Proxy

Proxy object enables you to create a proxy for another object, which can intercept and redefine fundamental operations for that object.

描述

A Proxy is created with two parameters:

  • target : the original object which you want to proxy
  • handler : an object that defines which operations will be intercepted and how to redefine intercepted operations.

For example, this code defines a simple target with just two properties, and an even simpler handler with no properties:

const target = {
  message1: "hello",
  message2: "everyone"
};
const handler1 = {};
const proxy1 = new Proxy(target, handler1);
					

Because the handler is empty, this proxy behaves just like the original target:

console.log(proxy1.message1); // hello
console.log(proxy1.message2); // everyone
					

To customise the proxy, we define functions on the handler object:

const target = {
  message1: "hello",
  message2: "everyone"
};
const handler2 = {
  get: function(target, prop, receiver) {
    return "world";
  }
};
const proxy2 = new Proxy(target, handler2);
					

Here we've provided an implementation of the get() handler, which intercepts attempts to access properties in the target.

Handler functions are sometimes called traps , presumably because they trap calls to the target object. The very simple trap in handler2 above redefines all property accessors:

console.log(proxy2.message1); // world
console.log(proxy2.message2); // world
					

With the help of the Reflect class we can give some accessors the original behavior and redefine others:

const target = {
  message1: "hello",
  message2: "everyone"
};
const handler3 = {
  get: function (target, prop, receiver) {
    if (prop === "message2") {
      return "world";
    }
    return Reflect.get(...arguments);
  },
};
const proxy3 = new Proxy(target, handler3);
console.log(proxy3.message1); // hello
console.log(proxy3.message2); // world
					

构造函数

Proxy()
创建新的 Proxy 对象。

静态方法

Proxy.revocable()
Creates a revocable Proxy 对象。

范例

Basic example

In this simple example, the number 37 gets returned as the default value when the property name is not in the object. It is using the get handler.

const handler = {
  get: function(obj, prop) {
    return prop in obj ?
      obj[prop] :
      37;
  }
};
const p = new Proxy({}, handler);
p.a = 1;
p.b = undefined;
console.log(p.a, p.b);
//  1, undefined
console.log('c' in p, p.c);
//  false, 37
					

No-op forwarding proxy

In this example, we are using a native JavaScript object to which our proxy will forward all operations that are applied to it.

const target = {};
const p = new Proxy(target, {});
p.a = 37;
//  operation forwarded to the target
console.log(target.a);
//  37
//  (The operation has been properly forwarded!)
					

Note that while this "no-op" works for JavaScript objects, it does not work for native browser objects like DOM Elements.

验证

With a Proxy , you can easily validate the passed value for an object. This example uses the set handler.

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;
    // Indicate success
    return true;
  }
};
const person = new Proxy({}, validator);
person.age = 100;
console.log(person.age); // 100
person.age = 'young';    // Throws an exception
person.age = 300;        // Throws an exception
					

Extending constructor

A function proxy could easily extend a constructor with a new constructor. This example uses the construct and apply handlers.

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

Manipulating DOM nodes

Sometimes you want to toggle the attribute or class name of two different elements. Here's how using the set handler.

let view = new Proxy({
  selected: null
},
{
  set: function(obj, prop, newval) {
    let oldval = obj[prop];
    if (prop === 'selected') {
      if (oldval) {
        oldval.setAttribute('aria-selected', 'false');
      }
      if (newval) {
        newval.setAttribute('aria-selected', 'true');
      }
    }
    // The default behavior to store the value
    obj[prop] = newval;
    // Indicate success
    return true;
  }
});
let i1 = view.selected = document.getElementById('item-1');  //giving error here, i1 is null
console.log(i1.getAttribute('aria-selected'));
//  'true'
let i2 = view.selected = document.getElementById('item-2');
console.log(i1.getAttribute('aria-selected'));
//  'false'
console.log(i2.getAttribute('aria-selected'));
//  'true'
Note: even if selected: !null, then giving oldval.setAttribute is not a function
					

Value correction and an extra property

products proxy object evaluates the passed value and converts it to an array if needed. The object also supports an extra property called latestBrowser both as a getter and a setter.

let products = new Proxy({
  browsers: ['Internet Explorer', 'Netscape']
},
{
  get: function(obj, prop) {
    // An extra property
    if (prop === 'latestBrowser') {
      return obj.browsers[obj.browsers.length - 1];
    }
    // The default behavior to return the value
    return obj[prop];
  },
  set: function(obj, prop, value) {
    // An extra property
    if (prop === 'latestBrowser') {
      obj.browsers.push(value);
      return true;
    }
    // Convert the value if it is not an array
    if (typeof value === 'string') {
      value = [value];
    }
    // The default behavior to store the value
    obj[prop] = value;
    // Indicate success
    return true;
  }
});
console.log(products.browsers);
//  ['Internet Explorer', 'Netscape']
products.browsers = 'Firefox';
//  pass a string (by mistake)
console.log(products.browsers);
//  ['Firefox'] <- no problem, the value is an array
products.latestBrowser = 'Chrome';
console.log(products.browsers);
//  ['Firefox', 'Chrome']
console.log(products.latestBrowser);
//  'Chrome'
					

Finding an array item object by its property

This proxy extends an array with some utility features. As you see, you can flexibly "define" properties without using Object.defineProperties . This example can be adapted to find a table row by its cell. In that case, the target will be table.rows .

let products = new Proxy([
  { name: 'Firefox', type: 'browser' },
  { name: 'SeaMonkey', type: 'browser' },
  { name: 'Thunderbird', type: 'mailer' }
],
{
  get: function(obj, prop) {
    // The default behavior to return the value; prop is usually an integer
    if (prop in obj) {
      return obj[prop];
    }
    // Get the number of products; an alias of products.length
    if (prop === 'number') {
      return obj.length;
    }
    let result, types = {};
    for (let product of obj) {
      if (product.name === prop) {
        result = product;
      }
      if (types[product.type]) {
        types[product.type].push(product);
      } else {
        types[product.type] = [product];
      }
    }
    // Get a product by name
    if (result) {
      return result;
    }
    // Get products by type
    if (prop in types) {
      return types[prop];
    }
    // Get product types
    if (prop === 'types') {
      return Object.keys(types);
    }
    return undefined;
  }
});
console.log(products[0]);          // { name: 'Firefox', type: 'browser' }
console.log(products['Firefox']);  // { name: 'Firefox', type: 'browser' }
console.log(products['Chrome']);   // undefined
console.log(products.browser);     // [{ name: 'Firefox', type: 'browser' }, { name: 'SeaMonkey', type: 'browser' }]
console.log(products.types);       // ['browser', 'mailer']
console.log(products.number);      // 3
					

A complete traps list example

Now in order to create a complete sample traps list, for didactic purposes, we will try to proxify a non-native object that is particularly suited to this type of operation: the docCookies global object created by the "little framework" published on the document.cookie page .

/*
  var docCookies = ... get the "docCookies" object here:
  https://developer.mozilla.org/en-US/docs/DOM/document.cookie#A_little_framework.3A_a_complete_cookies_reader.2Fwriter_with_full_unicode_support
*/
var docCookies = new Proxy(docCookies, {
  get: function (oTarget, sKey) {
    return oTarget[sKey] || oTarget.getItem(sKey) || undefined;
  },
  set: function (oTarget, sKey, vValue) {
    if (sKey in oTarget) { return false; }
    return oTarget.setItem(sKey, vValue);
  },
  deleteProperty: function (oTarget, sKey) {
    if (sKey in oTarget) { return false; }
    return oTarget.removeItem(sKey);
  },
  enumerate: function (oTarget, sKey) {
    return oTarget.keys();
  },
  ownKeys: function (oTarget, sKey) {
    return oTarget.keys();
  },
  has: function (oTarget, sKey) {
    return sKey in oTarget || oTarget.hasItem(sKey);
  },
  defineProperty: function (oTarget, sKey, oDesc) {
    if (oDesc && 'value' in oDesc) { oTarget.setItem(sKey, oDesc.value); }
    return oTarget;
  },
  getOwnPropertyDescriptor: function (oTarget, sKey) {
    var vValue = oTarget.getItem(sKey);
    return vValue ? {
      value: vValue,
      writable: true,
      enumerable: true,
      configurable: false
    } : undefined;
  },
});
/* Cookies test */
console.log(docCookies.my_cookie1 = 'First value');
console.log(docCookies.getItem('my_cookie1'));
docCookies.setItem('my_cookie1', 'Changed value');
console.log(docCookies.my_cookie1);
					

规范

规范
ECMAScript (ECMA-262)
The definition of 'Proxy' in that specification.

浏览器兼容性

更新 GitHub 上的兼容性数据
Desktop Mobile Server
Chrome Edge Firefox Internet Explorer Opera Safari Android webview Chrome for Android Firefox for Android Opera for Android Safari on iOS Samsung Internet Node.js
Proxy Chrome 49 Edge 12 Firefox 18 IE No Opera 36 Safari 10 WebView Android 49 Chrome Android 49 Firefox Android 18 Opera Android 36 Safari iOS 10 Samsung Internet Android 5.0 nodejs 6.0.0
Proxy() 构造函数 Chrome 49 Edge 12 Firefox 18 IE No Opera 36 Safari 10 WebView Android 49 Chrome Android 49 Firefox Android 18 Opera Android 36 Safari iOS 10 Samsung Internet Android 5.0 nodejs 6.0.0
handler.apply Chrome 49 Edge 12 Firefox 18 IE No Opera 36 Safari 10 WebView Android 49 Chrome Android 49 Firefox Android 18 Opera Android 36 Safari iOS 10 Samsung Internet Android 5.0 nodejs 6.0.0
handler.construct Chrome 49 Edge 12 Firefox 18 IE No Opera 36 Safari 10 WebView Android 49 Chrome Android 49 Firefox Android 18 Opera Android 36 Safari iOS 10 Samsung Internet Android 5.0 nodejs 6.0.0
handler.defineProperty Chrome 49 Edge 12 Firefox 18 IE No Opera 36 Safari 10 WebView Android 49 Chrome Android 49 Firefox Android 18 Opera Android 36 Safari iOS 10 Samsung Internet Android 5.0 nodejs 6.0.0
handler.deleteProperty Chrome 49 Edge 12 Firefox 18 IE No Opera 36 Safari 10 WebView Android 49 Chrome Android 49 Firefox Android 18 Opera Android 36 Safari iOS 10 Samsung Internet Android 5.0 nodejs 6.0.0
handler.get Chrome 49 Edge 12 Firefox 18 IE No Opera 36 Safari 10 WebView Android 49 Chrome Android 49 Firefox Android 18 Opera Android 36 Safari iOS 10 Samsung Internet Android 5.0 nodejs 6.0.0
handler.getOwnPropertyDescriptor Chrome 49 Edge 12 Firefox 18 IE No Opera 36 Safari 10 WebView Android 49 Chrome Android 49 Firefox Android 18 Opera Android 36 Safari iOS 10 Samsung Internet Android 5.0 nodejs 6.0.0
handler.getPrototypeOf Chrome 49 Edge 79 Firefox 49 IE No Opera 36 Safari 10 WebView Android 49 Chrome Android 49 Firefox Android 49 Opera Android 36 Safari iOS 10 Samsung Internet Android 5.0 nodejs 6.0.0
handler.has Chrome 49 Edge 12 Firefox 18 IE No Opera 36 Safari 10 WebView Android 49 Chrome Android 49 Firefox Android 18 Opera Android 36 Safari iOS 10 Samsung Internet Android 5.0 nodejs 6.0.0
handler.isExtensible Chrome 49 Edge 12 Firefox 31 IE No Opera 36 Safari 10 WebView Android 49 Chrome Android 49 Firefox Android 31 Opera Android 36 Safari iOS 10 Samsung Internet Android 5.0 nodejs 6.0.0
handler.ownKeys Chrome 49 Edge 12 Firefox 18
18
In Firefox 42, the implementation got updated to reflect the final ES2015 specification: The result is now checked if it is an array and if the array elements are either of type string or of type symbol. Enumerating duplicate own property names is not a failure anymore.
IE No Opera 36 Safari 10 WebView Android 49 Chrome Android 49 Firefox Android 18
18
In Firefox 42, the implementation got updated to reflect the final ES2015 specification: The result is now checked if it is an array and if the array elements are either of type string or of type symbol. Enumerating duplicate own property names is not a failure anymore.
Opera Android 36 Safari iOS 10 Samsung Internet Android 5.0 nodejs 6.0.0
handler.preventExtensions Chrome 49 Edge 12 Firefox 22 IE No Opera 36 Safari 10 WebView Android 49 Chrome Android 49 Firefox Android 22 Opera Android 36 Safari iOS 10 Samsung Internet Android 5.0 nodejs 6.0.0
handler.set Chrome 49 Edge 12 Firefox 18 IE No Opera 36 Safari 10 WebView Android 49 Chrome Android 49 Firefox Android 18 Opera Android 36 Safari iOS 10 Samsung Internet Android 5.0 nodejs 6.0.0
handler.setPrototypeOf Chrome 49 Edge 12 Firefox 49 IE No Opera 36 Safari 10 WebView Android 49 Chrome Android 49 Firefox Android 49 Opera Android 36 Safari iOS 10 Samsung Internet Android 5.0 nodejs 6.0.0
revocable Chrome 63 Edge 12 Firefox 34 IE No Opera 50 Safari 10 WebView Android 63 Chrome Android 63 Firefox Android 34 Opera Android 46 Safari iOS 10 Samsung Internet Android 8.0 nodejs 6.0.0

图例

完整支持
完整支持
不支持
不支持
见实现注意事项。

另请参阅

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