From 218934fa2ed1c702a6d3923d2aa2cc6b43c48684 Mon Sep 17 00:00:00 2001 From: Peter Bengtsson Date: Tue, 8 Dec 2020 14:43:23 -0500 Subject: initial commit --- .../reference/global_objects/proxy/index.html | 399 +++++++++++++++++++++ 1 file changed, 399 insertions(+) create mode 100644 files/uk/web/javascript/reference/global_objects/proxy/index.html (limited to 'files/uk/web/javascript/reference/global_objects/proxy/index.html') diff --git a/files/uk/web/javascript/reference/global_objects/proxy/index.html b/files/uk/web/javascript/reference/global_objects/proxy/index.html new file mode 100644 index 0000000000..b246c6e047 --- /dev/null +++ b/files/uk/web/javascript/reference/global_objects/proxy/index.html @@ -0,0 +1,399 @@ +--- +title: Проксі +slug: Web/JavaScript/Reference/Global_Objects/Proxy +translation_of: Web/JavaScript/Reference/Global_Objects/Proxy +--- +
+
{{JSRef}}
+
+ +

Proxy об'єкт використовується для визначення власної поведінки для базових операцій (наприклад, доступ до властивостей, присвоєння, обрахунок, виклик функцій та ін.).

+ +

Terminology

+ +
+
handler
+
Placeholder object which contains traps.
+
traps
+
The methods that provide property access. This is analogous to the concept of traps in operating systems.
+
target
+
Object which the proxy virtualizes. It is often used as storage backend for the proxy. Invariants (semantics that remain unchanged) regarding object non-extensibility or non-configurable properties are verified against the target.
+
+ +

Syntax

+ +
var p = new Proxy(target, handler);
+
+ +

Parameters

+ +
+
target
+
A target object (can be any sort of object, including a native array, a function or even another proxy) to wrap with Proxy.
+
handler
+
An object whose properties are functions which define the behavior of the proxy when an operation is performed on it.
+
+ +

Methods

+ +
+
{{jsxref("Proxy.revocable()")}}
+
Creates a revocable Proxy object.
+
+ +

Methods of the handler object

+ +

The handler object is a placeholder object which contains traps for Proxy.

+ +
{{page('/en-US/docs/Web/JavaScript/Reference/Global_Objects/Proxy/handler', 'Methods') }}
+ +

Examples

+ +

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.

+ +
var handler = {
+    get: function(target, name) {
+        return name in target ?
+            target[name] :
+            37;
+    }
+};
+
+var 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.

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

Validation

+ +

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;
+  }
+};
+
+let 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.sex = 'M';
+
+var Peter = new Boy('Peter', 13);
+console.log(Peter.sex);  // "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');
+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'
+ +

Value correction and an extra property

+ +

The 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);
+ +

Specifications

+ + + + + + + + + + + + + + + + + + + + + + + + + + + + + +
SpecificationStatusComment
{{SpecName('ES2015', '#sec-proxy-objects', 'Proxy')}}{{Spec2('ES2015')}}Initial definition.
{{SpecName('ES2016', '#sec-proxy-objects', 'Proxy')}}{{Spec2('ES2016')}}
{{SpecName('ES2017', '#sec-proxy-objects', 'Proxy')}}{{Spec2('ES2017')}}
{{SpecName('ESDraft', '#sec-proxy-objects', 'Proxy')}}{{Spec2('ESDraft')}}
+ +

Browser compatibility

+ + + +

{{Compat("javascript.builtins.Proxy", 2)}}

+ +

See also

+ + + +

Licensing note

+ +

Some content (text, examples) in this page has been copied or adapted from the ECMAScript wiki which content is licensed CC 2.0 BY-NC-SA.

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