单例 是一种创建型设计模式， 让你能够保证一个类只有一个实例， 并提供一个访问该实例的全局节点。

识别方法： 单例可以通过返回相同缓存对象的静态构建方法来识别（getInstance）。

/**
 * The Singleton class defines the `getInstance` method that lets clients access
 * the unique singleton instance.
 */
class Singleton {
    private static instance: Singleton;

    /**
     * The Singleton's constructor should always be private to prevent direct
     * construction calls with the `new` operator.
     */
    private constructor() { }

    /**
     * The static method that controls the access to the singleton instance.
     *
     * This implementation let you subclass the Singleton class while keeping
     * just one instance of each subclass around.
     */
    public static getInstance(): Singleton {
        if (!Singleton.instance) {
            Singleton.instance = new Singleton();
        }

        return Singleton.instance;
    }

    /**
     * Finally, any singleton should define some business logic, which can be
     * executed on its instance.
     */
    public someBusinessLogic() {
        // ...
    }
}

/**
 * The client code.
 */
function clientCode() {
    const s1 = Singleton.getInstance();
    const s2 = Singleton.getInstance();
    // 虽然通过getInstance()调用了两遍，但返回的是同一个实例
    if (s1 === s2) {
        console.log('s1 和 s2 是相同的实例');
    } else {
        console.log('s1 和 s2 不是相同的实例');
    }
}

clientCode();