function genGetInstance() {
  let instance = null
  class Singleton {}
  return () => {
    if (instance === null) {
      instance = new Singleton();
    }
    return instance
  }
}
const getInstance = genGetInstance();
const s1 = getInstance();
const s2 = getInstance();
console.log(s1, s2, s1 === s2) // Singleton类，Singleton类，true

let instance
class Singleton {}
export default () => {
  if (instance === null) {
    instance = new Singleton();
  }
  return instance
}

是否符合设计原则

• 内部封装 getInstance，高内聚，低耦合

注意：Java等多线程语言，单例要加线程锁