Understanding the Proxy Design Pattern in JavaScript

TL;DR: This article demystifies the Proxy design pattern in JavaScript, a structural pattern providing a substitute for another object. The Proxy controls access to the original object, optimizing performance and improving efficiency. We’ll discuss its use cases and demonstrate how to create a Just-In-Time Proxy.

Introduction

The Proxy pattern, a significant player in the world of design patterns, serves as an interface for another object. It is a software engineering tool used to control interactions between clients and the original object, which can significantly enhance application performance and efficiency.

Understanding the Proxy Design Pattern

Acting as an intermediary between the client and the object, the Proxy controls access to the object and can batch requests, store them for later use, or even delay expensive operations to enhance performance. One such use case is the Just-In-Time Proxy, which delays the instantiation of a slow or resource-intensive object until necessary.

Creating a Just-In-Time Proxy in JavaScript

Let’s walk through the creation of a Just-In-Time Proxy using JavaScript.

Step 1 – Create the slowObject.js

First, we define the slow object, which emulates a delay with a for loop to mimic a slow initialization process.

javascriptCopy code// JavaScript Document
define(function () {
	'use strict';
	
	var SlowObject = function () { //set up slowObject
		this.someMethod = function () {
			console.log('some method on the slow object was invoked'); //use method to log a statement
		};
	};
	
	return {
		init: function () { //add artificial initialization delay with for loop
			for (var x = 0, max = 1000; x < max; x++) { //for loop iterates 1000 times
				console.log('slowness...');
			}
			return new SlowObject(); //once loop finishes, returns new instance of slowObject
		}
	};
});

Step 2 – Create the slowObjectProxy.js

Next, we create the Proxy for the slow object. This Proxy checks if the slow object has been initialized, and if not, it initializes it. An interval is also set up to periodically check if the slowObject has been created yet.

javascriptCopy code// JavaScript Document
define(function (require) {//use require function
	'use strict';
	
	var SlowObjectProxy, slowObjectInstance, //initialize variables
		slowObject = require('proxy/slowObject');
		
	SlowObjectProxy = function () { //set up constructor for proxy object
		this.someMethod = function () { //add method that matches the slowObject method
			var interval; //add the new variable
			
			if (!slowObjectInstance) { //check if the slowObject has been initialized previously
				slowObjectInstance = slowObject.init(); //if return is falsy, we haven't initialized yet, so we use init method to initialize
			} else { //if it has been initialized
				slowObjectInstance.someMethod(); //we invoke the same method on the slowObject
			}
			
			interval = window.setInterval(invokeMethodWhenExists, 100); //set this up to poll the object and wait for initialization.
			
			function invokeMethodWhenExists() { //the above interval will invoke this method every 100 milliseconds to check if slowObject has been created yet
				if (slowObjectInstance) { //we check the slowObjectInstance variable, and if truthy
					console.log('proxying some method'); //use a log to know the proxy is working as expected
					window.clearInterval(interval); //clear the interval to stop the loop
					
					slowObjectInstance.someMethod(); //then invoke the method on the slow object
				}
			}
		};
	};
	
	return {
		init: function () {
			return new SlowObjectProxy();	
		}
	};
});

Step 3 – Set up init.js

This file initializes the slowObjectProxy and invokes the ‘someMethod’ from it.

javascriptCopy codedefine(function(require) {
	'use strict';

	return {
		init: function() {
			var myProxy, //set up variables
				slowObjectProxy = require('proxy/slowObjectProxy');
				
			myProxy = slowObjectProxy.init(); //initialize the slowObject, which is delayed by the interval
			myProxy.someMethod();
		}
	};
});

Step 4 – Define main.js

This is the main entry point for our Proxy example. It sets up the Proxy and provides a method to run our Proxy example.

javascriptCopy code// JavaScript Document
require (
	['proxy/init'],
	function ( proxy) {
		'use strict';

		var examples = {
			proxy: proxy
		};
		
		window.runExample = function (example) {
			examples[example].init();
		};
	}
);

Conclusion

The Proxy design pattern is a powerful tool in JavaScript, controlling access to objects and optimizing performance. By effectively delaying expensive operations and controlling how and when objects are accessed, this pattern enhances the efficiency of your applications. Implementing a Just-In-Time Proxy, as demonstrated, can greatly enhance your JavaScript applications. Happy coding!