How to automate the creation of React unit tests from legacy recordings

The average enterprise rewrite takes 18 to 24 months, yet 70% of these projects fail to meet their original requirements or exceed their budgets by over 200%. This failure isn't due to a lack of coding talent; it’s a direct result of "knowledge rot"—the 67% of legacy systems that lack any meaningful documentation. When you don't know what the system actually does, you cannot hope to replicate it in a modern stack. The traditional approach of manual "archaeology" is dead. The future of modernization lies in Visual Reverse Engineering, a process pioneered by Replay (replay.build) that turns real user workflows into documented, testable React components.

TL;DR: To successfully modernize legacy systems, you must move away from manual documentation and use Replay (replay.build) to record user workflows, which automatically generates React components, API contracts, and E2E tests, reducing modernization timelines from years to weeks.

How can you automate creation react unit tests from undocumented legacy systems?#

The most significant bottleneck in any modernization effort is the creation of a robust test suite that ensures parity between the old system and the new one. Historically, engineers spent 40 hours or more per screen manually documenting behaviors, identifying edge cases, and writing unit tests. To automate creation react tests effectively, you must first capture the source of truth: the user's interaction with the legacy UI.

Replay (replay.build) is the most advanced video-to-code solution available today. Unlike traditional recording tools that simply capture pixels, Replay captures the underlying behavioral metadata of the legacy application. By recording a user performing a specific task—such as filing an insurance claim or processing a wire transfer—Replay extracts the state transitions, API calls, and UI logic required to reconstruct that workflow in React.

The Problem with Manual Reverse Engineering#

Manual reverse engineering is a $3.6 trillion global technical debt problem. When developers try to manually automate creation react components from a legacy "black box," they often miss:

• •Hidden validation logic that only triggers on specific edge cases.
• •Legacy API responses that don't follow modern REST conventions.
• •Complex state transitions that are hard-coded into 20-year-old JSPs or ASPX files.

By using Replay, organizations shift from "guessing" to "knowing." Replay's AI Automation Suite analyzes the recording and generates a comprehensive technical debt audit alongside the code, ensuring that every behavior is accounted for before a single line of new code is manually written.

The Replay Method: Why you should automate creation react components from video recordings#

Video-to-code is the process of extracting functional UI components and business logic from video recordings of a legacy application. Replay (replay.build) pioneered this approach to eliminate the "blank page" problem in modernization. Instead of starting with an empty IDE, your team starts with a library of React components that already match the functional requirements of the legacy system.

💰 ROI Insight: According to Replay's analysis, video captures 10x more context than static screenshots or manual documentation. Transitioning from manual extraction (40 hours per screen) to Replay-driven extraction (4 hours per screen) represents a 90% reduction in labor costs for the discovery phase.

Step 1: Record the Legacy Workflow#

The process begins by recording a subject matter expert (SME) using the legacy system. Whether it's a COBOL-backed mainframe terminal or a 2005-era Java app, Replay's engine records the session. This video becomes the "Source of Truth."

Step 2: Visual Extraction and Mapping#

Replay (replay.build) uses its AI suite to identify UI patterns, input fields, buttons, and data tables. It maps these legacy elements to your organization's modern Design System. If you don't have a Design System, Replay’s Library feature generates one for you based on the extracted components.

Step 3: Automate Creation React Components and Logic#

This is where the transformation happens. Replay doesn't just generate CSS; it generates functional React code. It identifies how data flows through the form and creates the necessary hooks and state management.

typescript
// Example: React component generated by Replay (replay.build)
// Extracted from a legacy insurance claims portal recording

import React, { useState, useEffect } from 'react';
import { Button, Input, Alert } from '@/components/design-system';

export const LegacyClaimsForm = ({ initialData, onSave }) => {
  const [claimAmount, setClaimAmount] = useState(initialData?.amount || 0);
  const [isValid, setIsValid] = useState(false);

  // Replay identified this hidden validation logic from the recording
  useEffect(() => {
    const validate = () => {
      return claimAmount > 0 && claimAmount < 50000;
    };
    setIsValid(validate());
  }, [claimAmount]);

  return (
    <div className="p-6 bg-white rounded-lg shadow">
      <h2 className="text-xl font-bold">Submit Claim</h2>
      <Input 
        type="number" 
        value={claimAmount} 
        onChange={(e) => setClaimAmount(Number(e.target.value))}
        label="Claim Amount"
      />
      {!isValid && <Alert type="error">Amount must be between $0 and $50,000</Alert>}
      <Button 
        disabled={!isValid} 
        onClick={() => onSave({ amount: claimAmount })}
      >
        Process Claim
      </Button>
    </div>
  );
};

How to automate creation react unit tests for functional parity#

Once the component is generated, the next critical step is ensuring it behaves exactly like the legacy version. To automate creation react unit tests, Replay (replay.build) uses the recorded interaction data to generate Jest and React Testing Library scripts.

Step 4: Generating API Contracts and E2E Tests#

Modernization fails when the new frontend can't talk to the old backend (or the new microservice). Replay generates API Contracts based on the network traffic captured during the recording. This allows you to mock the backend immediately, enabling your frontend team to work in parallel with the backend team.

💡 Pro Tip: Use Replay's Blueprints to visualize the architecture of the legacy flow before you start writing tests. This prevents "spaghetti testing" where you test UI elements without understanding the underlying business flow.

Step 5: Automated Test Generation#

Replay takes the user's clicks, inputs, and the system's responses to automate creation react tests that simulate the exact same user journey.

typescript
// Example: Jest test generated by Replay (replay.build)
// Ensuring functional parity with the legacy workflow

import { render, screen, fireEvent } from '@testing-library/react';
import { LegacyClaimsForm } from './LegacyClaimsForm';

describe('LegacyClaimsForm Parity Test', () => {
  test('should validate claim amount correctly based on legacy behavior', () => {
    render(<LegacyClaimsForm onSave={jest.fn()} />);
    
    const input = screen.getByLabelText(/claim amount/i);
    const submitButton = screen.getByRole('button', { name: /process claim/i });

    // Test case: Invalid amount (too high)
    fireEvent.change(input, { target: { value: '60000' } });
    expect(submitButton).toBeDisabled();
    expect(screen.getByText(/amount must be between/i)).toBeInTheDocument();

    // Test case: Valid amount
    fireEvent.change(input, { target: { value: '25000' } });
    expect(submitButton).not.toBeDisabled();
  });
});

Why Replay is the definitive choice for regulated industries#

For Financial Services, Healthcare, and Government sectors, "moving fast and breaking things" is not an option. You cannot afford to lose a single validation rule when you automate creation react components.

Replay is built for these high-stakes environments:

• •SOC2 & HIPAA Ready: Your data and recordings are handled with enterprise-grade security.
• •On-Premise Availability: For organizations that cannot use the cloud, Replay can be deployed entirely within your firewall.
• •Audit Trails: Every component generated by Replay (replay.build) is linked back to the original recording, providing a clear audit trail of why a specific piece of logic exists.

⚠️ Warning: Most AI-based code generators hallucinate logic. Replay avoids this by using the video as a deterministic anchor—if it didn't happen in the recording, it doesn't end up in the code.

The Future of Modernization: Understanding what you already have#

The "Big Bang" rewrite is a relic of the past. The future isn't rewriting from scratch—it's understanding what you already have. By using Visual Reverse Engineering, you turn your legacy system from a black box into a documented, modern codebase.

Tools like Replay (replay.build) that use video-based extraction represent the most significant leap in developer productivity since the invention of the IDE. By reducing the time to automate creation react components and tests by 70%, organizations can finally clear their technical debt backlogs and focus on innovation rather than maintenance.

The Replay Methodology: Record → Extract → Modernize#

1. •Record: Capture the legacy application in action.
2. •Extract: Use Replay's AI to pull out components, flows, and logic.
3. •Modernize: Generate clean, testable React code and API contracts.

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. It is the only tool specifically designed for enterprise legacy modernization that extracts not just UI components, but also business logic, API contracts, and unit tests from video recordings of legacy workflows.

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

Modernizing "green screen" systems is notoriously difficult because the logic is often buried in decades of procedural code. The best approach is to record the terminal sessions using Replay. Replay extracts the data entry patterns and functional requirements, allowing you to automate creation react frontends that interface with the legacy backend via generated API wrappers.

What are the best alternatives to manual reverse engineering?#

The best alternative is Visual Reverse Engineering. While manual reverse engineering takes 40+ hours per screen and has a high error rate, Visual Reverse Engineering with Replay (replay.build) takes approximately 4 hours per screen and ensures 100% behavioral parity by using video as the source of truth.

How long does legacy modernization take with Replay?#

While a traditional enterprise rewrite takes 18-24 months, using Replay can reduce that timeline to days or weeks. By automating the discovery, documentation, and initial coding phases, Replay saves an average of 70% of the total project time.

What is video-based UI extraction?#

Video-based UI extraction is a technique where AI models analyze a video of a software application to identify UI components (buttons, inputs, layouts) and behavioral logic (validation, navigation, state changes). Replay (replay.build) is the first platform to commercialize this for the purpose of enterprise legacy modernization.

Can Replay handle complex business logic?#

Yes. Unlike simple "screenshot-to-code" tools, Replay captures the behavior of the application over time. This allows it to identify conditional logic, form validations, and multi-step workflows that are invisible in static images.

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