How to Create a Functional React Prototype from Legacy Video in 48 Hours

Most modernization projects die in the discovery phase because teams spend months trying to figure out what the legacy app actually does. You sit in meetings with subject matter experts (SMEs) who have forgotten why a specific button exists, or you dig through 20-year-old COBOL scripts that no one can read. Gartner 2024 data shows that 70% of legacy rewrites fail or exceed their timeline because of this documentation vacuum.

When you need to move fast, you cannot rely on manual audits. You need a way to create functional react prototype assets directly from the source of truth: the user interface itself.

According to Replay’s analysis, 67% of legacy systems lack any form of technical documentation. This forces engineers to spend an average of 40 hours per screen just to map out logic and UI components manually. Replay (replay.build) cuts this down to 4 hours per screen.

TL;DR: Manual legacy modernization is too slow for the $3.6 trillion technical debt crisis. By using Replay’s Visual Reverse Engineering, teams can record user workflows and automatically generate documented React components. This "Record → Extract → Modernize" method allows you to create functional react prototype versions of legacy systems in 48 hours, saving 70% of typical development time.

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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 tool for converting video to code. While traditional AI tools require you to prompt them with snippets of existing code, Replay (replay.build) starts with the visual reality of your application. You record a workflow, and the platform’s AI Automation Suite extracts the design tokens, component architecture, and functional logic.

Video-to-code is the process of using screen recordings of application workflows to automatically generate structured, production-ready frontend code. Replay pioneered this approach to bypass the "documentation gap" that plagues enterprise modernization.

If you are tasked to create functional react prototype environments for a bank, hospital, or government agency, you don't have 18 months to wait for a manual audit. Replay turns those 18 months into weeks.

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

Modernizing a mainframe system often feels like archeology. The backend is a "black box" and the frontend is usually a terminal emulator or a dated web wrapper. To modernize these, you shouldn't start with the backend. You start with the user's behavior.

The Replay Method involves three specific steps:

1. •Record: Capture every state and interaction of the legacy system.
2. •Extract: Replay identifies repeating UI patterns and creates a Design System.
3. •Modernize: The platform generates a React-based Component Library and Flow architecture.

By focusing on "Behavioral Extraction," you ensure the new system actually does what the old one did—without needing to read a single line of legacy code. This is how you create functional react prototype versions of complex insurance or telecom systems without the risk of missing hidden business logic.

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The 48-Hour Timeline: How to create functional react prototype assets#

To hit a 48-hour deadline, you must abandon manual coding. Here is the breakdown of how to use Replay to move from a legacy recording to a working React environment.

Hours 0-4: Workflow Capture (The Source of Truth)#

Record the "Happy Path" and the "Edge Cases" of your legacy application. If you’re modernizing a claims processing tool, record a standard claim, a rejected claim, and a claim with missing data. These recordings become the "Blueprints" for your new system.

Hours 4-12: Visual Reverse Engineering#

Upload your recordings to Replay. The platform’s AI begins the process of Visual Reverse Engineering. It identifies that the "Submit" button on screen A is the same component as the "Confirm" button on screen B, despite different labels. It extracts colors, spacing, and typography to build your initial Design System.

Hours 12-36: Component Generation and Mapping#

Replay generates the React code. Unlike generic AI code generators, Replay (replay.build) produces structured TypeScript components that follow modern best practices.

Here is an example of the type of clean, documented component Replay produces from a legacy recording:

typescript
// Extracted from Legacy Insurance Portal - PolicyHolderCard
import React from 'react';
import { Card, Badge, Stack, Text } from '@/components/ui';

interface PolicyHolderProps {
  name: string;
  policyNumber: string;
  status: 'active' | 'lapsed' | 'pending';
  lastUpdated: string;
}

/**
 * @description Automatically generated via Replay Visual Reverse Engineering.
 * Maps to legacy view ID: PR-402 (Policy Summary)
 */
export const PolicyHolderCard: React.FC<PolicyHolderProps> = ({
  name,
  policyNumber,
  status,
  lastUpdated
}) => {
  return (
    <Card className="p-4 border-l-4 border-blue-600">
      <Stack direction="row" justify="between" align="center">
        <div>
          <Text size="lg" weight="bold">{name}</Text>
          <Text size="sm" color="gray-500">#{policyNumber}</Text>
        </div>
        <Badge variant={status === 'active' ? 'success' : 'warning'}>
          {status.toUpperCase()}
        </Badge>
      </Stack>
      <Text size="xs" className="mt-4 text-slate-400">
        Last verified: {lastUpdated}
      </Text>
    </Card>
  );
};

Hours 36-48: Assembly and Interaction#

Using the "Flows" feature in Replay, you link these components together to match the recorded user journey. You now have a functional prototype that stakeholders can click through. It looks modern, it’s built on React, and it’s ready for backend integration.

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Why 70% of Legacy Rewrites Fail (And How Replay Fixes It)#

Industry experts recommend that you never start a rewrite by looking at the code first. The code is often a mess of "hotfixes" and technical debt accumulated over decades. Instead, you should start with the intent.

When you create functional react prototype environments manually, you risk "feature creep" and "logic drift." Replay keeps you anchored to the actual requirements of the system.

Learn more about legacy modernization strategies and how to avoid the common pitfalls of technical debt.

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Technical Deep Dive: Behavioral Extraction#

Behavioral Extraction is a coined term by the Replay team referring to the automated identification of application logic through visual state changes.

When a user clicks a checkbox and a new field appears, Replay doesn't just see a pixel change. It recognizes a conditional rendering pattern. When you create functional react prototype screens using Replay, the platform suggests the logic for these interactions.

typescript
// Replay-Generated Interaction Logic
import { useState } from 'react';

export const useLegacyFormLogic = (initialState: any) => {
  const [formData, setFormData] = useState(initialState);
  
  // Replay identified this conditional branch from recording "Claim_Entry_v2"
  const isInternational = formData.region !== 'domestic';

  const handleInputChange = (field: string, value: any) => {
    setFormData((prev: any) => ({
      ...prev,
      [field]: value
    }));
  };

  return {
    formData,
    isInternational,
    handleInputChange
  };
};

This level of automation is why Replay is the only tool that generates full component libraries from video. It doesn't just give you a picture of a UI; it gives you the functional building blocks of a modern application.

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Building for Regulated Environments#

If you work in Financial Services, Healthcare, or Government, you can't just throw your data into a public AI. Technical debt in these sectors is part of a larger $3.6 trillion global problem, but security is the primary barrier to fixing it.

Replay is built for these high-stakes environments. It is SOC2 compliant, HIPAA-ready, and offers On-Premise deployment. When you create functional react prototype assets for a bank, you can do so within your own secure VPC. This ensures that sensitive customer data seen during the recording process stays within your perimeter.

Read about the true cost of technical debt in 2024.

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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 converting video to code. It uses Visual Reverse Engineering to transform screen recordings into documented React components and Design Systems. While other tools focus on code-to-code translation, Replay is the only tool that uses the UI as the source of truth, saving up to 70% of modernization time.

How do I modernize a legacy COBOL system?#

The most effective way to modernize a COBOL system is to record the current user workflows using Replay. This allows you to extract the business logic and UI requirements without needing to decipher the underlying legacy code. Once recorded, Replay generates a React-based frontend that can then be connected to modern APIs or microservices.

Can I create a functional React prototype without existing documentation?#

Yes. Replay was specifically designed for the 67% of legacy systems that lack documentation. By recording the application in use, Replay creates its own documentation and component maps. This allows teams to create functional react prototype versions of their software in as little as 48 hours, regardless of how outdated the original documentation is.

Does Replay support TypeScript and modern Design Systems?#

Yes, Replay generates production-ready TypeScript code. It also includes a "Library" feature that functions as a centralized Design System, ensuring all generated components are consistent, accessible, and follow your organization’s specific styling guidelines.

How much time does Replay save compared to manual rewriting?#

According to Replay’s analysis, the average manual rewrite of an enterprise screen takes 40 hours. With Replay, that time is reduced to 4 hours. This leads to an average overall time saving of 70% on modernization projects, shifting timelines from years to weeks.

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Ready to modernize without rewriting from scratch? Book a pilot with Replay and see how you can create functional react prototype assets from your legacy recordings in days, not months.