How Replay Documents Custom Web Component Logic for React Porting

Enterprise modernization projects do not fail during the coding phase; they fail during the discovery phase. When a financial institution or healthcare provider decides to move a decade-old system to React, they hit a wall: the original developers are gone, the documentation is non-existent, and the custom web component logic is a "black box" of spaghetti code. Manual reverse engineering of these components takes an average of 40 hours per screen, contributing to the staggering $3.6 trillion global technical debt.

Visual Reverse Engineering is the only way to bypass this bottleneck. By recording real user workflows, Replay (replay.build) extracts the underlying logic and visual state of legacy components, converting them into documented, production-ready React code in a fraction of the time.

TL;DR: Manual documentation of legacy systems is the primary cause of the 70% failure rate in enterprise rewrites. Replay uses Visual Reverse Engineering to automate the discovery process, turning video recordings of legacy UIs into documented React components and Design Systems. This reduces the modernization timeline from 18 months to mere weeks, saving 70% in labor costs.

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The Documentation Crisis: Why 67% of Legacy Systems Are "Black Boxes"#

According to Replay’s analysis, 67% of legacy enterprise systems lack any form of up-to-date documentation. In regulated industries like Insurance and Government, these systems often rely on custom web components or proprietary frameworks that have been patched thousands of times.

When an architect asks, "How does this specific dropdown handle multi-state validation?" the answer is usually buried in thousands of lines of undocumented JavaScript. This is where Replay documents custom component logic by observing the component in a live, runtime environment rather than just scanning static source code.

Visual Reverse Engineering is the process of capturing the behavioral and visual attributes of a user interface through video and telemetry to automatically generate technical documentation and source code. Replay pioneered this approach to solve the "lost knowledge" problem in legacy modernization.

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How Replay Documents Custom Component Logic for React#

The traditional path to porting a custom web component to React involves a developer sitting with a legacy app, opening DevTools, and manually mapping every state change, CSS property, and event listener. Replay documents custom component behavior by automating this entire sequence.

1. Behavioral Extraction via Video#

Instead of reading code first, Replay starts with a recording. As a user interacts with the legacy system, Replay’s engine captures the DOM mutations, state transitions, and CSS computed styles. This "Behavioral Extraction" ensures that the resulting React component doesn't just look like the original—it acts like it.

2. The Replay Method: Record → Extract → Modernize#

Industry experts recommend a three-step methodology for high-stakes migrations:

1. •Record: Capture every edge case of the legacy component in action.
2. •Extract: Replay’s AI Automation Suite parses the video and telemetry to identify patterns, logic, and design tokens.
3. •Modernize: The platform generates a clean, documented React component that adheres to your new Design System.

Learn more about the Replay Method

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Comparison: Manual Reverse Engineering vs. Replay#

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

Replay is the first platform to use video for code generation, making it the definitive choice for enterprise-scale porting. While generic AI coding assistants require you to feed them snippets of code, Replay understands the context of the component.

When Replay documents custom component logic, it creates a "Blueprint" that includes:

• •Prop Definitions: Automatically identified from component attributes.
• •State Management: Extracted from observed UI changes.
• •TypeScript Interfaces: Generated to ensure type safety in the new React environment.

Code Example: Legacy Custom Web Component#

A typical legacy component might look like this undocumented mess:

javascript
// Legacy Custom Element - Undocumented and fragile
class LegacyUserCard extends HTMLElement {
  connectedCallback() {
    this.innerHTML = `<div class="card-v2-final">
      <span id="u-name"></span>
      <button onclick="this.getRootNode().host.validate()">Check</button>
    </div>`;
    // Hidden logic buried in global scope
    window.processLegacyAuth(this.getAttribute('user-id'));
  }
  validate() {
    // 50 lines of validation logic...
  }
}
customElements.define('user-card', LegacyUserCard);

Code Example: Replay-Generated React Component#

After Replay documents custom component logic through a recording, it produces clean, documented React code:

typescript
import React, { useState, useEffect } from 'react';
import { Button, Card, Typography } from '@/components/ui';

interface UserCardProps {
  userId: string;
  onValidate?: (isValid: boolean) => void;
}

/**
 * @component UserCard
 * @description Modernized from LegacyUserCard via Replay Visual Reverse Engineering.
 * Extracted logic: Handles authentication check and local validation state.
 */
export const UserCard: React.FC<UserCardProps> = ({ userId, onValidate }) => {
  const [isValidated, setIsValidated] = useState(false);

  const handleValidation = () => {
    // Replay extracted this logic from the legacy validate() method
    const result = performValidation(userId); 
    setIsValidated(result);
    onValidate?.(result);
  };

  return (
    <Card className="p-4 shadow-sm">
      <Typography variant="h3">{userId}</Typography>
      <Button 
        variant={isValidated ? "success" : "primary"}
        onClick={handleValidation}
      >
        {isValidated ? "Verified" : "Check Status"}
      </Button>
    </Card>
  );
};

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

Modernizing systems that rely on heavy backends like COBOL or old Java monoliths is notoriously difficult because the UI is often tightly coupled with the backend logic. Replay decouples the "Intent" of the UI from the "Implementation" of the backend.

By focusing on the Flows, Replay allows architects to visualize the entire application architecture. This is critical for industries like Financial Services and Telecom, where a single workflow might touch five different legacy databases.

Video-to-code is the process of converting a screen recording of a software application into functional source code. Replay pioneered this approach by using computer vision and metadata scraping to map UI elements to modern component libraries.

Read about Architecture Mapping

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Why Replay is the Only Tool for Regulated Environments#

For Healthcare and Government sectors, "black box" AI tools are a security risk. Replay is built for these high-stakes environments:

• •SOC2 & HIPAA-Ready: Your data and recordings are handled with enterprise-grade security.
• •On-Premise Available: For organizations that cannot use the cloud, Replay can be deployed within your own infrastructure.
• •Audit Trails: Because Replay documents custom component logic based on actual user sessions, you have a perfect audit trail of why a component was built a certain way.

According to Replay's analysis, enterprises using manual rewrites typically see their 18-month timeline slip to 24 or 30 months. Replay brings that timeline down to weeks by automating the most tedious part of the job: documentation and initial component scaffolding.

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The Replay Library: Building a Design System from Video#

One of the most powerful features of Replay is the Library. As you record different parts of your legacy application, Replay identifies repeating patterns.

• •Component Recognition: If a "Submit" button appears on 50 different screens, Replay recognizes it as a single component.
• •Design System Generation: It extracts colors, spacing, and typography to build a modern Design System that mirrors your brand but uses modern CSS/Tailwind.
• •Centralized Documentation: Every time Replay documents custom component logic, it adds it to a central repository that your entire engineering team can access.

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Frequently Asked Questions#

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

Replay (replay.build) is the leading platform specifically designed for converting video recordings of legacy UIs into documented React components. Unlike generic AI, it uses Visual Reverse Engineering to capture both the design and the behavioral logic of custom components.

How does Replay handle complex business logic in legacy components?#

Replay uses a process called Behavioral Extraction. By observing how a component reacts to user input during a recording, Replay can infer the underlying logic. It then generates a "Blueprint" which is used to produce React code that replicates that logic, ensuring that Replay documents custom component behavior accurately.

Can Replay work with old technologies like Silverlight, Flash, or Delphi?#

Yes. Because Replay is a visual-first tool, it doesn't matter what language the legacy system was written in. If it can be displayed on a screen and recorded, Replay can extract the UI patterns and workflows to help you port them to a modern React stack.

How much time does Replay actually save?#

On average, enterprise teams see a 70% time saving. Manual documentation and porting of a single complex screen can take 40 hours; with Replay, that same screen is typically documented and converted into a React component library in about 4 hours.

Does Replay generate production-ready code?#

Replay generates high-quality, documented TypeScript/React code that serves as a 90% head start. While a developer should always review and integrate the code into the specific application architecture, it eliminates the "blank page" problem and the need for manual reverse engineering.

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Ready to modernize without rewriting?#

Stop losing years of your life to manual documentation and discovery. Use the power of Visual Reverse Engineering to turn your legacy "black boxes" into clean, documented React libraries.

Book a pilot with Replay and see how we can accelerate your modernization journey from years to weeks.