JavaScript Proxies are a way to intercept and customize operations performed on objects.
Many libraries and frameworks (e.g. Solid.js) use proxies.
A few weeks ago, I wrote about a utility function to access environment variables in Node.js which caches environment variables and throws an error when the environment variable is undefined.
You could use a JavaScript proxy for the same purpose:
require("dotenv").config();
const cache = {};
const cacheHandler = {
get: (target, prop) => {
if (target[prop]) {
const value = Reflect.get(target, prop);
console.log(`GET cached ${prop} with value ${JSON.stringify(value)}`);
return value;
} else {
const env = process.env[prop];
if (!env) {
throw new ReferenceError(`Unknown environment variable: ${prop}`);
}
cache[prop] = { [prop]: env, time: new Date() };
console.log(
`GET initial ${prop} with value ${JSON.stringify(cache[prop])}`
);
return cache[prop];
}
},
};
const proxy = new Proxy(cache, cacheHandler);
const accessEnv = (key) => proxy[key];
accessEnv("DB_URL"); // GET initial DB_URL with value {"DB_URL":"mongodb://localhost:27017/db_name","time":"2022-04-06T07:22:04.303Z"}
console.log("-------");
accessEnv("DB_URL"); // GET cached DB_URL with value {"DB_URL":"mongodb://localhost:27017/db_name","time":"2022-04-06T07:22:04.303Z"}
console.log("-------");
accessEnv("DOES_NOT_EXIST"); // ReferenceError: Unknown environment variable: DOES_NOT_EXISTThe runnable code is on stackblitz.
You can also use proxies for validation via setters or to build a wrapper around HTTP calls.
