详解JavaScript的Proxy对象

// 语法
const p = new Proxy(target, handler)

// 1、找到合适的 handler 并编写代码
const handler = {
    get: function(obj, prop) {
        return prop in obj ? obj[prop] : 37;
    }
};
// 2、新建 Proxy 对象
const p = new Proxy({}, handler);
// 3、执行操作
p.a = 1;
p.b = undefined;
// 4、查看结果
console.log(p.a, p.b);      // 1, undefined
console.log('c' in p, p.c); // false, 37

let target = {};
let p = new Proxy(target, {});

p.a = 37;   // 操作转发到目标

console.log(target.a);    // 37. 操作已经被正确地转发

let validator = {
  set: function(obj, prop, value) {
    if (prop === 'age') {
      if (!Number.isInteger(value)) {
        throw new TypeError('The age is not an integer');
      }
      if (value > 200) {
        throw new RangeError('The age seems invalid');
      }
    }

    // The default behavior to store the value
    obj[prop] = value;

    // 表示成功
    return true;
  }
};

let person = new Proxy({}, validator);
person.age = 100;
console.log(person.age);
// 100
person.age = 'young';
// 抛出异常: Uncaught TypeError: The age is not an integer
person.age = 300;
// 抛出异常: Uncaught RangeError: The age seems invalid

function extend(sup, base) {
  var descriptor = Object.getOwnPropertyDescriptor(
    base.prototype, "constructor"
  );
  base.prototype = Object.create(sup.prototype);
  var handler = {
    construct: function(target, args) {
      var obj = Object.create(base.prototype);
      this.apply(target, obj, args);
      return obj;
    },
    apply: function(target, that, args) {
      sup.apply(that, args);
      base.apply(that, args);
    }
  };
  var proxy = new Proxy(base, handler);
  descriptor.value = proxy;
  Object.defineProperty(base.prototype, "constructor", descriptor);
  return proxy;
}

var Person = function (name) {
  this.name = name
};

var Boy = extend(Person, function (name, age) {
  this.age = age;
});

Boy.prototype.sex = "M";

var Peter = new Boy("Peter", 13);
console.log(Peter.sex);  // "M"
console.log(Peter.name); // "Peter"
console.log(Peter.age);  // 13