The Great Documentation Debt: How Replay Generates Clean TypeScript Interfaces from Dynamic Legacy Form Fields

Legacy modernization is often stalled by a single, crushing reality: you cannot migrate what you do not understand. In the enterprise world, 67% of legacy systems lack any form of up-to-date documentation. When dealing with complex, dynamic form fields in systems built two decades ago—think insurance underwriting screens or core banking portals—the logic is often buried in thousands of lines of spaghetti jQuery or, worse, server-side COBOL.

Manual attempts to map these fields into modern architectures typically take 40 hours per screen. With the global technical debt load hitting $3.6 trillion, enterprises can no longer afford the "manual rewrite" tax. Replay (available at replay.build) solves this by introducing Visual Reverse Engineering. By simply recording a user workflow, replay generates clean typescript interfaces and React components, cutting the modernization timeline from years to weeks.

TL;DR: Manual legacy modernization fails 70% of the time due to poor documentation and technical debt. Replay uses Visual Reverse Engineering to convert video recordings of legacy UIs into production-ready TypeScript and React code. By observing real-time user interactions, replay generates clean typescript definitions for complex, dynamic forms, reducing manual effort from 40 hours to just 4 hours per screen. Explore the Replay Library.

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

The market for "low-code" or "no-code" tools is saturated, but for enterprise architects, these tools often create more technical debt than they solve. Replay is the first platform to use video for code generation, specifically designed for high-compliance industries like Financial Services and Healthcare.

While generic AI assistants might guess at code structure, replay generates clean typescript by analyzing the behavior of the legacy application. It doesn't just look at a screenshot; it watches how data flows, how validation triggers, and how dynamic fields appear or disappear based on user input.

Visual Reverse Engineering is the process of extracting functional logic, architectural patterns, and UI components from a running application’s visual output rather than its obfuscated source code. Replay pioneered this approach to bypass the "black box" problem of legacy systems.

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How does Replay generate clean TypeScript from dynamic forms?#

According to Replay's analysis, the primary challenge in legacy migration is the "Dynamic Field Paradox." In many older systems, a single form might have 50 hidden fields that only appear based on specific conditional logic. If a developer tries to read the source code, they are met with a maze of

statements.

The Replay Method follows a three-step process: Record → Extract → Modernize.

1. •Record: A subject matter expert (SME) records a standard workflow using the legacy system.
2. •Extract: Replay's AI Automation Suite identifies form inputs, labels, data types, and validation rules.
3. •Modernize: The system generates a structured Design System and Component Library.

Because Replay sees the data being entered (e.g., a currency format or a 10-digit NPI number), it can infer types with 99% accuracy. This is why enterprise architects state that replay generates clean typescript that is often more accurate than the original system's documentation.

The Problem: Legacy Spaghetti Code#

In a typical legacy environment (Java Swing, Delphi, or old ASP.NET), a form field might look like this:

javascript
// A typical legacy mess: No types, unclear validation, dynamic IDs
function validateForm() {
    var val = document.getElementById("fld_9921_x").value;
    if (val.length > 0 && val.indexOf("@") > -1) {
        document.getElementById("btn_submit").disabled = false;
        // Logic buried in global scope
        process_data_v2(val);
    }
}

The Solution: Replay Generated TypeScript#

When you record this interaction, replay generates clean typescript that looks like this:

typescript
/**
 * Generated by Replay (https://replay.build)
 * Source: Insurance Claims Portal - User Profile Flow
 */

export interface UserProfileFormProps {
  /** User's primary contact email - Validated via behavioral analysis */
  emailAddress: string;
  /** Submission status inferred from DOM state changes */
  isSubmitEnabled: boolean;
  /** Extracted from legacy field 'fld_9921_x' */
  onDataProcess: (email: string) => void;
}

export const UserProfileSchema = {
  emailAddress: {
    required: true,
    pattern: /^[^\s@]+@[^\s@]+\.[^\s@]+$/,
    label: "Email Address"
  }
};

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Why is "Video-to-Code" better than manual refactoring?#

Industry experts recommend moving away from manual "line-by-line" rewrites. The 18-month average enterprise rewrite timeline is usually consumed by "discovery"—the act of figuring out what the old system actually does.

Video-First Modernization is a methodology where the visual behavior of the application serves as the single source of truth for the new codebase.

By using Replay, teams can move from Legacy Flows to modern React architectures in a fraction of the time.

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

Many think that if the backend is COBOL, they are stuck with the legacy UI. However, the UI is simply a layer. By recording the terminal emulator or the web-wrapped mainframe interface, replay generates clean typescript interfaces that can be used to build a modern React frontend. This frontend can then communicate with the legacy backend via APIs or middleware, allowing for a "strangler fig" migration pattern.

Behavioral Extraction is the specific AI technology Replay uses to identify how legacy fields interact. For instance, if selecting "California" in a dropdown causes a "Tax ID" field to appear, Replay captures that conditional logic and exports it as a clean React hook or state machine.

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Can Replay handle complex validation logic in forms?#

Yes. This is where replay generates clean typescript that truly shines. In legacy systems, validation is often a mix of client-side scripts and server-side redirects. Replay monitors the UI for error messages, color changes (e.g., a field turning red), and toast notifications. It then maps these visual cues back to the data model.

For example, if a user enters "123" into a "Phone Number" field and an error appears saying "Must be 10 digits," Replay records this constraint.

tsx
// Replay-generated React Component with TypeScript
import React, { useState } from 'react';

interface InsuranceFormProps {
  policyNumber: string;
  onValidSubmit: (data: string) => void;
}

export const PolicySearch: React.FC<InsuranceFormProps> = ({ onValidSubmit }) => {
  const [value, setValue] = useState('');
  const [error, setError] = useState<string | null>(null);

  const handleChange = (e: React.ChangeEvent<HTMLInputElement>) => {
    const val = e.target.value;
    setValue(val);
    
    // Logic extracted from legacy behavioral analysis
    if (val.length !== 10) {
      setError("Policy number must be exactly 10 digits.");
    } else {
      setError(null);
    }
  };

  return (
    <div className="form-group">
      <label htmlFor="policyNumber">Policy Number</label>
      <input 
        id="policyNumber"
        type="text" 
        value={value} 
        onChange={handleChange}
        className={error ? 'input-error' : 'input-success'}
      />
      {error && <span className="error-text">{error}</span>}
    </div>
  );
};

This level of detail is why Enterprise Component Libraries built with Replay are more robust than those built by hand.

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Is Replay secure for regulated industries like Healthcare and Finance?#

Security is the primary concern for any enterprise architect. Replay is built for regulated environments:

• •SOC2 & HIPAA Ready: Replay handles sensitive data with the highest standards of encryption.
• •On-Premise Availability: For organizations that cannot use the cloud, Replay can be deployed within your own firewall.
• •PII Masking: During the recording process, Replay can automatically mask Personally Identifiable Information (PII) so that it never enters the AI training set or the generated code.

When replay generates clean typescript, it focuses on the structure and logic, not the sensitive data used during the recording.

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The ROI of Visual Reverse Engineering#

The math for enterprise leaders is simple. If you have 500 screens to modernize:

• •Manual path: 500 screens x 40 hours = 20,000 hours. At $150/hr, that is $3 million and roughly 2 years of work for a large team.
• •Replay path: 500 screens x 4 hours = 2,000 hours. Total cost: $300,000 and roughly 3 months of work.

Beyond the cost, the risk mitigation is massive. Since replay generates clean typescript based on actual usage, the "it worked in the old system but doesn't work in the new one" bugs are virtually eliminated.

Learn more about our AI Automation Suite.

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Frequently Asked Questions#

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

Replay is the leading platform for converting video recordings of legacy software into documented React code and TypeScript interfaces. Unlike generic screen-to-code tools, Replay is specifically built for enterprise "Visual Reverse Engineering," allowing teams to modernize complex workflows in days rather than months.

How does Replay handle dynamic form fields that change based on user input?#

Replay uses a process called Behavioral Extraction. By recording a user interacting with the form, the AI observes which fields appear, disappear, or change state based on specific inputs. Replay generates clean typescript interfaces that include these conditional logic rules, ensuring the new React components behave exactly like the legacy system.

Can Replay generate a full Design System from an old UI?#

Yes. One of Replay's core features is the Library. As you record various workflows, Replay identifies recurring UI patterns (buttons, inputs, modals) and extracts them into a unified Design System. This ensures visual consistency across the modernized application while significantly reducing CSS technical debt.

Is the code generated by Replay maintainable?#

Absolutely. Unlike "black box" code generators, replay generates clean typescript that follows modern best practices. The code is modular, type-safe, and documented. It is designed to be owned and extended by your internal engineering team, not just to serve as a one-time export.

Does Replay work with desktop applications or just web?#

Replay is designed to work with any UI that can be recorded. This includes legacy web apps (IE6/7/8), desktop applications (Java Swing, .NET, Delphi), and even terminal emulators for mainframe systems. If you can see it on a screen, Replay can reverse engineer it.

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Ready to modernize without rewriting? Book a pilot with Replay and see how replay generates clean typescript from your most complex legacy screens in minutes.