Proxy revoking causing inaccessible objects

Proxy Revoking Causing Inaccessible Objects in JavaScript

Introduction to Proxy and Revocable Proxy

JavaScript provides the Proxy object, which allows you to define custom behavior for fundamental operations such as property lookup, assignment, enumeration, and function invocation. However, Proxy.revocable() is a special method that creates a revocable proxy, meaning the proxy can be disabled at any time, making the target object inaccessible.

When a revoked proxy is accessed, JavaScript throws a TypeError stating that the object is no longer accessible. This can lead to unexpected errors if you attempt to interact with the proxy after it has been revoked.

1. Understanding `Proxy.revocable()`

A revocable proxy is created using Proxy.revocable(), which returns an object containing two properties:

proxy: The actual proxy that wraps the target object.
revoke(): A function that, when called, disables the proxy.

Example: Creating and Revoking a Proxy

const target = { message: "Hello, world!" };

// Create a revocable proxy
const { proxy, revoke } = Proxy.revocable(target, {
    get(obj, prop) {
        return prop in obj ? obj[prop] : "Property not found";
    }
});

console.log(proxy.message); // Output: "Hello, world!"

// Revoke the proxy
revoke();

console.log(proxy.message); // TypeError: Cannot perform 'get' on a proxy that has been revoked

In this example:

The proxy works normally until revoke() is called.
After revocation, any attempt to access the proxy results in a TypeError.

2. Why Does Revoking Cause Inaccessibility?

Once a Proxy is revoked, it loses its connection to the target object. Any operation on it, whether reading, writing, or calling methods, will fail with an error.

Example of Different Operations Failing After Revocation:

const target = { value: 42 };
const { proxy, revoke } = Proxy.revocable(target, {});

console.log(proxy.value); // Output: 42

revoke();

console.log(proxy.value); // TypeError: Cannot perform 'get' on a proxy that has been revoked
proxy.value = 100; // TypeError: Cannot perform 'set' on a proxy that has been revoked
delete proxy.value; // TypeError: Cannot perform 'deleteProperty' on a proxy that has been revoked

Any operation (get, set, deleteProperty) on proxy after revocation leads to an error.
This behavior is enforced to prevent any further interaction with the proxy once it is revoked.

3. Common Issues and Debugging

Developers often encounter unexpected proxy revocations in complex applications. Here are some common mistakes:

Mistake 1: Calling `revoke()` Too Early

If you revoke a proxy too soon in your code, it will be unusable where you expect it to work.

const target = { username: "JohnDoe" };
const { proxy, revoke } = Proxy.revocable(target, {});

revoke(); // Proxy is revoked immediately

console.log(proxy.username); // TypeError: Cannot perform 'get' on a proxy that has been revoked

Solution: Ensure you only call revoke() when you are sure the proxy is no longer needed.

Mistake 2: Accessing a Revoked Proxy in Asynchronous Code

If revoke() is called before an async operation completes, it will cause runtime errors.

const target = { status: "active" };
const { proxy, revoke } = Proxy.revocable(target, {});

setTimeout(() => {
    console.log(proxy.status); // TypeError if revoke() has already been called
}, 2000);

revoke(); // Proxy is revoked before the async operation

Solution: Make sure the proxy is still valid before using it inside asynchronous code.

Mistake 3: Using a Revoked Proxy in a Loop

Using a revoked proxy inside a loop will result in errors as soon as revoke() is called.

const target = { count: 5 };
const { proxy, revoke } = Proxy.revocable(target, {});

for (let i = 0; i < 10; i++) {
    if (i === 5) revoke(); // Revokes at iteration 5
    console.log(proxy.count); // TypeError after revocation
}

Solution: Avoid revoking the proxy inside a loop unless it’s absolutely necessary.

4. Best Practices When Using `Proxy.revocable()`

Use revocable proxies only when necessary.
- If you don’t need to revoke access dynamically, a standard Proxy is a better choice.
Check if the proxy is still valid before accessing it.try { console.log(proxy.someProperty); } catch (error) { console.error("Proxy access error:", error.message); }
- This prevents runtime crashes when a revoked proxy is accessed.
Delay revoke() until the proxy is no longer needed.
- Only call revoke() when you are sure no further operations will be performed.
Wrap proxy access in a function that checks its state.function safeAccess(proxy, prop) { try { return proxy[prop]; } catch { return "Proxy has been revoked"; } } console.log(safeAccess(proxy, "name")); // "Proxy has been revoked"
- This ensures safe handling of revoked proxies.

5. Alternative Approaches

If your goal is to make an object inaccessible, consider these alternatives:

Manually Setting Object Properties to null Instead of Using Proxieslet user = { name: "Alice" }; user = null; // Makes the object inaccessible
- Instead of revoking a proxy, nullifying an object achieves a similar result.
Using WeakMap for Controlled Accessconst privateData = new WeakMap(); class User { constructor(name) { privateData.set(this, { name }); } getName() { return privateData.get(this)?.name || "No access"; } } const user = new User("Alice"); console.log(user.getName()); // "Alice"
- WeakMap can store private data without exposing it via a proxy.

6. Conclusion

Proxy.revocable() is useful for security-sensitive applications where you need to dynamically disable object access.
Revoked proxies cannot be restored, making them permanently inaccessible.
Common mistakes include revoking too early or inside asynchronous code, causing unintended errors.
Best practices involve checking if the proxy is revoked before accessing it, and considering alternative approaches like WeakMaps for better encapsulation.

By understanding how revocable proxies work, you can avoid unexpected TypeErrors and manage object access more effectively in JavaScript applications.