Ts Proxy
Proxy – JavaScript – MDN Web Docs
The Proxy object enables you to create a proxy for another object, which can intercept and redefine fundamental operations for that scriptionA 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:
(ssage1); // hello
(ssage2); // everyone
To customise the proxy, we define functions on the handler object:
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:
(ssage1); // world
(ssage2); // world
With the help of the Reflect class we can give some accessors the original behavior and redefine others:
const handler3 = {
get: function (target, prop, receiver) {
if (prop === “message2”) {
return “world”;}
return (guments);}, };
const proxy3 = new Proxy(target, handler3);
Constructor
Proxy()
Creates a new Proxy object.
Static methodsExamplesBasic exampleIn 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;
(p. a, p. b);
// 1, undefined
(‘c’ in p, p. c);
// false, 37
No-op forwarding proxyIn 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
(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 lidationWith 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 (! Integer(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);
= 100;
(); // 100
= ‘young’; // Throws an exception
= 300; // Throws an exception
Extending constructorA 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 = tOwnPropertyDescriptor(
ototype, ‘constructor’);
ototype = (ototype);
var handler = {
construct: function(target, args) {
var obj = (ototype);
(target, obj, args);
return obj;},
apply: function(target, that, args) {
(that, args);
(that, args);}};
var proxy = new Proxy(base, handler);
= proxy;
fineProperty(ototype, ‘constructor’, descriptor);
return proxy;}
var Person = function(name) {
= name;};
var Boy = extend(Person, function(name, age) {
= age;});
= ‘M’;
var Peter = new Boy(‘Peter’, 13);
(); // “M”
(); // “Peter”
(); // 13
Manipulating DOM nodesSometimes 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) {
tAttribute(‘aria-selected’, ‘false’);}
if (newval) {
tAttribute(‘aria-selected’, ‘true’);}}
obj[prop] = newval;
return true;}});
let i1 = lected = tElementById(‘item-1’); //giving error here, i1 is null
(tAttribute(‘aria-selected’));
// ‘true’
let i2 = lected = tElementById(‘item-2’);
// ‘false’
Note: even if selected:! null, then giving tAttribute is not a function
Value correction and an extra propertyThe 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’]},
// An extra property
if (prop === ‘latestBrowser’) {
return owsers[ – 1];}
// The default behavior to return the value
return obj[prop];},
(value);
return true;}
// Convert the value if it is not an array
if (typeof value === ‘string’) {
value = [value];}
(owsers);
// [‘Internet Explorer’, ‘Netscape’]
owsers = ‘Firefox’;
// pass a string (by mistake)
// [‘Firefox’] <- no problem, the value is an array
testBrowser = 'Chrome';
// ['Firefox', 'Chrome']
(testBrowser);
// 'Chrome'
Finding an array item object by its propertyThis proxy extends an array with some utility features. As you see, you can flexibly "define" properties without using fineProperties(). This example can be adapted to find a table row by its cell. In that case, the target will be
let products = new Proxy([
{ name: 'Firefox', type: 'browser'},
{ name: 'SeaMonkey', type: 'browser'},
{ name: 'Thunderbird', type: 'mailer'}],
// 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
if (prop === 'number') {
return;}
let result, types = {};
for (let product of obj) {
if ( === prop) {
result = product;}
if (types[]) {
types[](product);} else {
types[] = [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 (types);}
return undefined;}});
(products[0]); // { name: 'Firefox', type: 'browser'}
(products['Firefox']); // { name: 'Firefox', type: 'browser'}
(products['Chrome']); // undefined
(owser); // [{ name: 'Firefox', type: 'browser'}, { name: 'SeaMonkey', type: 'browser'}]
(); // ['browser', 'mailer']
(); // 3
A complete traps list exampleNow 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 a simple cookie framework.
/*
var docCookies =... get the "docCookies" object here:
*/
var docCookies = new Proxy(docCookies, {
get: function (oTarget, sKey) {
return oTarget[sKey] || tItem(sKey) || undefined;},
set: function (oTarget, sKey, vValue) {
if (sKey in oTarget) { return false;}
return tItem(sKey, vValue);},
deleteProperty: function (oTarget, sKey) {
if (! sKey in oTarget) { return false;}
return moveItem(sKey);},
enumerate: function (oTarget, sKey) {
return ();},
ownKeys: function (oTarget, sKey) {
has: function (oTarget, sKey) {
return sKey in oTarget || oTarget. hasItem(sKey);},
defineProperty: function (oTarget, sKey, oDesc) {
if (oDesc && 'value' in oDesc) { tItem(sKey, );}
return oTarget;},
getOwnPropertyDescriptor: function (oTarget, sKey) {
var vValue = tItem(sKey);
return vValue? {
value: vValue,
writable: true,
enumerable: true,
configurable: false}: undefined;}, });
/* Cookies test */
(_cookie1 = 'First value');
(tItem('my_cookie1'));
tItem('my_cookie1', 'Changed value');
(_cookie1);
SpecificationsSpecificationECMAScript Language Specification (ECMAScript)# sec-proxy-objectsBrowser compatibilityBCD tables only load in the browserSee also
"Proxies are awesome" Brendan Eich presentation at JSConf (slides)
Tutorial on proxies
Design Patterns: Proxy in TypeScript – Refactoring Guru
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Proxy is a structural design pattern that provides an object that acts as a substitute for a real service object used by a client. A proxy receives client requests, does some work (access control, caching, etc. ) and then passes the request to a service object.
The proxy object has the same interface as a service, which makes it interchangeable with a real object when passed to a client.
Usage of the pattern in TypeScript
Complexity:
Popularity:
Usage examples: While the Proxy pattern isn’t a frequent guest in most TypeScript applications, it’s still very handy in some special cases. It’s irreplaceable when you want to add some additional behaviors to an object of some existing class without changing the client code.
Identification: Proxies delegate all of the real work to some other object. Each proxy method should, in the end, refer to a service object unless the proxy is a subclass of a service.
Conceptual Example
This example illustrates the structure of the Proxy design pattern and focuses on the following questions:
What classes does it consist of?
What roles do these classes play?
In what way the elements of the pattern are related?
Conceptual example
/**
* The Subject interface declares common operations for both RealSubject and the
* Proxy. As long as the client works with RealSubject using this interface,
* you’ll be able to pass it a proxy instead of a real subject.
*/
interface Subject {
request(): void;}
* The RealSubject contains some core business logic. Usually, RealSubjects are
* capable of doing some useful work which may also be very slow or sensitive –
* e. g. correcting input data. A Proxy can solve these issues without any
* changes to the RealSubject’s code.
class RealSubject implements Subject {
public request(): void {
(‘RealSubject: Handling request. ‘);}}
* The Proxy has an interface identical to the RealSubject.
class Proxy implements Subject {
private realSubject: RealSubject;
* The Proxy maintains a reference to an object of the RealSubject class. It
* can be either lazy-loaded or passed to the Proxy by the client.
constructor(realSubject: RealSubject) {
alSubject = realSubject;}
* The most common applications of the Proxy pattern are lazy loading,
* caching, controlling the access, logging, etc. A Proxy can perform one of
* these things and then, depending on the result, pass the execution to the
* same method in a linked RealSubject object.
if (eckAccess()) {
quest();
this. logAccess();}}
private checkAccess(): boolean {
// Some real checks should go here.
(‘Proxy: Checking access prior to firing a real request. ‘);
return true;}
private logAccess(): void {
(‘Proxy: Logging the time of request. ‘);}}
* The client code is supposed to work with all objects (both subjects and
* proxies) via the Subject interface in order to support both real subjects and
* proxies. In real life, however, clients mostly work with their real subjects
* directly. In this case, to implement the pattern more easily, you can extend
* your proxy from the real subject’s class.
function clientCode(subject: Subject) {
//…
//… }
(‘Client: Executing the client code with a real subject:’);
const realSubject = new RealSubject();
clientCode(realSubject);
(”);
(‘Client: Executing the same client code with a proxy:’);
const proxy = new Proxy(realSubject);
clientCode(proxy);
Execution result
Client: Executing the client code with a real subject:
RealSubject: Handling request.
Client: Executing the same client code with a proxy:
Proxy: Checking access prior to firing a real request.
Proxy: Logging the time of request.
Proxy in Other Languages
What Are Proxy Objects – Win32 apps | Microsoft Docs
What Are Proxy Objects – Win32 apps | Microsoft Docs
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08/19/2020
2 minutes to read
In this article
A proxy object acts as an intermediary between the client and an accessible object. The purpose of the proxy object is to monitor the life span of the accessible object and to forward calls to the accessible object only if it is not destroyed.
When a client calls an IAccessible property to get information about an object, the proxy object must check whether the accessible object is still available. If it is, the proxy object passes the client’s request to the accessible object. If the accessible object is destroyed (for example, when a dialog box with custom controls is closed), the proxy object returns an error. To indicate that the object has been destroyed, it is recommended that servers return the error code CO_E_OBJNOTCONNECTED because this error is returned by Component Object Model (COM) after a server calls CoDisconnectObject.
The proxy object is transparent to the client. When the client calls AccessibleObjectFromEvent, AccessibleObjectFromPoint, or AccessibleObjectFromWindow, it receives back a pointer to an IAccessible interface. However, when the client uses this pointer to call any of the IAccessible properties or methods, the code that is executed is within the proxy object.
Frequently Asked Questions about ts proxy
What is typescript proxy?
Proxy is a structural design pattern that provides an object that acts as a substitute for a real service object used by a client. The proxy object has the same interface as a service, which makes it interchangeable with a real object when passed to a client. …
What are proxy objects?
A proxy object acts as an intermediary between the client and an accessible object. The purpose of the proxy object is to monitor the life span of the accessible object and to forward calls to the accessible object only if it is not destroyed.Aug 19, 2020
What is a proxy object JavaScript?
Proxy is an object in javascript which wraps an object or a function and monitors it via something called target. Irrespective of the wrapped object or function existence. Proxy are similar to meta programming in other languages. … Handler: The function that does something on Object or Function that is proxied.
