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
JavaScript Proxy Explained Clearly By Practical Examples
Summary: in this tutorial, you will learn about the JavaScript Proxy object in is a JavaScript Proxy objectA JavaScript Proxy is an object that wraps another object (target) and intercepts the fundamental operations of the target fundamental operations can be the property lookup, assignment, enumeration, and function invocations, eating a proxy objectTo create a new proxy object, you use the following syntax:let proxy = new Proxy(target, handler);
Code language: JavaScript (javascript)In this syntax:target – is an object to wrap. handler – is an object that contains methods to control the behaviors of the target. The methods inside the handler object are called traps. A simple proxy exampleFirst, define an object called user:const user = {
firstName: ‘John’,
lastName: ‘Doe’,
email: ”, }
Code language: JavaScript (javascript)Second, define a handler object:const handler = {
get(target, property) {
(`Property ${property} has been read. `);
return target[property];}}
Code language: JavaScript (javascript)Third, create a proxy object:const proxyUser = new Proxy(user, handler);
Code language: JavaScript (javascript)The proxyUser object uses the user object to store data. The proxyUser can access all properties of the user, access the firstName and lastName properties of the user object via the proxyUser (rstName);
(stName);
Code language: CSS (css)Output:Property firstName has been read.
John
Property lastName has been read.
Doe
When you access a property of the user object via the proxyUser object, the get() method in the handler object is, if you modify the original object user, the change is reflected in the rstName = ‘Jane’;
(rstName);
Code language: JavaScript (javascript)Output:Property firstName has been read.
Jane
Similarly, a change in the proxyUser object will be reflected in the original object (user)stName = ‘William’;
Code language: JavaScript (javascript)Output:William
Proxy TrapsThe get() trapThe get() trap is fired when a property of the target object is accessed via the proxy the previous example, a message is printed out when a property of the user object is accessed by the proxyUser nerally, you can develop a custom logic in the get() trap when a property is example, you can use the get() trap to define computed properties for the target object. The computed properties are properties whose values are calculated based on values of existing user object does not have a property fullName, you can use the get() trap to create the fullName property based on the firstName and lastName properties:const user = {
lastName: ‘Doe’}
const handler = {
return property === ‘fullName’?
`${rstName} ${stName}`:
target[property];}};
const proxyUser = new Proxy(user, handler);
(proxyUser. fullName);
Code language: JavaScript (javascript)Output:John Doe
The set() trapThe set() trap controls behavior when a property of the target object is ppose that the age of user must be greater than 18. To enforce this constraint, you develop a set() trap as follows:const user = {
age: 20}
set(target, property, value) {
if (property === ‘age’) {
if (typeof value! == ‘number’) {
throw new Error(‘Age must be a number. ‘);}
if (value < 18) {
throw new Error('The user must be 18 or older. ')}}
target[property] = value;}};
Code language: JavaScript (javascript)First, set the age of user to a = 'foo';
Code language: JavaScript (javascript)Output:Error: Age must be a number.
Code language: JavaScript (javascript)Second, set the age of the user to = '16';
Code language: JavaScript (javascript)Output:The user must be 18 or older.
Third, set the age of the user to = 21;
No error apply() trapThe () method is a trap for a function call. Here is the syntax:let proxy = new Proxy(target, {
apply: function(target, thisArg, args) {
//... }});
Code language: JavaScript (javascript)See the following example:const user = {
const getFullName = function (user) {
return `${rstName} ${stName}`;}
const getFullNameProxy = new Proxy(getFullName, {
apply(target, thisArg, args) {
return target(.. ). toUpperCase();}});
(getFullNameProxy(user)); //
Code language: JavaScript (javascript)OutputJOHN DOE
More trapsThe following are more traps:construct – traps usage of the new operatorgetPrototypeOf – traps an internal call to [[GetPrototypeOf]]setPrototypeOf – traps a call to tPrototypeOfisExtensible – traps a call to ExtensiblepreventExtensions – traps a call to eventExtensionsgetOwnPropertyDescriptor – traps a call to tOwnPropertyDescriptorIn this tutorial, you have learned about the JavaScript Proxy object used to wrap another object to change the fundamental behaviors of that this tutorial helpful?
How to Set Up a Proxy Server on Your PC, Mac, or Web Browser
As for what a proxy server is, think of your proxy as a gateway between you and the internet. When you visit a website, the proxy server communicates with it on behalf of your browser. Then, when the website answers, the proxy forwards the data to you.
Proxy servers can do many jobs. These include scanning for viruses, acting as a firewall, speeding up your connection by caching, and hiding your public IP address.
You have lots of choices when it comes to internet privacy. There’s a lot to learn, such as understanding what a reverse proxy is or distinguishing between VPNs, proxies, and Tor. If you want dependable protection and encryption for your internet traffic, consider a VPN. Avast SecureLine VPN hides your data from snoopers and protects your online identity, and it’s also much easier to set up than a proxy server.
There’s no similar guarantee of protection with a proxy, especially if you’re using one of the many web-based proxies available online. These public proxy services may slow down your service, and even worse, they may be insecure. Instead, we suggest you learn how to set up a proxy server on Windows, MacOS, and the most popular web browsers.
If you’re configuring your browser to use a proxy operated by a company, school, or other similar organization, some of these instructions may require you to contact your IT staff. This is the case particularly when you are asked to manually enter a script name, formally called a proxy auto-configuration (PAC) file. You may also need to type in the proxy’s IP and port numbers. In both cases, turn to your IT helpdesk if you haven’t been given this information. If you’re using a proxy that you’ve purchased or created on your own, you’ll already have these details.
How to set up a proxy server in Windows
Here’s how to set up your Windows PC to use a proxy server on Windows 8 or Windows 10:
Press the Windows + I keys simultaneously to access the Windows Settings menu.
On Windows 10, click Settings > Network & Internet > Proxy. On Windows 8, click Settings > Network Proxy.
Under the Manual Setup section, set the “Use a Proxy Server” toggle to On. Make sure the “Automatically detect settings” toggle is also on.
By default, Windows automatically checks to see if your business, school, or local network already has an automatic proxy server setup ready to run for you. If it does, Windows tells you its name, and encourages you to follow its instructions.
Assuming that Windows finds a PAC file, in the Automatic Proxy Setup, set the Use Setup Script switch to On.
Enter the script address and click Save.
Congratulations! You’re done and ready to proxy.
How to manually set up a proxy in Windows 8 or Windows 10
Not every organization sets up the process automatically. If you need to set up a proxy manually in Windows 8 or Windows 10, here’s how to do it.
To access the Windows Settings menu, press the Windows + I keys simultaneously.
On Windows 10, click Settings > Network & Internet > Proxy. On Windows 8 systems, click Settings > Network Proxy.
Scroll down to “Manual proxy setup” and set the “Use a Proxy Server” switch to On.
In the address field, type the proxy server name or IP address. In the Port field, enter the proxy port number. You can get these from your IT support tech.
If the proxy server is protected by a password, select the “Proxy server requires password” checkbox. Enter your account name and password in the Username and Password fields.
Click Save.
That’s all there is to it. You’re now set up to use your proxy server on Windows. The next time you use the internet, you’ll be using the proxy server.
How to set up a proxy server on a Mac running macOS
Here’s how to set up a proxy server on a Mac running macOS 10. 15 Catalina. Other macOS versions use essentially the same commands.
Open System Preferences. One way to get there is to click on the Apple menu > System Preferences.
Choose the Network icon to adjust connectivity settings.
Select the type of network you use, which is usually Ethernet or Wi-Fi.
Click Advanced > Proxies.
To automatically configure your proxy server settings, select Auto Proxy Discovery. Click on Secure Web Proxy (HTTPS). Type in the proxy server address and its port number in the fields on the right.
To use a proxy auto-configuration (PAC) file, select Automatic Proxy Configuration and enter the proxy address into the URL field.
Click OK to save the proxy setup.
You’re now ready to surf the internet via your proxy.
How to set up a proxy in Google Chrome
By default, Chrome uses your macOS or Windows proxy. To change your proxy settings from within Chrome, take the following steps:
Open the Chrome toolbar and select Settings.
Scroll down to the bottom of the display. Click on Show advanced settings…
Scroll down to “System” and choose Open your computer’s proxy settings.
Next, go back to the instructions for your operating system listed above to set up your proxy server settings and follow them.
In short, to set up a proxy server in Google Chrome, you’re just doing the first few steps in the browser, and then completing the process in your machine’s operating system.
How to set up a proxy server in Safari
Like Chrome, Safari’s default setting is to follow the proxy configurations within your OS. Here’s how to change your proxy settings from within Safari:
Click on Safari in the browser toolbar. Choose Preferences.
Click on the Advanced tab. Under “Proxies, ” click Change settings…
Next, you must go back to the instructions for macOS listed above to set up your proxy server settings and follow them.
How to set up a proxy server in Edge
Edge is Windows 10’s built-in web browser, and unless you tell it otherwise, it’ll use your proxy configurations in Windows. The latest versions of Edge are based on Chrome, so the configuration is similar. The following steps show you how to set up a proxy from within Edge:
In Microsoft Edge, at the top-right corner, click the Menu button. Select Settings.
Scroll down to the Advanced settings section and click the View advanced settings icon.
Click the Open proxy settings button.
Next, go back to the instructions for Windows 10 listed above, then follow them to configure your proxy server settings.
How to set up a proxy server in Firefox
Unlike other web browsers, Firefox doesn’t default to using your system’s proxy settings. You’ll need to change your proxy from within Firefox:
Open the Firefox menu and select Options.
Click on the Advanced icon.
Select the Network tab.
In the Connection Settings, select, Auto-detect proxy configuration for this network. When prompted, enter the proxy address.
Alternatively, in the Connection Settings, you can select, Use system proxy settings. If you choose that option, Firefox uses the proxy settings in your operating system.
Click OK to save your settings.
How to set up a proxy server in Internet Explorer
Please keep in mind that Internet Explorer is an outdated web browser. Instead, switch to a browser that’s still receiving regular updates and support, such as Avast Secure Browser, Chrome, Edge, Firefox, or Safari. Though since quite a few people and companies still use Internet Explorer (IE), you should know how to set things up when necessary.
As with other browsers, IE defaults to using your Windows proxy configurations. Here’s how to set your proxy up from within IE.
Open the IE toolbar and select Internet Options.
Open the Connections tab.
Select LAN Settings.
Check the “Use a proxy server for your LAN” box.
When prompted, enter the proxy server name and port number.
How to set up a proxy server in Android
With Android, you must configure proxy settings for each network you use. In addition, the proxy will only be used by your web browser. Other internet applications may choose to use or ignore the proxy based on their own internal settings. This is another reason why VPNs tend to be better choices — when you use one, it covers all the internet traffic from your device.
Open Android’s Settings and tap Wi-Fi to view a list of Wi-Fi networks.
Long-press the Wi-Fi network name for which you want to change the proxy settings. Tap Modify Network.
Tap Advanced Options.
Tap Manual to change your proxy’s settings. Enter your hostname and proxy port.
OR
If you have a Proxy Auto-Config (PAC) file, select Proxy Auto-Config and Android will prompt you to enter the PAC address.
Tap SAVE.
How to set up a proxy server in iOS
Go to Settings > Wi-Fi.
Select the active Wi-Fi connection.
Under HTTP proxy, click Configure Proxy.
Tap Manual to change your proxy’s settings. Enter your host name, proxy port, and, if needed, your authentication username and password.
If your provider supports Web Proxy Auto-Discovery Protocol (WPAD), tap Automatic. If required, enter the Proxy Auto-Config (PAC) file location.
Tap back to the Wi-Fi selection window.
Protect your online anonymity the easy way
Proxy servers have their uses, particularly within large organizations. However, setting one up requires a bit of work, and this is a job typically entrusted to network administrators and other IT professionals.
Avast SecureLine VPN is an internet privacy solution that’s both simpler to use and far more protective of your identity and online activities. A single click or tap of a button gets you encryption that keeps you anonymous online and prevents anyone from snooping on your sensitive personal info, such as your online banking, health info, or private photos. Enjoy true online privacy on your desktop, laptop or mobile device with a VPN that’s as convenient as it is secure.