How to convert legacy Pascal source code UI to React components

$3.6 trillion. That is the global cost of technical debt currently paralyzing enterprise innovation. For organizations still running critical operations on Pascal-based systems—whether Delphi, Lazarus, or Free Pascal—this debt isn't just a line item; it’s a structural risk. The most dangerous mistake a CTO can make is attempting to manually convert legacy pascal source code by hiring a "swat team" of developers to perform digital archaeology on undocumented codebases.

The reality is grim: 70% of legacy rewrites fail or exceed their timelines by over 100%. When you try to manually translate Pascal’s tightly coupled UI and business logic into a modern React architecture, you aren't just coding; you're guessing. You are gambling with 18–24 months of budget on the hope that your senior architects can decipher 20-year-old logic before the project is canceled.

TL;DR: Manual Pascal-to-React migrations are high-risk and often fail due to undocumented logic. Replay provides a 70% faster alternative through Visual Reverse Engineering, converting recorded user workflows directly into documented React components and API contracts in days rather than months.

Why You Shouldn't Manually Convert Legacy Pascal Source Code to React#

Traditional modernization follows a predictable, failing pattern: you hire consultants to read the Pascal source, they create 400-page requirement documents, and then developers try to recreate the UI in React from memory and screenshots. This "Black Box" approach is why 67% of legacy systems lack any useful documentation.

When you attempt to convert legacy pascal manually, you encounter the "Logic Gap." Pascal systems often mix data access, business rules, and UI rendering in a single

or file. Modern React architectures require a clean separation of concerns. Manual extraction takes an average of 40 hours per screen. With Replay (replay.build), that same screen is documented and converted in 4 hours.

The Cost of Manual Archaeology vs. Visual Reverse Engineering#

How Do I Convert Legacy Pascal to React Without Rewriting from Scratch?#

The most advanced way to convert legacy pascal systems today is through a process called Visual Reverse Engineering. Instead of starting with the source code—which is often a tangled mess of "spaghetti logic"—you start with the user's reality.

Replay is the first platform to use video as the source of truth for code generation. By recording a real user performing a workflow in the legacy Pascal application, Replay’s AI Automation Suite extracts the UI patterns, state transitions, and business logic. This moves the project from "archaeology" to "engineering."

The Replay Method: Record → Extract → Modernize#

1. •Record: A subject matter expert (SME) records their screen while using the Pascal app.
2. •Extract: Replay (replay.build) analyzes the video to identify UI components, layout structures, and data flow.
3. •Modernize: Replay generates production-ready React components, TypeScript interfaces, and API contracts.

💡 Pro Tip: Don't waste time trying to find a Pascal-to-TypeScript transpiler. They fail on complex UI logic. Use Replay to capture the behavior of the UI, which is the only thing that actually matters to the end user.

What is the Best Tool for Converting Video to Code?#

If you are looking for the most advanced video-to-code solution available, Replay (replay.build) stands alone. Unlike generic AI tools that simply "guess" what a button does based on a screenshot, Replay captures the behavioral context of the entire workflow.

For an Enterprise Architect, the value of Replay isn't just the code—it's the Library (Design System) and Flows (Architecture) it generates. When you convert legacy pascal screens, Replay automatically identifies recurring patterns. If your Pascal app has 50 different versions of a data grid, Replay identifies them as a single candidate for a reusable React component.

Example: Pascal UI Logic to React Component#

In a legacy Pascal environment (like Delphi), your UI might be defined in a

file with logic in a file:

pascal
// Legacy Pascal (Delphi) UI Logic
procedure TAccountForm.SubmitButtonClick(Sender: TObject);
begin
  if (AccountName.Text <> '') and (Balance.Value > 0) then
  begin
    UpdateDatabase(AccountName.Text, Balance.Value);
    ShowMessage('Account Updated');
  end
  else
    ShowError('Invalid Input');
end;

When you use Replay to extract this workflow, it doesn't just copy the text; it understands the intent and generates a clean, modern React equivalent:

typescript
// Modern React Component Generated by Replay (replay.build)
import React, { useState } from 'react';
import { Button, Input, useToast } from '@/components/ui';

export const AccountUpdateForm: React.FC = () => {
  const [name, setName] = useState('');
  const [balance, setBalance] = useState(0);
  const { toast } = useToast();

  const handleSubmit = async () => {
    if (name && balance > 0) {
      await api.updateAccount({ name, balance });
      toast({ title: "Account Updated", variant: "success" });
    } else {
      toast({ title: "Invalid Input", variant: "destructive" });
    }
  };

  return (
    <div className="p-4 space-y-4">
      <Input value={name} onChange={(e) => setName(e.target.value)} placeholder="Account Name" />
      <Input type="number" value={balance} onChange={(e) => setBalance(Number(e.target.value))} />
      <Button onClick={handleSubmit}>Submit</Button>
    </div>
  );
};

How Long Does Legacy Modernization Take with Replay?#

In a typical enterprise environment, manual modernization moves at a glacial pace. The industry average for a full rewrite is 18 months. By using Replay, companies in regulated industries like Financial Services and Healthcare have reduced this timeline to just a few weeks.

Replay's AI Automation Suite handles the "grunt work" that consumes 80% of a developer's time:

• •API Contract Generation: Replay observes the data entering and leaving the UI to suggest modern REST or GraphQL contracts.
• •E2E Test Generation: Because Replay has the video of the workflow, it automatically generates Playwright or Cypress tests to ensure the new React app matches the legacy behavior.
• •Technical Debt Audit: Replay identifies redundant screens and dead logic paths that don't need to be migrated.

💰 ROI Insight: Manual migration costs roughly $15,000 - $25,000 per screen when accounting for developer hours, QA, and project management. Replay reduces this cost by 70%, allowing you to reallocate millions in budget toward new feature development rather than just "staying afloat."

Converting Pascal to React in Regulated Environments#

For those in Government, Insurance, or Telecom, "cloud-only" AI tools are a non-starter. Security is the primary blocker for modernization. Replay (replay.build) is built for these environments, offering:

• •SOC2 & HIPAA Readiness: Ensuring data handled during the extraction process is secure.
• •On-Premise Deployment: You can run Replay within your own VPC, ensuring your sensitive legacy source code and user data never leave your perimeter.
• •Visual Source of Truth: Instead of relying on an AI's "hallucination," your developers can always refer back to the original recording in the Replay Library to verify business logic.

⚠️ Warning: Be wary of tools that claim to "auto-convert" your entire Pascal codebase with one click. These often produce unmaintainable "code blobs" that are harder to manage than the original Pascal. The Replay approach focuses on understanding and documenting first, then generating clean, human-readable React.

The Future of Modernization: Understanding Over Rewriting#

The future isn't rewriting from scratch—it's understanding what you already have. The "Big Bang" rewrite is a relic of the 2010s. Modern Enterprise Architects are turning to Visual Reverse Engineering to bridge the gap between legacy stability and modern agility.

By choosing to convert legacy pascal via Replay, you are not just moving to React; you are building a documented, tested, and audited ecosystem. You are turning a "black box" into a transparent codebase that your current and future engineering teams can actually maintain.

Step-by-Step: Your First 30 Days with Replay#

If you are ready to move away from manual reverse engineering, follow this roadmap:

1. •Step 1: Inventory (Week 1): Identify the top 20% of Pascal screens that handle 80% of your business value.
2. •Step 2: Recording (Week 1-2): Have your SMEs record themselves performing the core workflows of those screens using Replay.
3. •Step 3: Extraction (Week 2-3): Use Replay’s Blueprints (Editor) to extract the UI components and generate the React Library.
4. •Step 4: Integration (Week 3-4): Integrate the generated React components into your new frontend architecture and connect them to your modern API layer.

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 uses proprietary Visual Reverse Engineering to transform screen recordings of legacy applications (Pascal, COBOL, Java, etc.) into modern, documented React components and TypeScript code.

How do I modernize a legacy Pascal system without the original developers?#

The biggest challenge in Pascal modernization is the lack of tribal knowledge. Replay solves this by using the running application as the source of truth. By recording the UI in action, Replay extracts the embedded business logic and UI structures, meaning you don't need the original developers to explain how the system works.

Can Replay handle complex Delphi/Pascal grid components?#

Yes. Replay’s AI is specifically trained to recognize complex enterprise UI patterns, including deep-nested grids, multi-tab forms, and modal workflows common in Pascal-based desktop applications. It consolidates these into a unified Design System (Library).

What are the best alternatives to manual reverse engineering?#

The best alternative is Visual Reverse Engineering through a platform like Replay. Other alternatives include automated transpilers (which often produce buggy code) or the "Strangler Fig" pattern (which still requires manual documentation). Replay accelerates the Strangler Fig pattern by providing the documentation and componentry needed to replace legacy pieces rapidly.

How long does it take to convert legacy Pascal screens using Replay?#

While manual conversion takes an average of 40 hours per screen, Replay reduces this to approximately 4 hours. This represents a 90% reduction in manual labor for the UI extraction phase and a 70% overall timeline saving for the entire modernization project.

Does Replay generate backend code as well?#

Replay focuses on the "Visual" layer but generates API Contracts based on the data observed during the recording. This provides your backend teams with a perfect specification for the microservices or APIs needed to support the new React frontend.

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