.log( "The public can see me!" );
      },
 
      publicProperty: "I am also public",
 
      getPrivateVariable: function() {
        return privateVariable;
      }
    };
  }
 
  return {
    getInstance: function () {
      if (instance) return instance;

      instance = init();
      return instance;
    }
  };
})();
 
// Usage:
var singleA = mySingleton.getInstance();
var singleB = mySingleton.getInstance();
console.log(singleA.getPrivateVariable() === singleB.getPrivateVariable()); // true

/* ========================= */

var Singleton = (function () {
  var instance;

  return function Construct () {
    if (instance) return instance;
    if (this && this.constructor === Construct) {
      instance = this;
    } else {
      return new Construct();
    }
  }
}());