ERP Custom Logic Preservation: Moving Proprietary Workflows to React

Every enterprise has a "Black Box" screen—a complex, mission-critical interface for risk assessment, claims processing, or inventory reconciliation that hasn't been touched since 2004. The original developer is long gone, the documentation is non-existent, and the source code is a spaghetti-mess of COBOL or legacy Java. Yet, this screen contains twenty years of edge-case handling and proprietary business rules that are the lifeblood of the company.

The fear of losing these nuances is why 70% of legacy rewrites fail or exceed their timelines. When you decide to modernize, you aren't just building a prettier UI; you are performing a delicate extraction of business intelligence. Custom logic preservation moving to modern frameworks like React requires more than just a fresh coat of CSS—it requires a forensic approach to software engineering.

TL;DR: Modernizing legacy ERP systems often fails because proprietary business logic is trapped in the UI layer. Manual extraction takes roughly 40 hours per screen and carries high risk. Replay uses Visual Reverse Engineering to capture these workflows, reducing modernization time by 70% and ensuring that "custom logic preservation moving" to React is documented, deterministic, and accurate.

The $3.6 Trillion Technical Debt Crisis#

The global technical debt has ballooned to $3.6 trillion. For most organizations in financial services, healthcare, and manufacturing, this debt is concentrated in ERP systems that have been customized beyond recognition. According to Replay's analysis, 67% of these legacy systems completely lack up-to-date documentation.

When teams attempt a manual rewrite, they often follow a "look and guess" methodology. Developers watch a user interact with the legacy system and try to replicate the behavior in React. This is where the errors creep in. A hidden validation rule on line 4,000 of a legacy script might only trigger when a specific combination of "Country Code" and "Tax ID" is entered. If the new React component misses that rule, the entire ERP integrity is compromised.

Visual Reverse Engineering is the process of recording real user sessions within legacy applications to automatically generate documented React components and state logic, ensuring no edge case is left behind.

The Strategy for Custom Logic Preservation Moving to Modern Stacks#

Moving proprietary workflows is not a 1:1 translation. It is an architectural shift from monolithic, server-side rendering or thick-client logic to a decoupled, state-driven React architecture.

1. Identifying the "Hidden" Logic#

Proprietary logic in legacy ERPs usually lives in three places:

1. •Input Masking & Validation: How data is cleaned before it hits the database.
2. •Conditional Visibility: Which fields appear based on previous selections (the "If-This-Then-That" of the UI).
3. •Calculated Fields: Real-time totals, risk scores, or interest rates calculated on the fly.

Industry experts recommend a "Capture First" approach. Instead of reading the code, record the behavior. By using Replay, architects can record a user performing a complex workflow—like an insurance claim adjustment—and the platform’s AI Automation Suite extracts the underlying logic.

2. Mapping State Transitions#

In a legacy environment, state is often global and mutable. In React, we want state to be predictable and encapsulated. Custom logic preservation moving to a modern stack involves mapping these legacy triggers to React hooks or state management libraries (like Redux or Zustand).

3. The Modernization Comparison: Manual vs. Replay#

The following table illustrates the stark difference between traditional manual rewrites and the Visual Reverse Engineering approach offered by Replay.

Technical Implementation: From Legacy Script to React Hook#

Let’s look at a practical example. Imagine a legacy ERP screen for a bank that calculates a "Risk Adjusted Rate." The original logic is buried in a 2,000-line jQuery file.

The Legacy Problem#

In the legacy system, the logic might look like this:

javascript
// Legacy Spaghetti Logic
function updateRate() {
    var base = parseFloat($('#base_rate').val());
    var score = parseInt($('#credit_score').val());
    var region = $('#region_code').val();
    
    if (score > 700) {
        base -= 0.5;
    } else if (score < 600 && region === 'NY') {
        base += 1.2;
    }
    // ... 50 more conditions ...
    $('#display_rate').text(base.toFixed(2));
}

The Modernized React Approach#

When custom logic preservation moving to React is handled via Replay, the platform identifies these dependencies and generates a clean, testable hook. This ensures that the proprietary calculation remains intact while becoming maintainable.

typescript
// Modernized React Hook generated via Replay Blueprints
import { useState, useEffect } from 'react';

interface RiskParams {
  baseRate: number;
  creditScore: number;
  regionCode: string;
}

export const useRiskAdjustment = ({ baseRate, creditScore, regionCode }: RiskParams) => {
  const [adjustedRate, setAdjustedRate] = useState(baseRate);

  useEffect(() => {
    let newRate = baseRate;

    // Preserved Custom Logic from Legacy System
    if (creditScore > 700) {
      newRate -= 0.5;
    } else if (creditScore < 600 && regionCode === 'NY') {
      newRate += 1.2;
    }

    // Logic extracted via Replay's AI Automation Suite
    // ensures that even obscure regional rules are kept.
    setAdjustedRate(newRate);
  }, [baseRate, creditScore, regionCode]);

  return adjustedRate;
};

By isolating the logic into a hook, you can unit test the proprietary business rules independently of the UI. This is a core pillar of Legacy Modernization Strategies that prioritize stability over speed.

Implementing a Design System During the Move#

One of the biggest mistakes in ERP modernization is focusing solely on logic and ignoring the Design System. If you move the logic but keep a fragmented UI, you haven't solved the technical debt; you've just moved it.

Replay's Library feature allows teams to define a Design System (using Tailwind, Material UI, or custom CSS-in-JS) and automatically map legacy UI elements to modern components. This means that while you are focused on custom logic preservation moving your workflows, the visual layer is being standardized simultaneously.

Video-to-code is the process of converting screen recordings into functional React code. It bypasses the need for manual Figma mockups for every single legacy state, allowing developers to focus on the high-level architecture.

Ensuring Compliance in Regulated Environments#

For industries like Healthcare (HIPAA) and Financial Services (SOC2), "custom logic preservation moving" to a new cloud-native stack isn't just about functionality—it's about auditability.

Legacy systems often have "hidden" compliance checks. For example, a screen might prevent a user from clicking "Approve" unless they have scrolled through a specific legal disclaimer. If this behavior isn't preserved in the React rewrite, the company could face massive regulatory fines.

Replay is built for these high-stakes environments. With options for On-Premise deployment and SOC2 compliance, it allows enterprise architects to record workflows without sensitive data leaving their secure perimeter. This ensures that the "Flows" (architectural maps) generated by the platform are both accurate and secure.

Example: Complex Workflow Mapping#

A standard ERP workflow involves multiple steps:

1. •Data Entry
2. •Validation
3. •Manager Approval
4. •Ledger Posting

Manual documentation of these flows takes weeks. Replay's Flows feature visualizes these paths automatically.

typescript
// Example of a Flow-based Component Structure
import React from 'react';
import { useWorkflowState } from './hooks/useWorkflowState';
import { StepIndicator, DataGrid, ActionPanel } from './components/ui';

const ERPWorkflowContainer: React.FC = () => {
  const { currentStep, data, transitionTo } = useWorkflowState();

  // Replay captured that the 'Approval' step is only reachable 
  // if 'Validation' returns a status of 200.
  const handleValidation = async () => {
    const isValid = await validateLegacyRules(data);
    if (isValid) transitionTo('APPROVAL');
  };

  return (
    <div className="erp-container">
      <StepIndicator current={currentStep} />
      {currentStep === 'ENTRY' && <DataGrid onComplete={handleValidation} />}
      {/* ... other steps ... */}
    </div>
  );
};

Why Traditional Rewrites Fail (and How to Avoid It)#

According to Replay's analysis, the primary reason for the 18-month average enterprise rewrite timeline is "Discovery Debt." Teams spend the first six months simply trying to understand what the current system does.

By the time they start coding, the business requirements have changed. This creates a cycle of endless catch-up. Custom logic preservation moving to React becomes a moving target.

To break this cycle, architects must:

1. •Stop Manual Discovery: Use Visual Reverse Engineering to create a source of truth.
2. •Modularize the Monolith: Don't rewrite the whole ERP at once. Use Replay to extract one "Flow" at a time.
3. •Automate Component Creation: Use Replay’s Blueprints to generate the boilerplate code, so developers can focus on the complex integration logic.

For more on how to structure these projects, check out our guide on Scaling Component Libraries.

Frequently Asked Questions#

How does Replay handle complex calculations that happen on the server?#

While Replay focuses on the UI and client-side logic, it captures the data contracts (API requests and responses) that occur during a workflow. This allows developers to see exactly what inputs the legacy server requires and what outputs it produces, making it easier to recreate or wrap those backend services.

Can Replay work with legacy technologies like Silverlight, Delphi, or Mainframes?#

Yes. Because Replay uses Visual Reverse Engineering (recording the screen and DOM interactions where available), it can document workflows from any system that a user can interact with via a browser or terminal emulator. It turns the "visual" into "code," regardless of the underlying legacy language.

How is "custom logic preservation moving" to React different from a standard refactor?#

A refactor assumes you have clean, understandable code to start with. Most ERP modernizations are not refactors; they are "re-platforming" projects. Preservation is about ensuring that the business intent and edge-case handling are maintained even when the underlying technology changes entirely.

Does Replay generate production-ready code?#

Replay generates highly accurate React components, hooks, and TypeScript definitions. While a Senior Developer should always review the output to ensure it aligns with specific internal coding standards, Replay typically handles 70-80% of the heavy lifting, including state management and UI structure.

Is my data secure during the recording process?#

Replay is designed for the enterprise. We offer SOC2 and HIPAA-ready environments, and for highly sensitive sectors like Government or Defense, we provide On-Premise deployment options so that no proprietary logic or data ever leaves your network.

Conclusion#

The transition from a legacy ERP to a modern React-based architecture is the most significant hurdle an IT department will face. The stakes are high: lose the custom logic, and you lose the competitive advantage of the business.

By shifting from manual, error-prone discovery to an automated, visual-first approach, enterprises can finally retire their technical debt. Custom logic preservation moving to modern frameworks doesn't have to be a multi-year gamble. With the right tools, you can transform 18 months of work into a matter of weeks.

Ready to modernize without rewriting? Book a pilot with Replay