I had the idea of writing this article a few months ago, but when I thought of this knowledge point,VueJsis used a lot, and I am aReactdeveloper. Or, if I were asked to choose another framework, I might be more inclined toangular, but recently I was learningNestJsand found thatReflectis widely used at the bottom, as follows:

Object->Proxy->Reflectwill be explained in the order, learn it quickly!

ES5, we You can define an object and perform operations on it (add or search), as shown in the following code:

const moment = { age: 18, address: "西安", }; moment["hobby"] = "basketball"; moment.hight = 1.59; console.log(moment.age); // 18

Attribute Descriptor.

ES5, all attributes have attribute descriptors. The specific usage is as shown in the figure below:

18, it also contains three other characteristics, which are :

• writable;
• enumerable;
• configurable;

Object.defineProperty(...)to add a new property or modify an existing property (when the When the value of attributeConfigurableistrue) and set the characteristics, the details are as shown in the following code:

const moment = { age: 18, address: "西安", }; Object.defineProperty(moment, "address", { value: "肇庆", writable: true, configurable: true, enumerable: true, }); console.log(moment.address); // 肇庆

The object to define the attribute;
The name of the attribute to be defined or modified or
• Symbol;
The attribute to be defined or modified Descriptor;

Object.definePropertyprovides two properties, they aresetandget. The use of the two properties is as follows:

const moment = { age: 18, address: "西安", }; Object.keys(moment).forEach((key) => { let value = moment[key]; Object.defineProperty(moment, key, { get: function () { console.log(`监听到了对象的 ${key} 属性被访问了`); return value; }, set: function (params) { console.log(`监听到了对象的 ${key} 属性被修改了`); value = params; }, }); }); moment.age = 22; const foo = moment.address;

moment.age, we will call the method on thesetattribute, and eventually the output will beThe age attribute of the object has been modified.

moment.address, the method on thegetattribute will be called, and finally the output will beThe address attribute of the object is monitored. Visited.

Object.defineProperty(. ..)It can’t be done

Proxysolves this pain point very well.

ES6, a newProxyclass has been added. This class can be seen from the name and is used to help us Create a proxy. It is a special object created by you. It encapsulates another ordinary object or blocks in front of this ordinary object. For example, if you want to modify an object, it mainly has the following flow chart:

const moment = { age: 18, address: "西安", }; const proxy = new Proxy(moment, {}); console.log(proxy); // { age: 18, address: '西安' }

false.

PeoxyThere are 13 capturers in total, but there are only a few commonly used ones. Here is a full explanation of them. You can check them out for details.MDN official website

const moment = { age: 18, address: "西安", }; function foo(x, y) { return x + y; } const proxy = new Proxy(moment, { has: function (target, prop) { console.log(`使用 in 访问了 ${prop} 是否存在于 moment 对象`); }, get: function (target, property, receiver) { console.log(`通过读取 moment 对象中的 ${property} 属性`); }, set: function (target, property, value, receiver) { console.log(`通过设置 moment 对象中的 ${property} 属性为 ${value}`); }, }); const fProxy = new Proxy(foo, { apply: function (target, _, params) { return target(...params) * 10; }, construct: function (target, argumentsList, newTarget) { console.log(target); // [Function: foo] console.log(argumentsList); // [ 1, 2 ] console.log(newTarget); // [Function: foo] return {}; }, }); "age" in proxy; // 使用 in 访问了 age 是否存在于 moment 对象 proxy.age; // 通过读取 moment 对象中的 age 属性 proxy.address = "肇庆"; // 通过设置 moment 对象中的 address 属性为 肇庆 console.log(foo(1, 2)); // 3 console.log(fProxy(1, 2)); // 30 new fProxy(1, 2);

target === momentandreceiver === proxyall returntrue, which meanstargetcorresponds to the object to be modified, andreceiverisProxyOr an object that inheritsProxy.

操作Proxy的同时会修改moment对象。

可取消代理

普通对象总是陷入到目标对象,并且在创建之后不能改变,只要还保持着对这个代理的引用,代理的机制就将维持下去。

但是可能会存在这样的情况,比如你想要创建一个在你想要停止它作为代理时便可被停用的代理,解决的方案是创建可取消代理,具体代码如下所示:

const moment = { age: 18, address: "西安", }; const { proxy, revoke } = Proxy.revocable(moment, { get: function (target, key, receiver) { console.log("get 捕获器"); }, }); proxy.address; revoke(); proxy.address;

最终的输出如下图所示:

一旦可取消代理被取消,任何对他的访问都会抛出TypeError错误。

Proxy的问题与不足

尽管现在Proxy已经做得很好了,但是在某些情况下,代理也不能与现在的ECMAScript机制很好地协同。

Proxy中的this

Peoxy潜在的一个问题来源是this值,我们知道方法中的this通常执行调用这个方法的对象,具体代码如下所示:

const target = { moment() { return this === proxy; }, }; const proxy = new Proxy(target, {}); console.log(target.moment()); // false console.log(proxy.moment()); // true

按照正常的理解这是没有问题的调用Proxy上的任何方法,而这个方法进而又会调用另一个方法,实际上都会调用Proxy内部的方法,这是符合预期的行为,但是,如果目标对象依赖于对象表示,那就可能碰到意料之外的问题。

举个例子:

const wm = new WeakMap(); class User { constructor(userId) { wm.set(this, userId); } set id(userId) { wm.set(this, userId); } get id() { return wm.get(this); } }

由于这个实现依赖User实例的对象标识,在这个实例被代理的情况下就会出现问题:

const user = new User(1); console.log(user.id); // 1 const proxy = new Proxy(user, {}); console.log(proxy.id); // undefined

这是因为User实例一开始使用目标对象作为WeakMap的键,代理对象却尝试从自身取得这个实例,要解决这个问题,就需要重新设置代理,把代理User实例改为代理User类本身,之后再创建代理的实例就会创建代理实例作为WeakMap的键了:

const UserProcess = new Proxy(User, {}); const proxy = new UserProcess(1); console.log(proxy.id); // 1

Proxy与内部槽位

代理与内置引用类型的实例通常可以很好地协同,但有些ECMAScript内置类型可能会依赖代理无法控制的机制,结果导致在代理上调用某些方法会出错,具体代码如下所示:

const target = new Date(); const proxy = new Proxy(target, {}); console.log(proxy instanceof Date); // true proxy.getDate(); // TypeError: this is not a Date object.

在上面的代码中,根据ECMAScript规范,Date类型方法的执行依赖this值上的内部曹伟[[NumberDate]],代理对象上不存在这个内部槽位,而且这个内部槽位的值也不能通过普通的get()和set()操作访问到,于是代理拦截后本应该转发给目标对象的方法会抛出TypeRrror的错误。

这篇文章到此结束,下一篇将会详细讲解Reflect,敬请期待。