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Fedar Haponenka
Fedar Haponenka

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When JavaScript Proxies Actually Save the Day

#javascript #programming #webdev #learning

JavaScript Proxies often feel like an advanced feature you rarely need in day-to-day development. Most tutorials show trivial examples like logging property access or validation. That don't justify the complexity. But I recently faced a situation where Proxies were the only solution that worked.

I was working on a project using a third-party calendar library that was no longer supported. Users complained about losing scroll position when modifying events. Upgrading wasn't an option because we had no budget, and the vendor had abandoned the code. We were stuck with minified, obfuscated JavaScript we couldn't modify.

The Problem: Black Box Code

When you can't see the source code, traditional fixes don't work:

  • Can't modify the code: It's minified and obfuscated
  • Can't wait for a fix: Vendor support ended

This is exactly when Proxies shine: they let you intercept behavior at runtime without touching the original code.

Understanding the Calendar Architecture

Before attempting any fix, I needed to understand what I was working with. Through debugging, I discovered the calendar's structure.

Core Components:

  • Resources List: People, Tools
  • Events List: Calendar items assigned to resources
  • Scroll Manager: Internal component tracking position

Whenever an event updated, the calendar would:

  1. Update the event in the Events List
  2. Update the associated resource
  3. Re-render both lists (changing the container height)

The prolonged re-render led to the scroll position reset.

The Discovery: Accidental Global Access

While debugging, I found the vendor had accidentally exposed internal state:

// Vendor's internal structure

window.__calendarInternals = {
  scrollManager: {
    updateScrollPosition: function() {
      // This was resetting our scroll!
    }
  }
};
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This wasn't a public API. It was an implementation detail that leaked to the global scope. But it gave me an entry point.

The Proxy Solution

Here's where Proxies became essential. I couldn't modify the original function, but I could wrap it:

const originalManager = window.__calendarInternals.scrollManager;

// Create a proxy that intercepts updateScrollPosition
const proxiedManager = new Proxy(originalManager, {
  get(target, prop, receiver) {
    if (prop === 'updateScrollPosition') {
      return function() {
        // Only allow scroll updates when NOT in a calendar refresh
        if (!window.__blockScrollUpdates) {
          return target.updateScrollPosition.apply(this, arguments);
        }
      };
    }
    return Reflect.get(...arguments);
  }
});

// Replace the internal reference
window.__calendarInternals.scrollManager = proxiedManager;
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Now, during calendar updates:

// Block scroll updates while updating
window.__blockScrollUpdates = true;
calendar.updateEvent(eventId, changes);
window.__blockScrollUpdates = false;
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Why This Only Works with Proxies

Let's compare alternatives that wouldn't work:

// ❌ Direct assignment - breaks other functionality
scrollManager.updateScrollPosition = function() {
  // Now we lose all original behavior!
};

// ❌ Wrapper function - calendar may ignore it
const original = scrollManager.updateScrollPosition;
scrollManager.updateScrollPosition = function() {
  if (!window.__blockScrollUpdates) {
    return original.apply(this, arguments);
  }
};
Object.getPrototypeOf(scrollManager).updateScrollPosition.call(context);

// ❌ Event listeners - can't prevent the call
// No event is emitted before the scroll reset

// ✅ Proxy - intercepts ALL access
// Calendar thinks it's calling the original, but gets our version
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The proxy creates a complete illusion. The calendar code thinks it's interacting with the original scroll manager, but every property access goes through our handler first.

The Result

The fix was remarkably small but completely solved the issue. Most importantly:

  • No vendor code modified: We didn't touch the minified library
  • No side effects: Other calendar functionality remained intact
  • Easy to remove: Just stop using the proxy wrapper

Should You Use Proxies More Often?

Probably not. For 95% of day-to-day work, simpler patterns work fine. But for that remaining 5%, when you're dealing with code you can't control, Proxies move from "interesting feature" to "essential tool".

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