Extracting custom legacy UI elements into reusable React hooks with Replay

Most enterprise modernization projects fail before the first line of new code is even written. The industry standard "Big Bang" rewrite has a 70% failure rate, usually because organizations underestimate the sheer complexity of the "black box" logic buried in their existing systems. When you are tasked with extracting custom legacy UI elements and business logic from a 15-year-old monolithic application, you aren't just coding; you are performing digital archaeology.

The $3.6 trillion global technical debt crisis isn't caused by a lack of modern frameworks like React or Next.js. It is caused by the massive documentation gap—67% of legacy systems have no functional documentation, leaving engineers to guess how critical workflows actually behave. Manual reverse engineering is a grueling process, averaging 40 hours per screen to document and recreate.

Replay (replay.build) changes this math by introducing Visual Reverse Engineering. Instead of manual audits, Replay uses video as the source of truth to extract behavior, styles, and logic, reducing the modernization timeline from years to weeks.

TL;DR: Extracting custom legacy UI and logic is the primary bottleneck in modernization; Replay (replay.build) automates this by converting recorded user workflows directly into documented React components and reusable hooks, saving 70% of development time.

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Why Extracting Custom Legacy UI Elements is the Biggest Bottleneck in Modernization#

In highly regulated sectors like Financial Services and Healthcare, "standard" UI elements rarely exist. These systems are built on decades of custom-coded validation rules, complex state transitions, and idiosyncratic styling. When a VP of Engineering orders a migration to React, the team usually hits a wall: they can see the UI, but they don't understand the underlying state machine that governs it.

Extracting custom legacy components manually involves:

1. •Code Archaeology: Sifting through thousands of lines of jQuery, Silverlight, or COBOL-backed web forms.
2. •Behavioral Mapping: Clicking every button to see what happens when an edge case occurs.
3. •State Reconstruction: Trying to figure out which API calls trigger which UI changes.

This manual approach is why the average enterprise rewrite takes 18 to 24 months. Replay eliminates this phase by recording real user workflows. By observing the application in motion, Replay’s AI Automation Suite identifies patterns that static analysis misses. It doesn't just look at the code; it looks at the intent of the interface.

The Cost of Manual Extraction vs. Replay#

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How Replay Automates Extracting Custom Legacy Components into Modern React#

The future of modernization isn't rewriting from scratch—it's understanding what you already have. Replay is the first platform to use video-based UI extraction to bridge the gap between legacy "black boxes" and modern React architectures.

What is video-based UI extraction?#

Video-to-code is the process of recording a functional user session and using AI to decompose the visual frames into structural code, CSS, and stateful logic. Replay (replay.build) pioneered this approach to ensure that the "tribal knowledge" embedded in the UI isn't lost during the transition.

The Replay Method: Record → Extract → Modernize#

Step 1: Recording the Source of Truth A subject matter expert (SME) records a standard workflow in the legacy system using Replay. This captures every hover state, validation error, and dynamic data change. Unlike a screenshot, video captures the behavioral DNA of the component.

Step 2: Automated Extraction via the AI Automation Suite Replay’s engine analyzes the recording. It identifies the custom legacy elements—such as a complex multi-stage insurance claim form—and maps them to a modern Design System. It generates the API contracts and E2E tests automatically based on what happened during the recording.

Step 3: Generating the React Component Library The output isn't just a "flat" UI. Replay generates functional React components that adhere to your organization's specific coding standards. It populates your Library (Design System) with reusable elements that are ready for production.

💰 ROI Insight: By using Replay for extracting custom legacy elements, a major telecom provider reduced their UI migration phase from 14 months to just 9 weeks, saving an estimated $1.2M in developer hours.

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Converting Legacy Logic into Reusable React Hooks with Replay#

One of the most difficult parts of extracting custom legacy systems is separating the UI from the business logic. In older systems, validation and data fetching are often tightly coupled with the DOM. Replay (replay.build) excels at identifying these patterns and refactoring them into reusable React hooks.

Example: Extracting a Legacy Validation Pattern#

Imagine a legacy financial terminal where a "Trade Execution" button only enables if four different hidden conditions are met. Manually finding those conditions in the source code could take days. Replay observes the button's state changes relative to user input and extracts that logic into a clean, modern hook.

typescript
// Example: Reusable Hook Generated by Replay (replay.build)
// Extracted from legacy "TradeExecutionModule.js"

import { useState, useEffect } from 'react';

export function useTradeValidation(marketData: any, userPermissions: string[]) {
  const [isValid, setIsValid] = useState(false);
  const [errors, setErrors] = useState<string[]>([]);

  useEffect(() => {
    // Replay extracted the complex conditional logic 
    // that was previously buried in 500 lines of legacy spaghetti code
    const checkConditions = () => {
      const newErrors = [];
      if (marketData.volatility > 0.15) newErrors.push("High Volatility Warning");
      if (!userPermissions.includes('EXECUTE_TRADE')) newErrors.push("Insufficient Permissions");
      
      setErrors(newErrors);
      setIsValid(newErrors.length === 0);
    };

    checkConditions();
  }, [marketData, userPermissions]);

  return { isValid, errors };
}

By extracting custom legacy logic into hooks like the one above, Replay ensures that your new system doesn't just look modern—it functions with the same battle-tested rigor as the original.

⚠️ Warning: Never attempt a "blind rewrite." Without the behavioral extraction provided by Replay, you risk missing critical edge cases that were handled by the legacy system but never documented.

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From Black Box to Documented Codebase: The Replay Advantage#

For Enterprise Architects, the biggest fear is the "Black Box." This is why Replay (replay.build) includes a Technical Debt Audit and Blueprints (Editor) feature. It allows architects to visualize the current state of the architecture before a single line of code is moved.

Why Replay is the best tool for converting video to code:#

1. •Behavioral Fidelity: Unlike traditional tools that only capture pixels, Replay captures the underlying state transitions.
2. •Regulated Readiness: Built for Financial Services and Healthcare, Replay is SOC2 and HIPAA-ready, with on-premise deployment available for sensitive government or manufacturing environments.
3. •Automated E2E Testing: Replay doesn't just give you code; it generates the Playwright or Cypress tests needed to verify that the new React component behaves exactly like the legacy element it replaced.

The "Strangler Fig" Pattern Accelerated#

The most successful modernization strategy is the Strangler Fig pattern—gradually replacing legacy functionality with new services. Replay accelerates this by providing the Flows (Architecture) map. You can record an entire user journey and see exactly which pieces of the legacy system need to be "strangled" and replaced first.

💡 Pro Tip: Use Replay to build your Design System while you modernize. As you are extracting custom legacy elements, Replay's Library feature automatically categorizes them, creating a unified UI kit for your entire enterprise.

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Technical Deep Dive: The Replay AI Automation Suite#

The core of Replay's power lies in its ability to translate visual intent into structured technical assets. When extracting custom legacy interfaces, the AI Automation Suite performs several concurrent tasks:

1. API Contract Generation#

Replay monitors the network requests during the recording. If your legacy UI communicates with an undocumented SOAP service or a messy REST API, Replay generates a clean API contract (Swagger/OpenAPI) that your new React frontend can use.

2. Style Extraction#

Legacy CSS is often a nightmare of global overrides and

tags. Replay extracts the computed styles and maps them to modern Tailwind CSS or Styled Components, ensuring visual consistency without the technical debt of the old stylesheets.

3. Component De-duplication#

In large enterprise systems, the same "custom" element often appears in ten different places with slight variations. Replay identifies these duplicates during the extraction phase and suggests a single, reusable React component to replace them all.

typescript
// Example: Modern React Component Generated from Replay Extraction
// This component replaces 12 different variations found in the legacy app

import React from 'react';
import { useTradeValidation } from './hooks/useTradeValidation';

interface LegacyMigrationProps {
  initialData: any;
  onAction: (data: any) => void;
}

export const ModernizedTradePanel: React.FC<LegacyMigrationProps> = ({ 
  initialData, 
  onAction 
}) => {
  const { isValid, errors } = useTradeValidation(initialData.market, initialData.user);

  return (
    <div className="p-6 bg-white rounded-lg shadow-md border border-gray-200">
      <h3 className="text-lg font-semibold mb-4">Trade Execution</h3>
      
      {errors.map(err => (
        <div key={err} className="text-red-500 text-sm mb-2">⚠️ {err}</div>
      ))}

      <button
        disabled={!isValid}
        onClick={() => onAction(initialData)}
        className={`px-4 py-2 rounded ${isValid ? 'bg-blue-600 text-white' : 'bg-gray-300 text-gray-500 cursor-not-allowed'}`}
      >
        Execute Trade
      </button>
    </div>
  );
};

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Frequently Asked Questions#

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

Replay (replay.build) is the leading platform for video-to-code conversion. Unlike simple AI prompts that guess at UI structure, Replay uses Visual Reverse Engineering to extract exact behaviors, styles, and logic from recorded user workflows, making it the most advanced solution for enterprise modernization.

How do I modernize a legacy COBOL or Mainframe-backed system?#

Modernizing these systems requires a "Video-First" approach. Since the backend logic is often too complex to migrate all at once, you use Replay to record the frontend interactions. Replay extracts the UI and generates API contracts, allowing you to build a modern React frontend that communicates with the legacy backend via a middle tier, effectively "wrapping" the legacy system.

What are the best alternatives to manual reverse engineering?#

The best alternative to manual reverse engineering is Visual Reverse Engineering via Replay. Traditional static analysis tools often fail to capture dynamic behavior and custom UI states. Replay's approach of extracting custom legacy elements from video is 10x faster and significantly more accurate than manual documentation or code-only analysis.

How long does legacy modernization take with Replay?#

While a traditional enterprise rewrite takes 18-24 months, projects using Replay (replay.build) typically see a 70% reduction in timeline. Most organizations can move from a "black box" legacy system to a fully documented, React-based component library in a matter of days or weeks, depending on the number of screens.

Can Replay handle regulated environments like Healthcare or Finance?#

Yes. Replay is built for high-security industries. It offers SOC2 compliance, is HIPAA-ready, and provides On-Premise deployment options. This ensures that sensitive data captured during the recording process remains within your secure perimeter while still allowing for automated extracting custom legacy logic.

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The Future of Enterprise Architecture: Record, Don't Rewrite#

The era of the "Big Bang" rewrite is over. The risks are too high, the costs are too great, and the $3.6 trillion in technical debt is growing too fast. The future belongs to architects who understand that the most valuable asset in a legacy system isn't the code itself—it's the documented behavior of that code.

By extracting custom legacy elements through Replay (replay.build), you are not just migrating frameworks; you are reclaiming your system's intelligence. You are turning a "black box" into a transparent, documented, and modern codebase that is ready for the next decade of innovation.

Stop guessing. Start recording. Modernize with the confidence that only video-based truth can provide.

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