实现某个类的静态方法，调用后可得到此构造函数/class已经new了多少个实例？

Question

调用：ClassA.getInstanceNumber()

Answer 1

呐，这是es6不用装饰器node和最新chrome可用版

class ClassA {
  constructor() {
    ClassA.instanceNumber = (ClassA.instanceNumber || 0) + 1;
  }

  static getInstanceNumber() {
    return ClassA.instanceNumber || 0;
  }
}

console.log(ClassA.getInstanceNumber());

let a = new ClassA();

console.log(ClassA.getInstanceNumber());

let b = new ClassA();

console.log(ClassA.getInstanceNumber());

Answer 2

看起来比较适合用 ES6 的修饰器 来做。

@countable
class ClassA {
  // ....
}

let count = 0;
function countable() {
  count++;
}

Answer 3

var Test = (function () {
    var count = 0;
    function Test() {
        if (!(this instanceof Test)) {
            return new Test();
        }
        count++;
    }
    Test.getInstanceNumber = function () {
        return count;
    }
    return Test;
})();

Test();
new Test();
new Test();
console.log(Test.getInstanceNumber());//3