if (a == 1 && a == 2 && a == 3) {
  console.log("done");
}

let a = {
  [Symbol.toPrimitive]: ((i) => () => ++i)(0)
}

if (a == 1 && a == 2 && a == 3) {
  console.log("done");
}
// done

let obj = {
  [Symbol.toPrimitive](hint) {
    switch (hint) {
      case "number":
        return 123;
      case "string":
        return "str";
      case "default":
        return "default";
    }
  }
}
console.log(1-obj); // -122
console.log(1+obj); // 1default
console.log(`${obj}`); // str

let leon = {
  age: 30
}
const validator = {
  get: function(target, key){
    // 若没这个属性返回37
    return key in target "number" || Number.isNaN(value)){
      throw new Error("年龄得是数字");
    }
  }
}
const proxy = new Proxy(leon,validator);
console.log(proxy.name);
// 37
proxy.age = "hi";
// Error: 年龄得是数字

require("reflect-metadata")
class C {
  // @Reflect.metadata(metadataKey, metadataValue)
  method() {
  }
}
Reflect.defineMetadata("name", "jix", C.prototype, "method");

let metadataValue = Reflect.getMetadata("name", C.prototype, "method");
console.log(metadataValue);
// jix

let array = [1, 2, 3];

array = new Proxy(array, {
 get(target, prop, receiver) {
  if (prop < 0) {
   console.log(prop, 'prop')
   prop = +prop + target.length;
  }
  return Reflect.get(target, prop, receiver);
 }
});


console.log(array[-1]); // 3
console.log(array[-2]); // 2

let handlers = Symbol('handlers');

function makeObservable(target) {
 // 初始化存储 handler 的数组
 target[handlers] = [];

 // 存储 handler 函数到数组中以便于未来调用
 target.observe = function(handler) {
  this[handlers].push(handler);
 };

 // 创建代理以处理更改
 return new Proxy(target, {
  set(target, property, value, receiver) {
   // 转发写入操作到目标对象
   let success = Reflect.set(...arguments);
   // 如果设置属性的时候没有报错
   if (success) {
    // 调用所有 handler
    target[handlers].forEach(handler => handler(property, value));
   }
   return success;
  }
 });
}

let user = {};

user = makeObservable(user);

user.observe((key, value) => {
 console.log(`SET ${key}=${value}`);
});

user.name = "John";
// SET name=John