The Secret Life of JavaScript: The Proxy

Source: Dev.to

Timothy sat at his desk, looking a bit overwhelmed. He had a simple user object, but his code was cluttered with if statements.

let user = {
    name: "Timothy",
    age: 25
};

// Timothy's validation code
if (user.age

“I want to protect my data,” Timothy said softly, “but I don’t want to clutter my object with validation logic in every single function. Is there a cleaner way?”

Margaret smiled warmly. She pulled a chair up next to him, not as a critic, but as a guide.

“That is a very mature instinct, Timothy,” she said. “You are looking for Metaprogramming. Specifically, a tool called the Proxy.”

The Middleman

Margaret drew a circle on the chalkboard and labeled it Target.

“Normally, when you interact with an object, you are speaking directly to it,” she explained. “The Proxy allows you to place a ‘Middleman’ in front of that object.”

const proxy = new Proxy(target, handler);

The Target is your original object. The Handler is an object that defines the rules. It contains traps that intercept your interactions.

Trap 1: Intercepting Reads (get)

“Let’s try a harmless prank to see how it works,” Margaret said. “Let’s teach the engine to lie.”

const user = { name: "Timothy" };

const handler = {
    get(target, prop) {
        if (prop === "name") {
            return "The Great " + target[prop];
        }
        return target[prop];
    }
};

const proxyUser = new Proxy(user, handler);

console.log(proxyUser.name); // "The Great Timothy"

Timothy’s eyes widened. “I didn’t change the object. I just changed how I read it.”

“Exactly,” Margaret nodded. “In the real world, developers use this to log data access or create ‘private’ properties. The Proxy decides what to return, not the object.”

Trap 2: Validation (set)

“Now, for your problem,” Margaret said gently. “You want to stop invalid data from ever touching your object. We can use the set trap.”

const validator = {
    set(target, prop, value) {
        if (prop === "age" && value

“Precisely,” Margaret said. “You have separated your Logic (the validator) from your Data (the user). That is the hallmark of clean architecture.”

The Toolkit: Reflect and Other Traps

“Is that all it can do?” Timothy asked.

“Hardly,” Margaret laughed. “You can intercept almost anything.”

• has trap: Intercepts the in operator (hiding keys).
• deleteProperty trap: Prevents properties from being deleted.
• apply trap: Intercepts function calls.

“And to make this easier,” she added, “we use the Reflect API. It allows us to perform the default behavior without manually modifying the target.”

set(target, prop, value) {
    if (value

“Thank you, Margaret,” he said. “I feel like I have superpowers. I can change how the language behaves.”

Margaret patted him on the shoulder. “You do, Timothy. But remember the Golden Rule of Metaprogramming.”

“Magic comes with a cost. Proxies add a small performance overhead to every operation. Use them for critical validation, not for every single object in your system. Use your powers for clarity, not confusion.”