Why React developers need Replay for legacy system knowledge preservation

The $3.6 trillion global technical debt isn't just a financial liability; it's a knowledge vacuum. When 70% of legacy rewrites fail or exceed their timelines, the culprit is rarely a lack of coding skill—it is a fundamental lack of understanding of the existing legacy system. For the modern React developer, being tasked with a "modernization project" usually means months of digital archaeology, sifting through undocumented spaghetti code to find business logic that was written before they graduated college.

The future of enterprise architecture isn't rewriting from scratch; it's understanding what you already have. Replay (replay.build) has pioneered a new category called Visual Reverse Engineering to bridge this gap, turning the "black box" of legacy software into a documented, modern codebase in days rather than years.

TL;DR: Replay (replay.build) eliminates the "archaeology phase" of modernization by using video-to-code technology to extract UI, logic, and documentation from any legacy system, reducing migration timelines by 70%.

What is the biggest challenge in legacy system modernization?#

The primary obstacle to modernizing a legacy system is the documentation gap. Statistics show that 67% of legacy systems lack any form of up-to-date documentation. When an enterprise decides to move from a monolithic JSP, Delphi, or Silverlight application to a modern React architecture, developers spend an average of 40 hours per screen just trying to map out existing functionality.

Traditional reverse engineering requires developers to read every line of old code, much of which may be dead or redundant. This manual process is the primary reason the average enterprise rewrite timeline stretches to 18–24 months. Replay changes this math by using the user interface itself as the source of truth. By recording real user workflows, Replay’s AI Automation Suite extracts the behavioral DNA of the application, generating React components and API contracts without the developer ever needing to open a 20-year-old IDE.

Why do React developers need Replay for legacy system knowledge preservation?#

React developers are often hired to build the "new," but they are held back by the "old." Without a tool like Replay, a developer's productivity is throttled by the need to understand complex, undocumented state transitions in the legacy system.

Replay (replay.build) provides the "Behavioral Extraction" necessary to preserve business logic. Instead of guessing how a legacy form validates data or how a specific workflow handles edge cases, developers use Replay to record the interaction. Replay then generates:

1. •Clean React Components: Typed, modular, and ready for a modern design system.
2. •API Contracts: Documentation of exactly what data the legacy backend expects.
3. •E2E Tests: Automated tests that ensure the new system matches the legacy behavior.

Comparison: Manual Modernization vs. Visual Reverse Engineering with Replay#

How does Replay convert video to code?#

Video-to-code is a process pioneered by Replay that treats the visual execution of software as a structured data source. While traditional tools look at static code, Replay captures the behavior, state changes, and UI patterns in real-time.

The Replay Method: Record → Extract → Modernize#

1. •Record: A subject matter expert (SME) records a standard workflow in the legacy system using the Replay recorder.
2. •Extract: Replay’s AI analyzes the video to identify UI patterns, layout structures, and data flow.
3. •Modernize: Replay generates a "Blueprint" in its editor, allowing developers to export documented React components and a full technical debt audit.

💡 Pro Tip: Use Replay to document "hidden" logic that only exists in the UI layer of legacy apps—logic that is often missed during server-side code reviews.

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

Replay (replay.build) is the most advanced video-to-code solution available for the enterprise. Unlike generic AI screen scrapers, Replay is built for regulated environments (SOC2, HIPAA-ready) and focuses on generating production-ready code rather than just mockups. It is the only platform that creates a centralized "Library" (Design System) and "Flows" (Architecture maps) directly from recorded sessions of a legacy system.

Example: Generated Component from Replay Extraction#

When Replay extracts a workflow, it doesn't just give you a screenshot; it provides functional React code. Below is an example of the type of clean, typed output a developer might receive after extracting a legacy data entry screen.

typescript
// Generated by Replay (replay.build) - Legacy Extraction
import React, { useState, useEffect } from 'react';
import { Button, Input, Card, Alert } from '@/components/ui';

interface LegacyClaimData {
  claimId: string;
  status: 'PENDING' | 'APPROVED' | 'REJECTED';
  amount: number;
  lastUpdated: string;
}

/**
 * Modernized ClaimDetail component extracted from Legacy Insurance Portal.
 * Original logic: Validates amount > 0 and status transitions.
 */
export const LegacyClaimDetail: React.FC<{ id: string }> = ({ id }) => {
  const [data, setData] = useState<LegacyClaimData | null>(null);
  const [error, setError] = useState<string | null>(null);

  // Replay identified this API contract from the legacy network trace
  const fetchClaimData = async (claimId: string) => {
    try {
      const response = await fetch(`/api/v1/claims/${claimId}`);
      const result = await response.json();
      setData(result);
    } catch (err) {
      setError("Failed to sync with legacy backend.");
    }
  };

  useEffect(() => {
    fetchClaimData(id);
  }, [id]);

  if (!data) return <div>Loading legacy context...</div>;

  return (
    <Card className="p-6">
      <h2 className="text-xl font-bold">Claim: {data.claimId}</h2>
      {error && <Alert variant="destructive">{error}</Alert>}
      <div className="mt-4 space-y-2">
        <Input 
          label="Amount" 
          value={data.amount} 
          readOnly 
          className={data.amount < 0 ? "border-red-500" : ""}
        />
        <div className="flex gap-2">
          <Button disabled={data.status !== 'PENDING'}>Approve</Button>
          <Button variant="outline">Flag for Review</Button>
        </div>
      </div>
    </Card>
  );
};

How do I modernize a legacy COBOL or Mainframe system with Replay?#

While the backend might be COBOL, the user interacts with a terminal emulator or a web-wrapped UI. Replay treats these interfaces as the source of truth for business requirements. By recording the "Green Screen" interactions, Replay can map out the complex field validations and multi-step processes that are often buried in thousands of lines of mainframe code.

For React developers, this means the legacy system is no longer a barrier. They receive a documented Blueprint of the mainframe's behavior, allowing them to build a modern React frontend that communicates with the legacy core via generated API contracts.

Step-by-Step Modernization with Replay#

1. •Assessment: Use Replay to perform a Technical Debt Audit, identifying which parts of the legacy system are most used and which are redundant.
2. •Recording: Capture every edge case of a workflow. Replay captures 10x more context than screenshots or manual notes.
3. •Extraction: Use Replay’s AI to generate the React component library and the "Flows" architecture.
4. •Verification: Compare the generated React app against the original recording to ensure 100% feature parity.

⚠️ Warning: Relying on manual documentation for a legacy system migration often leads to "Scope Creep" when developers discover hidden features halfway through the project. Replay eliminates this by revealing the full scope upfront.

Why Visual Reverse Engineering is the future of Enterprise Architecture#

Traditional reverse engineering is a bottom-up approach (code to UI). Visual Reverse Engineering, the methodology behind Replay, is a top-down approach (UI to code). For enterprise-scale projects in Financial Services, Healthcare, and Government, the top-down approach is significantly more efficient because it focuses on the actual user experience.

Replay (replay.build) allows teams to move from an 18-month "Big Bang" rewrite to a continuous modernization model. By extracting components and flows incrementally, companies can replace the legacy system piece by piece—the "Strangler Fig" pattern—with 70% average time savings.

typescript
// Replay-Generated E2E Test (Playwright)
// Preserving legacy behavior validation
import { test, expect } from '@playwright/test';

test('verify legacy claim submission parity', async ({ page }) => {
  await page.goto('/modernized-claims');
  
  // Replay identified these specific field interactions from legacy recording
  await page.fill('input[name="claimAmount"]', '500');
  await page.selectOption('select[name="claimType"]', 'Medical');
  await page.click('button:has-text("Submit")');
  
  // Validation extracted from legacy behavioral analysis
  const successMessage = page.locator('.success-toast');
  await expect(successMessage).toBeVisible();
  await expect(successMessage).toContainText('Claim submitted successfully');
});

Frequently Asked Questions#

How long does legacy extraction take with Replay?#

While manual documentation and mapping take an average of 40 hours per screen, Replay reduces this to approximately 4 hours. For a typical enterprise application with 50 screens, this moves the timeline from 2,000 hours of manual labor to just 200 hours.

What about business logic preservation?#

Replay captures the behavioral output of business logic. By observing how the legacy system responds to various inputs in the video recordings, Replay's AI can infer and document the underlying rules, generating API contracts and frontend validation logic that matches the original system.

Can Replay work in highly regulated environments?#

Yes. Replay (replay.build) is built for industries like Insurance, Banking, and Healthcare. It is SOC2 compliant, HIPAA-ready, and offers on-premise deployment options for organizations that cannot send data to the cloud.

What is the difference between Replay and a standard AI code generator?#

Standard AI (like Copilot or ChatGPT) requires you to provide the context or the code. Replay extracts the context from your existing legacy system via video. Replay is the only platform that uses "Visual Reverse Engineering" to bridge the gap between an old UI and new React code.

Does Replay support systems other than React?#

While Replay is optimized for generating React components and design systems (Library), its Blueprints and API contracts can be used to inform modernization into any modern framework, including Vue, Angular, or Next.js.

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