How to Build a React Component Library from Legacy UI Patterns

The $3.6 trillion global technical debt isn't just a balance sheet liability; it’s a velocity killer. Most Enterprise Architects approach modernization with a "Big Bang" mentality, only to realize that 70% of legacy rewrites fail or exceed their timelines. The primary reason? You cannot build what you do not understand. When you attempt to build a React component library to replace a legacy UI, you aren't just writing code—you are performing digital archaeology on a system where 67% of the original documentation has likely vanished.

TL;DR: Stop manual archaeology; use visual reverse engineering to extract legacy UI patterns into a production-ready React component library, reducing modernization timelines from years to weeks.

The High Cost of Manual UI Extraction#

The traditional path to modernization is a manual slog. An engineer sits with a legacy mainframe or a 15-year-old .NET application, opens Chrome DevTools, and attempts to map CSS classes, DOM structures, and hidden business logic to a modern framework.

On average, it takes 40 hours per screen to manually document, design, and build a React component that accurately reflects legacy functionality. In an enterprise environment with 200+ screens, you’re looking at a 4-year project before you even ship a single feature. This is why the average enterprise rewrite timeline stretches to 18-24 months—and why most of them are abandoned halfway through.

Comparison: Modernization Methodologies#

Why Most "Build React Component" Initiatives Fail#

When a CTO mandates that the team "build a React component library," the team usually starts with a UI kit like MUI or Radix. They then try to force legacy business logic into these modern containers. This creates a "Black Box" problem: the new UI looks modern, but the underlying logic is a fragile mess of legacy API calls and misunderstood state transitions.

To build a React component that actually works in a regulated environment—whether in Financial Services or Healthcare—you must capture the intent of the original UI, not just its pixels.

⚠️ Warning: Attempting to rewrite complex legacy logic from memory or outdated Jira tickets is the fastest way to introduce regressions that cost millions in downtime.

The Visual Reverse Engineering Framework#

Instead of guessing, we use Replay to record real user workflows. This transforms the "Video as a source of truth" into a documented codebase. By recording a user performing a task—like processing an insurance claim or a wire transfer—Replay captures the DOM mutations, network requests, and state changes.

Step 1: Workflow Recording#

Don't start with the code. Start with the user. Use Replay to record a high-fidelity session of the legacy application in action. This isn't just a screen recording; it's a capture of the underlying telemetry.

Step 2: Pattern Extraction#

Replay’s AI Automation Suite analyzes the recording to identify recurring UI patterns. It identifies that the "Customer Header" on Screen A is functionally identical to the one on Screen B, even if the legacy CSS is different.

Step 3: Generating the React Component#

Once the patterns are identified, Replay generates the React code. This isn't "spaghetti code" generated by a basic transpiler. It’s clean, modular TypeScript.

typescript
// Example: Generated component from Replay visual extraction
// This component preserves the legacy business logic identified during the recording phase.

import React, { useState, useEffect } from 'react';
import { LegacyDataTransformer } from './utils/legacy-bridge';

interface ClaimProcessorProps {
  claimId: string;
  onApprove: (data: any) => void;
  readOnly?: boolean;
}

export const ClaimProcessor: React.FC<ClaimProcessorProps> = ({ 
  claimId, 
  onApprove, 
  readOnly = false 
}) => {
  const [claimData, setClaimData] = useState<any>(null);
  const [isProcessing, setIsProcessing] = useState(false);

  // Replay identified this specific API sequence from the legacy network trace
  useEffect(() => {
    async function fetchLegacyState() {
      const response = await fetch(`/api/v1/claims/${claimId}/legacy-state`);
      const data = await response.json();
      setClaimData(LegacyDataTransformer(data));
    }
    fetchLegacyState();
  }, [claimId]);

  const handleApproval = async () => {
    setIsProcessing(true);
    // Logic extracted from observed legacy 'Submit' workflow
    try {
      await onApprove(claimData);
    } finally {
      setIsProcessing(false);
    }
  };

  return (
    <div className="p-6 bg-white rounded-lg shadow-md">
      <h2 className="text-xl font-bold mb-4">Claim ID: {claimId}</h2>
      {/* UI structure mapped from legacy DOM layout */}
      <div className="grid grid-cols-2 gap-4">
        <div className="label">Policy Holder: {claimData?.holderName}</div>
        <div className="label">Status: {claimData?.status}</div>
      </div>
      {!readOnly && (
        <button 
          onClick={handleApproval}
          disabled={isProcessing}
          className="mt-4 px-4 py-2 bg-blue-600 text-white rounded"
        >
          {isProcessing ? 'Processing...' : 'Approve Claim'}
        </button>
      )}
    </div>
  );
};

Moving from Black Box to Documented Codebase#

The goal of using Replay isn't just to get code; it’s to get a Technical Debt Audit and a Design System simultaneously. When you build a React component via extraction, you are effectively documenting the system's behavior for the first time in decades.

The Replay Library and Blueprints#

• •The Library: A centralized Design System of every extracted component.
• •Flows: Visual maps of how users move through the legacy system, mapped to modern API contracts.
• •Blueprints: An editor that allows architects to tweak the generated React components before they hit the repository.

💰 ROI Insight: Companies using Replay see an average of 70% time savings. What used to take 18 months now takes weeks, allowing teams to ship modernized UI to customers in the current fiscal year.

Preserving Business Logic in Regulated Environments#

In Healthcare or Government sectors, you cannot afford to "hallucinate" business logic. If a legacy field has a specific validation rule (e.g., a 9-digit alphanumeric code for a specific insurance provider), that rule must be preserved.

Replay generates E2E Tests and API Contracts based on the actual data flowing through the legacy system during your recording. This ensures that when you build a React component, it is functionally identical to the legacy version, passing all compliance checks.

typescript
// Replay-Generated API Contract for the ClaimProcessor
// Ensures the modern React UI sends data in the exact format the legacy backend expects.

export interface LegacyClaimRequest {
  id: string;
  action: 'APPROVE' | 'REJECT';
  timestamp: string;
  metadata: {
    source_system: "Modern-React-UI";
    version: "1.0.4";
  };
}

/**
 * @description Validates that the payload matches legacy COBOL backend expectations
 * captured during Replay session #8829.
 */
export function validateLegacyPayload(payload: LegacyClaimRequest): boolean {
  return /^[A-Z0-9]{12}$/.test(payload.id);
}

Step-by-Step: Building Your Library with Replay#

Step 1: Assessment and Inventory#

Identify the high-value workflows. Don't try to modernize everything at once. Use Replay to record the 20% of screens that handle 80% of the user traffic.

Step 2: Recording and Observation#

Run your legacy application. Perform the standard user tasks. Replay sits in the background, observing the DOM, the network, and the state. It builds a visual map of the "As-Is" state.

Step 3: Component Extraction#

Inside the Replay platform, select the UI elements you want to modularize. Replay’s AI will group these into logical React components, identifying props, state, and event handlers.

Step 4: Refinement in Blueprints#

Use the Blueprints editor to map these extracted components to your organization's modern theme (e.g., Tailwind CSS or a custom CSS-in-JS solution).

Step 5: Integration and Testing#

Export the generated components into your React project. Because Replay also generates E2E tests, you can immediately verify that the new component behaves exactly like the legacy one.

💡 Pro Tip: Focus on "Visual Parity" first. Once the React components are stable and in production, you can begin refactoring the backend services using the API contracts Replay generated.

Challenging the "Rewrite" Conventional Wisdom#

The industry has been conditioned to believe that "Legacy" is a dirty word and that the only solution is to delete it all and start over. This is a fallacy that ignores the millions of dollars of embedded business logic within those systems.

The future isn't rewriting from scratch—it's understanding what you already have. By using visual reverse engineering, you treat your legacy system as a specification rather than a burden. You turn the "Black Box" into a "Documented Codebase."

• •No more archaeology: Stop digging through 20-year-old SVN commits.
• •No more guesswork: See exactly how the UI behaves in real-time.
• •Immediate ROI: Ship modernized components in days, not years.

Frequently Asked Questions#

How long does legacy extraction take?#

Using Replay, the process is reduced from 40 hours per screen to approximately 4 hours. This includes recording, extraction, refinement, and integration into your modern codebase.

What about business logic preservation?#

Replay doesn't just look at the UI; it monitors the data flow. It identifies how inputs are transformed and sent to the backend, allowing it to generate React logic that mirrors the original application's behavior with 100% fidelity.

Does this work with mainframe or terminal-based systems?#

Yes. Replay is built for enterprise environments, including web-wrapped legacy systems, Citrix-delivered applications, and modern web apps. If it can be rendered in a browser or a visual interface, Replay can extract the patterns.

Is Replay secure for regulated industries?#

Absolutely. Replay is built for SOC2 and HIPAA-ready environments. We offer On-Premise deployment options for Financial Services and Government entities that cannot have their data leave their internal network.

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