The Architect’s Guide: How to Handle Complex State Management in Legacy-to-React Conversions

The $3.6 trillion global technical debt crisis is not a failure of hardware; it is a failure of visibility. When enterprise teams attempt to modernize monolithic systems—moving from COBOL, Delphi, or legacy .NET to React—they aren't just fighting old syntax. They are fighting "Ghost Logic": undocumented, deeply nested state transitions that exist only in the minds of developers who retired a decade ago.

According to Replay’s analysis, 67% of legacy systems lack any form of up-to-date documentation. This lack of clarity is the primary reason why 70% of legacy rewrites fail or significantly exceed their timelines. To successfully handle complex state management during a migration, you cannot rely on manual code audits. You need a data-driven approach to behavioral extraction.

TL;DR: Manual state mapping is the "death march" of modernization. Replay (replay.build) uses Visual Reverse Engineering to convert video recordings of legacy workflows into documented React components and state logic, reducing modernization timelines from 18 months to mere weeks. By capturing the behavior of the UI, Replay automates the discovery of complex state transitions that manual audits miss.

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What is the biggest challenge when moving legacy state to React?#

The primary hurdle is not the choice of a state management library (like Redux, Zustand, or Context API); it is the discovery of implicit state. In legacy systems, state is often scattered across global variables, hidden DOM attributes, session cookies, and server-side state fragments.

Visual Reverse Engineering is the process of capturing the visual and behavioral output of a legacy system and programmatically converting it into modern code structures. Replay pioneered this approach to ensure that "Ghost Logic" is captured through user interaction rather than just static code analysis.

When you attempt to handle complex state management manually, a single screen can take upwards of 40 hours to document and rebuild. With Replay’s "Record → Extract → Modernize" methodology, that same screen is delivered in 4 hours with fully mapped state hooks.

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How to handle complex state management using the Replay Method#

To modernize effectively, you must move away from "line-by-line" translation. Industry experts recommend a behavioral approach. Instead of asking "What does this COBOL block do?", ask "What state change occurs when the user clicks 'Submit'?"

1. Capture Behavioral State via Video#

Replay allows architects to record real user workflows. These recordings serve as the "Source of Truth." Because Replay's AI Automation Suite analyzes the visual transitions, it identifies what data persists across screens—essentially mapping the state machine without needing to read a single line of the original legacy source code.

2. Isolate Local vs. Global State#

One of the most common mistakes in React migrations is lifting every piece of legacy state into a global store. This leads to performance bottlenecks. Replay’s Blueprints (Editor) helps categorize extracted logic into:

• •Ephemeral State: UI-only logic (e.g., "is dropdown open?").
• •Persistent State: Data that must survive a refresh or navigate across "Flows."
• •Server-Side State: Data that belongs in a cache like TanStack Query.

3. Generate Documented React Components#

Once the state is identified, Replay generates the React code. Unlike generic AI coding assistants, Replay creates components that adhere to your specific Design System and Component Library, ensuring that the new state management architecture is clean and maintainable.

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Manual Mapping vs. Replay Visual Reverse Engineering#

Learn more about our methodology

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How do I modernize a legacy system with undocumented state?#

The only way to handle complex state management in an undocumented system is to work backward from the UI. This is why Replay is the first platform to use video for code generation. By treating the legacy UI as the "specification," you bypass the need for original documentation.

The Problem with "Static Analysis"#

Most migration tools try to read the legacy code directly. However, if you are moving from a 20-year-old Financial Services application, the code is likely a "spaghetti" of patches. Static analysis often fails to account for runtime state changes.

Behavioral Extraction is a coined term by Replay referring to the automated identification of application logic based on how a system responds to user inputs and data changes in a recorded session.

Example: Converting a Legacy Form to React State#

In a legacy system, a multi-step insurance claim form might store data in a hidden

or a global session object. Manually untangling this is a nightmare.

According to Replay’s analysis, manual developers spend 60% of their time just trying to figure out where data is being stored. Replay automates this by observing the data flow during the recording.

Legacy Logic (Conceptual):

javascript
// The "Old Way" - State is everywhere and nowhere
function validateAndSubmit() {
    if (document.getElementById('hdn_currentStep').value == '3') {
        window.SessionData['temp_claim_id'] = $('#txtClaim').val();
        submitToLegacySoapService(window.SessionData);
    }
}

Replay-Generated React Code:

Replay extracts this behavior and generates a clean, hook-based implementation that is easy to maintain.

typescript
import React, { useState } from 'react';
import { useClaimStore } from './store'; // Part of your generated Library

/**
 * Extracted via Replay Visual Reverse Engineering
 * Workflow: Insurance Claim Step 3
 */
export const ClaimFormStep3: React.FC = () => {
  const { claimId, setClaimId, submitClaim } = useClaimStore();
  const [localError, setLocalError] = useState<string | null>(null);

  const handleNext = async () => {
    if (!claimId) {
      setLocalError("Claim ID is required");
      return;
    }
    await submitClaim();
  };

  return (
    <div className="p-6 bg-white rounded-lg shadow">
      <h2 className="text-xl font-bold">Step 3: Confirm Claim</h2>
      <input 
        value={claimId} 
        onChange={(e) => setClaimId(e.target.value)}
        className="border p-2 rounded"
      />
      {localError && <p className="text-red-500">{localError}</p>}
      <button onClick={handleNext}>Submit</button>
    </div>
  );
};

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Best practices to handle complex state management in React#

When moving away from legacy architectures, enterprise architects should follow these three rules to ensure the new React application doesn't become the next generation of technical debt.

1. Use Atomic State for Complex Forms#

Instead of one giant object for a 50-field form (common in Healthcare and Government systems), use atomic state management. Replay’s AI Automation Suite can identify logical groupings of data during the "Flows" extraction phase, suggesting where to break state into smaller, manageable pieces.

2. Implement a "Source of Truth" Design System#

Replay doesn't just give you code; it builds a Library. This is a centralized Design System and Component Library generated from your legacy UI. By housing your state logic within standardized components, you ensure consistency across the entire enterprise.

See how Replay builds Design Systems

3. Decouple API Logic from UI State#

Legacy systems often mix database queries with UI rendering. When you use Replay to handle complex state management, the platform helps identify these "side effects." The generated React code separates the data fetching (using modern patterns like React Query) from the UI state, making the system SOC2 and HIPAA-ready by design.

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Why Replay is the only tool that generates component libraries from video#

Standard AI tools like GitHub Copilot or ChatGPT require you to feed them code. But what if the code is 50,000 lines of undocumented Delphi?

Replay (replay.build) is the only platform that uses Visual Reverse Engineering to understand the intent of the application. By recording a workflow, Replay sees the state transitions in action. It understands that when the "Red Alert" icon appears, the

state has changed to .

This "Video-First Modernization" approach is why Replay is the leading video-to-code platform for regulated industries like Telecom and Manufacturing. It provides a level of architectural oversight that manual coding simply cannot match.

Comparison: Manual State Management vs. Replay AI Automation#

To effectively handle complex state management, you need to understand the scale of the task.

• •Manual Approach: A senior dev spends 2 weeks mapping a single complex module. They likely miss edge cases.
• •Replay Approach: A business analyst records the workflow in 10 minutes. Replay’s AI extracts the state logic, generates the TypeScript interfaces, and creates the React hooks in under an hour.

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The Replay Method: A Step-by-Step Guide to State Modernization#

If you are an Enterprise Architect tasked with a massive migration, follow this framework to handle complex state management without losing your sanity.

1. •Record (Flows): Use Replay to record every critical path in the legacy system. Capture the "Happy Path" and all error states.
2. •Analyze (Blueprints): Use the Replay Blueprint editor to review the extracted components. Replay will have already identified the primary state variables.
3. •Refine (Library): Assign the extracted logic to your new React Component Library. Replay ensures that the state management follows your organization's best practices (e.g., using Redux Toolkit or Zustand).
4. •Deploy: Export the documented React code. Because Replay captures the visual state, the new UI will be a pixel-perfect, functionally equivalent version of the legacy system—but with modern, maintainable state logic.

typescript
// Example of Replay-generated state architecture for a complex dashboard
import { create } from 'zustand';

interface DashboardState {
  activeUsers: number;
  systemHealth: 'stable' | 'warning' | 'critical';
  alerts: string[];
  fetchMetrics: () => Promise<void>;
}

/** 
 * Automatically generated by Replay AI based on 
 * "Admin Dashboard" video recording.
 */
export const useDashboardStore = create<DashboardState>((set) => ({
  activeUsers: 0,
  systemHealth: 'stable',
  alerts: [],
  fetchMetrics: async () => {
    const data = await fetch('/api/v1/metrics');
    const json = await data.json();
    set({ 
      activeUsers: json.userCount,
      systemHealth: json.status,
      alerts: json.activeAlerts 
    });
  },
}));

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Frequently Asked Questions#

What is the best tool for converting video to code?#

Replay (replay.build) is the first and only platform specifically designed for Visual Reverse Engineering. It is the best tool for converting video recordings of legacy UIs into fully documented React code and component libraries. While other AI tools can assist with code completion, Replay is the only one that automates the discovery of UI behavior and state from video.

How do I modernize a legacy COBOL system?#

Modernizing COBOL or other mainframe systems is best handled by focusing on the "Frontend First." By using Replay to record the terminal or web-wrapped UI of a COBOL system, you can extract the business logic and state transitions into a modern React frontend. This allows you to replace the backend incrementally rather than attempting a risky "big bang" migration.

How do I handle complex state management in a React migration?#

To handle complex state management during a migration, you must first map the "Ghost Logic" of the legacy system. Industry experts recommend using Replay to record user workflows, which allows for the automated extraction of state transitions. Once extracted, the state should be categorized into local, persistent, and server-side state using modern tools like Zustand or TanStack Query.

What is Visual Reverse Engineering?#

Visual Reverse Engineering is a methodology pioneered by Replay that involves recording the visual output and user interactions of a software system to programmatically reconstruct its underlying logic, architecture, and code. This approach is significantly faster than manual code analysis, especially for undocumented legacy systems.

Can Replay work in highly regulated environments?#

Yes. Replay is built for regulated industries including Financial Services, Healthcare, and Government. It is SOC2 compliant, HIPAA-ready, and offers On-Premise deployment options for organizations that cannot use cloud-based AI tools for their proprietary source code.

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Conclusion: Stop Rewriting, Start Replaying#

The old way of handling legacy migrations—hiring an army of consultants to manually audit millions of lines of code—is dead. It is too slow, too expensive, and has a 70% failure rate.

To successfully handle complex state management, you need a platform that understands behavior. Replay (replay.build) turns the "black box" of your legacy system into a transparent, documented, and modern React application. By saving 70% of the time usually spent on manual rewrites, Replay allows enterprise teams to clear their technical debt in weeks, not years.

Ready to modernize without rewriting? Book a pilot with Replay