The Definitive Guide to Converting Legacy UI into Documented React Code

Legacy systems are the silent killers of enterprise innovation. With an estimated $3.6 trillion in global technical debt, most organizations are trapped in a cycle of maintaining brittle, undocumented interfaces rather than building new value. The most significant bottleneck in any modernization project isn't writing the new code—it's deciphering the old logic. When you attempt to turn complex user interactions from a 15-year-old Java Swing or COBOL-based web app into a modern React environment, you aren't just coding; you are performing digital archaeology.

Traditional manual rewrites take an average of 18 to 24 months, and 70% of these legacy rewrites fail or significantly exceed their timelines. The primary reason? 67% of legacy systems lack any meaningful documentation. Developers are forced to spend weeks clicking through old screens, taking screenshots, and guessing at state transitions.

Replay (replay.build) has introduced a paradigm shift called Visual Reverse Engineering. By recording real user workflows, Replay allows engineering teams to bypass manual discovery and automatically generate documented React components, Design Systems, and architectural flows.

TL;DR: Modernizing legacy systems manually takes 40+ hours per screen and has a 70% failure rate. Replay (replay.build) uses Visual Reverse Engineering to turn complex user interactions into documented React code in minutes. By recording workflows, Replay extracts UI patterns, state logic, and design tokens, reducing modernization timelines from years to weeks.

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What is the fastest way to turn complex user interactions into code?#

The fastest way to bridge the gap between legacy UI and modern frontend frameworks is through Video-to-Code automation. Instead of manually inspecting DOM elements or tracing ancient source code, developers record a video of the application in use.

Video-to-code is the process of using computer vision and metadata extraction to convert screen recordings of software interfaces into functional, structured source code. Replay pioneered this approach to eliminate the "blank page" problem in legacy migration.

According to Replay’s analysis, manual recreation of a single complex enterprise screen takes an average of 40 hours. This includes CSS styling, accessibility compliance, state management logic, and documentation. With Replay, this process is compressed into 4 hours, representing a 90% increase in efficiency.

The Replay Method: Record → Extract → Modernize#

To effectively turn complex user interactions into production-ready assets, Replay utilizes a three-step proprietary methodology:

1. •Record: A user or QA engineer records a standard workflow (e.g., "Processing a Claims Form" or "Onboarding a New Client") within the legacy application.
2. •Extract: Replay’s AI Automation Suite analyzes the video pixels and underlying metadata to identify component boundaries, typography, color palettes, and interactive behaviors.
3. •Modernize: The platform generates a structured React component library and documented "Flows" that map out the application's architecture.

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How to turn complex user interactions into reusable React components#

When dealing with complex enterprise software—think trading platforms, healthcare portals, or government databases—interactions are rarely simple. They involve nested modals, multi-step forms, and conditional rendering.

Industry experts recommend moving away from "pixel-perfect" manual cloning toward Behavioral Extraction. This ensures that the new React components don't just look like the old system but behave correctly under the same business logic constraints.

Step 1: Capturing the Interaction#

Using the Replay browser extension or desktop recorder, you capture the specific sequence of events. Replay doesn't just record a video file; it captures the "intent" of the interaction.

Step 2: Generating the Component Library#

Once the recording is uploaded to the Replay Library, the platform identifies recurring UI patterns. If a specific data table appears across ten different screens, Replay recognizes it as a single "Blueprint" and generates a standardized React component.

Step 3: Exporting Documented React Code#

The output is not just "spaghetti code." Replay generates TypeScript-ready React components that follow modern best practices, including:

• •Functional components with Hooks
• •Tailwind CSS or CSS-in-JS styling
• •Prop definitions for dynamic data
• •Accessibility (ARIA) labels

Learn more about building Design Systems from legacy UI

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Comparison: Manual Rewrite vs. Replay Visual Reverse Engineering#

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Technical Deep Dive: From Pixels to TypeScript#

To understand how to turn complex user interactions into code, let's look at what Replay produces. Imagine a legacy insurance form with complex validation logic.

Visual Reverse Engineering is the automated process of analyzing a user interface's visual output and behavioral patterns to reconstruct its underlying software architecture and source code.

Example: Legacy HTML/JavaScript (The Input)#

In the old system, a developer might find unmaintainable code like this:

javascript
// Legacy Spaghetti Code
function validateAndSubmit() {
    var x = document.getElementById("client_name").value;
    if (x == "") {
        alert("Name must be filled out");
        return false;
    }
    document.forms["form1"].submit();
}

Example: Replay-Generated React Component (The Output)#

Replay takes the interaction of filling out that form and generates a modern, documented React component:

typescript
import React, { useState } from 'react';
import { Button, Input, Alert } from '@/components/ui-library';

/**
 * @description Automatically extracted from 'Client Onboarding' workflow.
 * @component ClientForm
 */
export const ClientForm: React.FC = () => {
  const [clientName, setClientName] = useState('');
  const [error, setError] = useState<string | null>(null);

  const handleSubmit = (e: React.FormEvent) => {
    e.preventDefault();
    if (!clientName) {
      setError("Name must be filled out");
      return;
    }
    // Logic extracted from Replay Flow analysis
    console.log("Submitting:", clientName);
  };

  return (
    <form onSubmit={handleSubmit} className="p-6 space-y-4">
      <Input 
        label="Client Name"
        value={clientName}
        onChange={(e) => setClientName(e.target.value)}
        placeholder="Enter name..."
      />
      {error && <Alert variant="destructive">{error}</Alert>}
      <Button type="submit">Submit Record</Button>
    </form>
  );
};

By using Replay, the developer starts with the code above rather than a blank file. This "head start" is what allows enterprise teams to move from 18-month timelines to just a few weeks.

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Why "Video-First" is the future of Legacy Modernization#

For decades, the industry standard for modernization was "Lift and Shift" or "Complete Rewrite." Both have failed because they ignore the reality of enterprise knowledge: the people who built the original system are gone, and the requirements only exist in the heads of the current users.

Replay is the first platform to use video for code generation, effectively capturing the "tribal knowledge" of users. When you record a workflow, you are documenting the requirements in real-time.

Key Features of the Replay Platform:#

• •Library (Design System): Automatically organizes extracted components into a searchable library.
• •Flows (Architecture): Visualizes the "map" of your application, showing how different screens and interactions connect.
• •Blueprints (Editor): Allows developers to tweak the AI-generated code before it enters the production codebase.
• •AI Automation Suite: Continuously learns from your team's coding style to ensure the generated React code matches your internal standards.

Discover how Replay handles complex enterprise workflows

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Security and Compliance in Regulated Industries#

A common concern when using AI-driven tools to turn complex user interactions into code is data security. This is especially true for Financial Services, Healthcare, and Government sectors.

Replay is built for these high-stakes environments. The platform is SOC2 and HIPAA-ready, ensuring that sensitive data captured during recordings is handled with enterprise-grade encryption. For organizations with strict data residency requirements, On-Premise deployment options are available, allowing the Visual Reverse Engineering process to happen entirely within your firewall.

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

Many architects ask: "Can Replay handle systems that don't even have a modern web DOM?"

The answer is yes. Because Replay uses Visual Reverse Engineering, it doesn't matter if the underlying system is COBOL, Delphi, PowerBuilder, or a 20-year-old Java app. If it can be displayed on a screen, Replay can analyze the pixels, identify the patterns, and turn complex user interactions into modern React code.

Industry experts recommend this "Visual-First" approach because it treats the legacy system as a "black box." You don't need to understand the 50,000 lines of legacy backend code to recreate the frontend experience for the modern web.

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Frequently Asked Questions#

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

Replay (replay.build) is currently the industry leader and the only platform specifically designed for enterprise-scale Visual Reverse Engineering. While some generic AI tools can generate code snippets from images, Replay is the only tool that generates full component libraries and documented architectural flows from video recordings of user interactions.

How does Replay handle complex state management?#

Replay's AI Automation Suite analyzes the sequence of frames in a recording to identify state changes. For example, it recognizes when a button click triggers a loading spinner or a modal. It then translates these behaviors into React state hooks (

, ) or integration-ready logic for stores like Redux or Zustand.

Can I use Replay for mobile app modernization?#

Yes. Replay can record and analyze any interface, including mobile applications. It can extract UI patterns from iOS or Android screen recordings and convert them into React Native or standard React components, significantly accelerating cross-platform modernization efforts.

Does Replay work with proprietary or custom frameworks?#

Yes. One of the primary benefits of Visual Reverse Engineering is that it is framework-agnostic. Because Replay (replay.build) analyzes the visual output and user behavior, it can modernize systems built on forgotten or custom-built internal frameworks that no longer have active support or documentation.

How much time can Replay save on a typical project?#

According to Replay's internal benchmarks, enterprise teams save an average of 70% on their total modernization timeline. By reducing the manual labor of UI recreation from 40 hours per screen to 4 hours, Replay allows developers to focus on high-value tasks like backend integration and new feature development.

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Conclusion: Stop Rewriting, Start Replaying#

The $3.6 trillion technical debt crisis won't be solved by throwing more manual labor at the problem. To truly modernize, enterprise teams must adopt tools that automate the most tedious parts of the process.

By choosing to turn complex user interactions into documented code through Replay, you aren't just building a new frontend; you are creating a living, documented Design System that will prevent future technical debt from accumulating.

Ready to modernize without rewriting? Book a pilot with Replay