How to Turn a 2,000-Page Legacy Manual Into Production-Ready React Code

The average enterprise rewrite takes 18 to 24 months, and 70% of those projects either fail or significantly exceed their original timelines. The primary culprit isn't a lack of engineering talent; it’s the "archaeology phase." When you are tasked to modernize a legacy system—whether it’s a 20-year-old COBOL-backed banking portal or a sprawling healthcare ERP—you aren't just writing code. You are excavating business logic buried in 2,000-page manuals that haven't been updated since the Bush administration.

TL;DR: Stop manual archaeology; use Replay to record legacy workflows and automatically generate documented React components, slashing modernization timelines by 70%.

The $3.6 Trillion Documentation Gap#

Global technical debt has reached a staggering $3.6 trillion. For the Enterprise Architect, this isn't just a number; it’s the reality of managing systems where 67% of the codebase lacks any form of reliable documentation. When leadership asks for a "modern web experience," they often underestimate the cost of discovery.

Traditionally, you have two bad options:

1. •The Big Bang Rewrite: Attempting to replicate every feature from scratch. This is where most projects die because the "source of truth" is a combination of outdated PDFs and the tribal knowledge of developers who are nearing retirement.
2. •The Strangler Fig Pattern: Incrementally replacing pieces. While safer, it often gets bogged down in the sheer complexity of mapping legacy state transitions to modern React hooks.

Replay introduces a third path: Visual Reverse Engineering. Instead of reading the manual, you record the system in action.

How to Turn Legacy Workflows into Production Code: A Step-by-Step Guide#

The goal is to move from a "black box" to a documented, modular React codebase without the manual labor of hand-coding every form field and validation rule. Manual UI reconstruction takes an average of 40 hours per screen. With Replay, that drops to 4 hours.

Step 1: Mapping the "Source of Truth" via Video#

Stop relying on the 2,000-page manual. It’s likely wrong. Instead, use Replay to record a real user performing a mission-critical workflow—for example, a complex insurance claim entry. This video becomes the "Source of Truth." Replay’s engine analyzes the frames, network calls, and state changes to understand what the legacy system is actually doing, not what the manual says it’s doing.

Step 2: Extracting the Component Architecture#

Once the workflow is captured, Replay’s AI Automation Suite identifies UI patterns. It doesn't just give you "spaghetti code"; it maps elements to your modern Design System (Library). If the legacy system has a data grid, Replay identifies the data structure and maps it to a clean, functional React component.

Step 3: Generating the React Code#

Replay generates production-ready TypeScript code. This isn't a low-code abstraction; it’s actual code you can commit to your repository.

typescript
// Example: Generated React Component from a Legacy Insurance Claim Screen
import React, { useState, useEffect } from 'react';
import { Button, TextField, DataGrid, Alert } from '@acme-corp/design-system';
import { validateClaimSchema } from './schemas/claimValidation';

interface ClaimData {
  policyNumber: string;
  incidentDate: string;
  claimAmount: number;
  claimantDetails: Record<string, any>;
}

/**
 * @generated Generated via Replay Visual Reverse Engineering
 * Legacy Source: Claims_Entry_Module_V4.exe
 * Workflow: Standard Property Damage Claim
 */
export const LegacyClaimForm: React.FC = () => {
  const [formData, setFormData] = useState<Partial<ClaimData>>({});
  const [error, setError] = useState<string | null>(null);

  const handleFieldChange = (field: keyof ClaimData, value: any) => {
    setFormData(prev => ({ ...prev, [field]: value }));
  };

  const handleSubmit = async () => {
    const { isValid, errors } = validateClaimSchema(formData);
    if (!isValid) {
      setError(`Validation Failed: ${errors.join(', ')}`);
      return;
    }
    // Business logic preserved from legacy network trace
    await submitToLegacyBridge(formData);
  };

  return (
    <div className="p-6 space-y-4">
      <TextField 
        label="Policy Number" 
        value={formData.policyNumber} 
        onChange={(v) => handleFieldChange('policyNumber', v)} 
      />
      <TextField 
        type="date" 
        label="Incident Date" 
        value={formData.incidentDate} 
        onChange={(v) => handleFieldChange('incidentDate', v)} 
      />
      {error && <Alert severity="error">{error}</Alert>}
      <Button onClick={handleSubmit} variant="primary">Submit Modernized Claim</Button>
    </div>
  );
};

💡 Pro Tip: Use the "Blueprints" feature in Replay to adjust the layout visually before exporting. This allows you to fix UX debt while preserving the underlying business logic.

Step 4: Automating the API Contract#

One of the biggest pain points in modernization is the "Middle-Tier Gap." Replay tracks the network traffic during your recording to generate OpenAPI/Swagger specifications. This ensures your new React frontend speaks the exact same language as the legacy mainframe or SOAP service.

yaml
# Generated API Contract from Replay Flow
openapi: 3.0.0
info:
  title: Legacy Claims API
  version: 1.0.0
paths:
  /api/v1/claims/submit:
    post:
      summary: Preserved endpoint from legacy trace
      requestBody:
        content:
          application/json:
            schema:
              $ref: '#/components/schemas/Claim'
      responses:
        '200':
          description: Success

Why Understanding beats Rewriting#

The "Future of Modernization" isn't about hiring 100 developers to rewrite code for two years. It's about high-fidelity understanding. When you use Replay, you are creating a "Digital Twin" of your legacy system.

• •Library (Design System): Ensures the new React components match your company's modern branding from day one.
• •Flows (Architecture): Provides a visual map of how data moves through the system—something a 2,000-page manual fails to do effectively.
• •Technical Debt Audit: Replay identifies redundant fields and dead logic paths that don't need to be migrated, reducing the surface area of your new application.

⚠️ Warning: Many AI tools attempt to "guess" logic from code snippets. In regulated industries like Banking or Healthcare, "guessing" is a compliance nightmare. Replay uses the actual execution (video and network traces) as the source of truth, ensuring 100% logic parity.

Built for the Regulated Enterprise#

We understand that you can't just upload your core banking logic to a public cloud. Replay is built for:

• •Financial Services: Complex state management and audit trails.
• •Healthcare: HIPAA-ready workflows and data privacy.
• •Government/Defense: On-premise deployment options for air-gapped environments.
• •Manufacturing: Bridging the gap between legacy SCADA interfaces and modern web dashboards.

💰 ROI Insight: By moving from 40 hours per screen to 4 hours, an enterprise migrating a 200-screen application saves approximately 7,200 engineering hours—roughly $1.1M in labor costs alone.

Frequently Asked Questions#

How does Replay handle "Black Box" logic?#

Replay records the inputs and outputs of the legacy system. By analyzing the state changes in the UI and the corresponding network payloads, it reconstructs the business rules in the generated React components and API contracts. You don't need the original source code to understand the logic.

Does this work with Mainframe or Terminal-based systems?#

Yes. If the system has a visual interface (even a green-screen terminal via an emulator), Replay can record the workflow, identify the data fields, and map them to modern React inputs.

Is the generated code maintainable?#

Unlike "no-code" platforms, Replay generates standard, human-readable TypeScript and React. It uses your existing component library and follows industry best practices for state management (Hooks, Context, or Redux).

How long does a typical pilot take?#

Most organizations can see their first legacy screen extracted and running in a modern React environment within 48 hours.

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Ready to modernize without rewriting? Book a pilot with Replay - see your legacy screen extracted live during the call.