單例模式

Singleton 模式是一種設計模式，它將類的例項化限制為一個物件。建立第一個物件後，每當呼叫一個物件時，它將返回對同一個物件的引用。

var Singleton = (function () {
        // instance stores a reference to the Singleton
        var instance;
    
        function createInstance() {
            // private variables and methods
            var _privateVariable = 'I am a private variable';
            function _privateMethod() {
                console.log('I am a private method');
            }

            return {
                // public methods and variables
                publicMethod: function() {
                    console.log('I am a public method');
                },
                publicVariable: 'I am a public variable'
            };
        }
         
        return {
            // Get the Singleton instance if it exists
            // or create one if doesn't
            getInstance: function () {
                if (!instance) {
                    instance = createInstance();
                }
                return instance;
            }
        };
    })();

用法：

// there is no existing instance of Singleton, so it will create one
var instance1 = Singleton.getInstance();
// there is an instance of Singleton, so it will return the reference to this one
var instance2 = Singleton.getInstance();
console.log(instance1 === instance2); // true