JavaScript Proxies represent a powerful mechanism for intercepting and customizing operations executed on objects.
Many libraries and frameworks, such as Solid.js, have harnessed the capabilities of proxies to enhance their functionality.
A few weeks ago, I shared an article about a utility function designed to access environment variables in Node.js. This utility efficiently caches environment variables and raises an error when an environment variable is undefined.
Interestingly, you can achieve the same objective by employing JavaScript proxies:
1require("dotenv").config();
2
3const cache = {};
4const cacheHandler = {
5 get: (target, prop) => {
6 if (target[prop]) {
7 const value = Reflect.get(target, prop);
8 console.log(`GET cached ${prop} with value ${JSON.stringify(value)}`);
9 return value;
10 } else {
11 const env = process.env[prop];
12 if (!env) {
13 throw new ReferenceError(`Unknown environment variable: ${prop}`);
14 }
15 cache[prop] = { [prop]: env, time: new Date() };
16 console.log(
17 `GET initial ${prop} with value ${JSON.stringify(cache[prop])}`
18 );
19 return cache[prop];
20 }
21 },
22};
23
24const proxy = new Proxy(cache, cacheHandler);
25
26const accessEnv = (key) => proxy[key];
27
28accessEnv("DB_URL"); // GET initial DB_URL with value {"DB_URL":"mongodb://localhost:27017/db_name","time":"2022-04-06T07:22:04.303Z"}
29console.log("-------");
30accessEnv("DB_URL"); // GET cached DB_URL with value {"DB_URL":"mongodb://localhost:27017/db_name","time":"2022-04-06T07:22:04.303Z"}
31console.log("-------");
32accessEnv("DOES_NOT_EXIST"); // ReferenceError: Unknown environment variable: DOES_NOT_EXISTYou can run the code on stackblitz to see it in action.
Additionally, JavaScript proxies find utility in various scenarios, including validation through setters or constructing wrappers for HTTP calls.
