Beyond the Green Screen: Generating Clean React Code from Recorded Mainframe Sessions

Legacy systems are often described as "black boxes," but for the engineers tasked with modernizing them, they feel more like concrete bunkers. You know the business logic is in there, buried under decades of COBOL and CICS transactions, but extracting it into a modern web architecture usually requires a multi-year migration project that carries a 70% failure rate. The "green screen" terminal—the ubiquitous 3270 or 5250 interface—remains the primary barrier to digital transformation.

Traditional modernization involves screen scraping or manual rewrites. Screen scraping produces brittle, unmaintainable "spaghetti" code, while manual rewrites lose critical edge-case logic that hasn't been documented since 1988. Replay introduces a third way: Visual Reverse Engineering. By recording a live mainframe session, Replay’s engine analyzes the visual patterns, data flows, and user interactions to output production-ready, modular code.

If your goal is generating clean react code that looks like it was written by a senior frontend architect rather than a machine, you need to move beyond simple character-mapping and toward semantic visual analysis.

TL;DR: Modernizing Mainframes with Replay#

• •The Problem: Mainframe modernization is stalled by undocumented legacy logic and brittle screen-scraping tools.
• •The Solution: Replay records terminal sessions and uses computer vision + LLMs to map terminal fields to modern React components.
• •The Result: You get a documented Design System and a React frontend that mirrors legacy functionality without the legacy technical debt.
• •Key Benefit: Drastically reduces "Time to First Byte" for modernization projects by generating clean react code directly from UI recordings.

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The Modernization Gap: Why Manual Rewrites Fail#

For decades, the standard approach to mainframe modernization has been "The Great Rewrite." This involves hiring a team of analysts to watch users interact with terminal emulators, document every field, and then tasking developers with recreating those screens in React or Angular.

This approach fails for three specific reasons:

1. •Implicit Logic: Mainframe screens often hide complex validation logic that only triggers on specific key combinations (like
   text

   PA1
   or
   text

   SysReq
   ). If the analyst misses one interaction, the new app is broken on day one.
2. •Inconsistent UI: Over 40 years, different teams build different screens. Manually standardizing these into a cohesive Design System is a Herculean task.
3. •Code Quality: Even when the logic is captured, the resulting code is often a direct port of the mainframe’s procedural flow, leading to "God Components" that are impossible to test or maintain.

By focusing on generating clean react code through visual recording, Replay bypasses the documentation phase entirely. The recording is the documentation.

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The Technical Architecture of Generating Clean React Code#

How do you go from a monochromatic 80x24 grid of characters to a responsive, TypeScript-ready React component? It requires a multi-stage pipeline that treats the terminal recording as a series of semantic signals.

1. Visual Pattern Recognition#

Replay doesn't just look at the text; it looks at the structure. It identifies clusters of fields that represent a "Customer Profile" or an "Order Entry Table." By using computer vision, the system can distinguish between a header, a protected input field, and a status message.

2. State Inference#

Every interaction in a mainframe session—every

, , and —represents a state transition. Replay tracks these transitions to build a state machine. This state machine ensures that the generated React code handles navigation and data persistence correctly, utilizing modern hooks like or state management libraries like XState.

3. Semantic Mapping to Design Systems#

The "clean" in generating clean react code comes from the abstraction layer. Instead of generating a

for every terminal coordinate, Replay maps identified patterns to your organization’s Design System. A terminal input becomes a , and a list of items becomes a .

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Comparison: Modernization Methodologies#

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From Terminal Stream to TypeScript: A Practical Example#

To understand the value of generating clean react code, let's look at what the raw data looks like versus the output provided by Replay.

The Legacy Input (Conceptual 3270 Stream)#

In a traditional terminal, an "Account Summary" screen might be represented as a series of coordinates and attributes:

text
ROW 05, COL 10: "ACCOUNT NUMBER:" (PROTECTED, INTENSIFIED)
ROW 05, COL 30: "123456789"       (UNPROTECTED, NUMERIC)
ROW 07, COL 10: "BALANCE:"          (PROTECTED)
ROW 07, COL 30: "$1,250.00"        (PROTECTED, GREEN)

The Replay Output: Generating Clean React Code#

Replay processes this recording and generates a modular React component. Notice the use of TypeScript interfaces, functional components, and a structured Design System.

tsx
import React from 'react';
import { Card, Label, Input, Badge, Stack } from '@/components/ui';

interface AccountSummaryProps {
  accountNumber: string;
  balance: number;
  currency: string;
}

/**
 * AccountSummary Component
 * Generated via Replay Visual Reverse Engineering
 * Source: Mainframe Transaction ACCT-01
 */
export const AccountSummary: React.FC<AccountSummaryProps> = ({
  accountNumber,
  balance,
  currency = 'USD',
}) => {
  const isOverdrawn = balance < 0;

  return (
    <Card className="p-6 max-w-md">
      <Stack gap={4}>
        <div className="flex flex-col gap-1">
          <Label htmlFor="account-num">Account Number</Label>
          <Input 
            id="account-num" 
            value={accountNumber} 
            readOnly 
            className="font-mono"
          />
        </div>

        <div className="flex justify-between items-center">
          <span className="text-sm font-medium text-gray-500">Current Balance</span>
          <Badge variant={isOverdrawn ? 'destructive' : 'success'}>
            {new Intl.NumberFormat('en-US', { style: 'currency', currency }).format(balance)}
          </Badge>
        </div>
      </Stack>
    </Card>
  );
};

In this example, the process of generating clean react code transformed static coordinates into a functional, accessible, and themed component. The logic for the "Green" text in the terminal was intelligently mapped to a

component with a variant.

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Best Practices for Generating Clean React Code via Visual Reverse Engineering#

When using replay.build to modernize your legacy stack, following these best practices ensures that the output remains scalable and developer-friendly.

1. Define Your Component Library Early#

Replay works best when it knows the "target" language. By providing your existing React component library (or a standard like Shadcn/UI), the engine can map terminal elements to your specific UI patterns. This prevents the generation of "one-off" styled components and ensures consistency across the entire modernized application.

2. Capture Multiple User Flows#

A single recording of a terminal session only shows one path through the application. To succeed in generating clean react code that handles all edge cases, record multiple sessions:

• •The "Happy Path" (Standard transaction)
• •The "Error Path" (Invalid input, system timeouts)
• •The "Administrative Path" (Overriding defaults)

Replay aggregates these recordings to build a robust TypeScript interface that accounts for optional fields and varied state transitions.

3. Prioritize Semantic Naming#

Mainframe fields often have cryptic names like

. During the Replay ingestion process, you can provide a mapping layer that renames these to . This is a critical step in generating clean react code that is readable by human developers who have never seen a green screen.

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Bridging the Gap Between COBOL Logic and React State#

The hardest part of modernization isn't the UI—it's the state management. Mainframes are inherently stateful; the server knows exactly where your cursor is. Modern web apps are often stateless or manage state on the client.

Replay bridges this gap by generating a "Controller Layer" alongside the React components. This layer handles the communication between the new React frontend and the legacy mainframe backend (often via a TN3270 bridge or a REST API).

typescript
// Generated Service Layer for Account Operations
import { api } from '@/lib/api-client';

export const useAccountData = (accountId: string) => {
  // Replay infers the necessary mainframe screen navigation
  // to fetch this specific data set.
  const fetchAccount = async () => {
    const response = await api.post('/execute-transaction', {
      transId: 'ACCT01',
      inputs: { 'MAP-ACC-ID': accountId },
      action: 'ENTER'
    });
    
    return response.data;
  };

  // Standard React Query or S hooks can then be used
  // to manage this data in the clean React UI.
};

By generating clean react code that includes these service hooks, Replay ensures that the "plumbing" of the application is just as modern as the "porcelain."

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The Role of AI in Visual Reverse Engineering#

We are entering an era where AI can "understand" user interfaces. Replay leverages Large Language Models (LLMs) specifically fine-tuned on UI/UX patterns. When the system encounters an ambiguous terminal screen, the AI asks: "Based on the surrounding text 'Invoice Date' and 'Due Date', is this 8-character field a DatePicker or a plain text string?"

This contextual intelligence is what makes generating clean react code possible. It moves the technology from "OCR for terminals" to "Cognitive Modernization."

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Why Choose Replay for Your Modernization Journey?#

The traditional path to modernization is paved with broken budgets and abandoned repositories. Replay offers a visual-first approach that respects the complexity of legacy systems while embracing the elegance of modern React development.

When you focus on generating clean react code through visual recordings, you achieve:

• •Zero-Knowledge Onboarding: New developers don't need to learn 3270 protocols to update the UI.
• •Instant Documentation: The recording serves as a living spec for how the system actually works.
• •Design Consistency: Automatically apply your modern brand guidelines to 40-year-old workflows.

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FAQ: Generating Clean React Code from Legacy Systems#

Can Replay handle complex terminal grids and tables?#

Yes. Replay’s visual engine is specifically designed to identify repeating patterns in terminal streams. It can distinguish between header rows, data rows, and pagination controls (like "More..." or "Bottom"), converting them into responsive React DataTables with sorting and filtering capabilities.

Does the generated code include testing utilities?#

Absolutely. One of the hallmarks of generating clean react code is testability. Replay generates Jest or Vitest snapshots for components and Playwright scripts for the inferred user flows, ensuring that your modernized UI remains functional as the backend evolves.

How does Replay handle mainframe "Function Keys" (F1-F12)?#

In the generated React code, function keys are mapped to semantic actions. For example, if

is "Exit" in the terminal, Replay generates a button component or a keyboard shortcut listener that triggers a function. This preserves the "power user" workflow of legacy systems while providing a modern click-based interface for new users.

Is the code proprietary or locked into the Replay platform?#

No. The primary goal of replay.build is to accelerate your development. The output is standard TypeScript/React code that follows industry best practices. Once generated, the code is yours to modify, host, and maintain in your own version control systems (GitHub, GitLab, etc.).

How does this approach compare to low-code platforms?#

Traditional low-code platforms often hide the complexity in a "black box" runtime. Replay is different because it focuses on generating clean react code that your developers can actually read and edit. It is a "pro-code" tool that uses automation to handle the tedious parts of reverse engineering, leaving the creative architecture to your team.

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Transform Your Legacy Today#

The "Green Screen" doesn't have to be the end of the road for your enterprise applications. By leveraging visual reverse engineering, you can turn decades of terminal interactions into a modern, documented, and high-performance React ecosystem.

Ready to start generating clean react code from your mainframe sessions?

Visit replay.build to request a demo and see visual reverse engineering in action.