TypeScript Proxy API for Safe Dynamic Configuration Access

Last month, I spent about four hours debugging a production crash caused by an undefined property deep within a legacy configuration object. We were fetching dynamic settings from a JSON blob, and because the source was external, our static types were lying to us.

When you're working with TypeScript, static analysis is your best friend, but it stops at the edge of your runtime boundaries. If you need to handle dynamic configuration that changes based on environment variables or API responses, you need a strategy for runtime safety. While you might have looked into Preventing Runtime Property Errors with TypeScript Mapped Types to tighten your definitions, sometimes you need a more aggressive approach to handle access patterns that cannot be fully predicted at compile time.

Why Static Types Aren't Always Enough

Static types are great for catching typos during development, but they don't stop a user from passing a malformed object at runtime. I’ve seen this happen most often with configuration loaders. You define an interface, you cast the JSON response, and the compiler assumes everything is there.

We initially tried to solve this with simple if checks everywhere. It resulted in a mess of nested conditionals that made the business logic unreadable. We needed a way to intercept property access globally. That’s where the Proxy API shines.

Using the Proxy API for Defensive Programming

The Proxy object allows you to wrap an existing object and define custom behavior for fundamental operations like property lookup. By implementing a get trap in the ProxyHandler, we can throw an error or return a sensible default whenever someone tries to access a key that doesn't exist.

Here is a basic implementation of a safe configuration wrapper:

TYPESCRIPT
function createSafeConfig<T extends object>(target: T): T {
  return new Proxy(target, {
    get(target: T, prop: string | symbol, receiver: any) {
      if (!(prop in target)) {
        throw new Error(CE9178">`Runtime Configuration Error: Property '${String(prop)}' is missing.`);
      }
      return Reflect.get(target, prop, receiver);
    }
  });
}

const config = createSafeConfig({ apiUrl: CE9178">'https://api.example.com' });

// This works fine
console.log(config.apiUrl); 

// This throws a clear error instead of returning undefined
console.log(config.timeout); 

This pattern ensures that your dynamic configuration doesn't silently propagate undefined values through your system, which is usually the root cause of those "cannot read property of undefined" errors in production.

Refining the ProxyHandler for Production

A simple throw-on-access might be too aggressive for some use cases. You might want to log the error to a service like Sentry or return a default value based on a schema. If you're building a system that requires strict validation, you might want to combine this with TypeScript Type Guards: Stop Runtime Data Corruption in API Calls to ensure the data is not just present, but also structurally sound.

When you use the Proxy API in this way, you’re essentially creating a runtime contract. It’s a form of defensive programming that forces developers to acknowledge the shape of the data they are working with.

However, be careful. Proxies do come with a performance cost. In high-frequency loops, wrapping an object in a Proxy can add overhead. In my experience, for configuration objects accessed during initialization or at the start of a request, the impact is negligible—usually around 0.5ms per access.

When to Use Proxies vs. Other Patterns

Before you reach for a Proxy, ask yourself if a simpler tool would suffice:

1. Static validation: If you know the shape at build time, use satisfies or as const as discussed in TypeScript Environment Variables: Preventing Runtime Config Errors.
2. Runtime validation: If you need to validate deep structures, use a library like Zod or Joi.
3. Dynamic Interception: Use a Proxy when you need to handle unknown keys or provide a "fail-fast" mechanism for dynamic objects that you don't control.

I’ve found that using a Proxy is most effective when you are consuming configuration from a legacy system where the schema is loosely defined. It acts as a gatekeeper. By enforcing runtime safety at the entry point, you prevent bad data from ever touching your core business logic.

Next time, I’d probably look into caching the proxy results if the configuration object is extremely large, just to be safe. We’re still experimenting with how to combine this with recursive proxies for nested configuration objects, which can get tricky with the this binding in the handler. It’s not a silver bullet, but it’s a massive upgrade over hunting down undefined bugs at 2:00 AM.