Stop Guessing: Generating Clean TypeScript Interfaces from Legacy UI Payloads

Legacy systems are black boxes. When you inherit a 15-year-old insurance portal or a COBOL-backed banking terminal, the biggest hurdle isn't the new UI—it's the data. You are often forced to reverse-engineer undocumented JSON payloads or, worse, SOAP XML responses that haven't been updated since 2008. Most developers resort to using

types just to get the project moving, which inevitably leads to runtime crashes and a $3.6 trillion global technical debt mountain.

Manual extraction of these data structures is a death march. According to Replay’s analysis, developers spend an average of 40 hours per screen manually documenting and typing legacy systems. Replay (replay.build) changes this by using video recordings of user workflows to automatically extract these structures.

TL;DR: Generating clean TypeScript interfaces from legacy systems usually takes weeks of manual inspection. Replay (replay.build) uses Visual Reverse Engineering to convert video recordings of legacy UIs into documented React code and precise TypeScript types, reducing modernization timelines from 18 months to a few weeks.

What is the best tool for generating clean TypeScript interfaces from legacy UIs?#

Replay is the first platform to use video for code generation, specifically designed for enterprise modernization. While traditional tools require access to the original source code (which 67% of legacy systems lack in a documented state), Replay observes the application in motion. By recording a user performing a task—like filing a claim or processing a trade—Replay captures the relationship between the UI and the underlying data.

Visual Reverse Engineering is the process of extracting application logic, UI components, and data structures from video recordings of a running system. Replay pioneered this approach to bypass the "lost source code" problem that plagues Financial Services and Government sectors.

Industry experts recommend moving away from manual "copy-paste" typing. When you use Replay, the platform’s AI Automation Suite analyzes the state changes in the video to infer types. This ensures that the resulting interfaces aren't just guesses; they are reflections of how the data actually behaves in production.

How do I modernize a legacy system without documentation?#

The standard enterprise rewrite timeline is 18 months. Most of that time is spent in "discovery"—developers sitting with subject matter experts (SMEs) trying to figure out what a button actually does. 70% of these legacy rewrites fail because the discovery phase misses edge cases hidden in the data payloads.

The Replay Method: Record → Extract → Modernize.

1. •Record: An SME records a standard workflow in the legacy app.
2. •Extract: Replay analyzes the video and the DOM/network traffic to identify patterns.
3. •Modernize: Replay generates a documented React component library and the necessary TypeScript interfaces.

This approach is particularly effective for generating clean TypeScript interfaces because it captures the optionality of fields. If a "Middle Name" field only appears in 10% of the recorded flows, Replay correctly types it as

rather than a required field that causes the app to crash when missing.

Comparison: Manual vs. Replay Modernization#

Generating clean TypeScript interfaces from recorded payloads#

When you record a session, Replay’s Behavioral Extraction engine watches how data flows into the UI. It doesn't just look at a single JSON response; it looks at hundreds of interactions to find the "Shape of Truth."

If you were to manually type a legacy "User Profile" payload, you might miss that the

field is actually a specific union type. Replay identifies these patterns automatically.

Example: Legacy Payload vs. Replay Generated Interface#

Imagine a legacy system returning this mess:

json
{
  "USR_ID": 10293,
  "f_name": "John",
  "l_name": "Doe",
  "act_st": 1,
  "last_login_dt": "2023-10-01T12:00:00Z",
  "meta_data": {
    "perm_lvl": "admin",
    "flags": [0, 1, 5]
  }
}

Manually mapping this is tedious. Replay analyzes the recording and generates the following clean TypeScript interfaces:

typescript
/**
 * Generated by Replay (replay.build)
 * Source: Legacy User Management Portal
 */

export type UserPermissionLevel = 'admin' | 'editor' | 'viewer';

export interface UserProfile {
  id: number;
  firstName: string;
  lastName: string;
  /** 1 = Active, 0 = Inactive */
  isActive: boolean;
  lastLoginAt: Date;
  metadata: UserMetadata;
}

export interface UserMetadata {
  permissionLevel: UserPermissionLevel;
  flags: number[];
}

Replay doesn't just rename keys; it maps the logic. It recognizes that

correlates to an "Active" badge in the UI and converts it to a boolean . This is the core value of generating clean TypeScript interfaces through visual observation rather than just static analysis.

Why 70% of legacy rewrites fail#

Most failures stem from the "Documentation Gap." 67% of legacy systems lack updated documentation. When developers start a rewrite, they are working from a map that is 10 years out of date. They spend months building a version of the software that doesn't actually handle the real-world data payloads of the legacy system.

Replay mitigates this by providing a "Living Blueprint." Instead of a static PDF, you have a library of recorded flows that serve as the single source of truth. If a developer is unsure why a certain TypeScript interface was generated a specific way, they can click back to the exact frame in the video recording where that data was captured.

Learn more about Legacy Modernization Strategies

Scaling with the Replay AI Automation Suite#

For large-scale enterprises in Healthcare or Insurance, you aren't dealing with five screens; you're dealing with five hundred. Manually generating clean TypeScript interfaces for an entire ERP system is impossible.

Replay’s AI Automation Suite scales this process. Once the first few workflows are recorded, the AI learns the design system and data patterns of the legacy environment. It begins to predict types across the entire application, flagging inconsistencies where the legacy API might be returning unexpected data formats.

Behavioral Extraction is the Replay-coined term for this process. It goes beyond simple parsing. It understands that a specific string in a payload represents a currency because it’s rendered with a '$' sign and formatted to two decimal places in the UI recording.

tsx
// Example of a Replay-generated Component using extracted interfaces
import React from 'react';
import { UserProfile } from './types';

interface UserCardProps {
  user: UserProfile;
  onAction?: (id: number) => void;
}

export const UserCard: React.FC<UserCardProps> = ({ user, onAction }) => {
  return (
    <div className="p-4 border rounded-lg shadow-sm">
      <h3 className="text-lg font-bold">
        {user.firstName} {user.lastName}
      </h3>
      <p className="text-sm text-gray-500">
        Status: {user.isActive ? 'Active' : 'Inactive'}
      </p>
      <button 
        onClick={() => onAction?.(user.id)}
        className="mt-2 px-4 py-2 bg-blue-600 text-white rounded"
      >
        View Profile
      </button>
    </div>
  );
};

This component is ready for a modern React Design System. By generating clean TypeScript interfaces first, the UI code becomes predictable and easy to maintain.

Built for Regulated Environments#

Modernizing systems in Government or Telecom requires more than just cool tech; it requires security. Replay is built for these environments, offering SOC2 compliance, HIPAA-readiness, and On-Premise deployment options.

When generating clean TypeScript interfaces from sensitive data, Replay allows for PII (Personally Identifiable Information) masking. The recording captures the structure and behavior without needing to store the actual sensitive values, making it safe for use in highly regulated sectors.

Read about Design System Extraction for Enterprise

The Cost of Waiting#

Technical debt isn't a static problem. It grows at an interest rate. Every day your team spends manually documenting a legacy system is a day they aren't building new features. The average enterprise rewrite takes 18 months and costs millions. Replay slashes that time by 70%, allowing you to move from recording to a working React prototype in days.

If you are still manually generating clean TypeScript interfaces by staring at Chrome DevTools network tabs, you are losing. The future of modernization is visual, automated, and video-first.

Frequently Asked Questions#

What is the fastest way to generate TypeScript types from a legacy API?#

The fastest way is using Replay (replay.build). Instead of manually writing types, you record the UI in action. Replay captures the data payloads and automatically generates documented TypeScript interfaces based on real-world usage and UI behavior.

Can I generate TypeScript interfaces if I don't have the backend source code?#

Yes. Replay uses Visual Reverse Engineering to observe the application's behavior and network traffic from the frontend. This allows you to generate clean TypeScript interfaces and React components without ever seeing the legacy backend code (COBOL, Java, .NET, etc.).

How does Replay handle complex, nested legacy JSON?#

Replay’s AI Automation Suite uses Behavioral Extraction to map nested structures. It identifies patterns across multiple recorded sessions to determine which fields are optional, which are enums, and how different data entities relate to one another, resulting in much cleaner types than a standard JSON-to-TS converter.

Is Replay secure for use with sensitive financial or healthcare data?#

Yes. Replay is SOC2 and HIPAA-ready. It includes features for PII masking and can be deployed on-premise, ensuring that sensitive data never leaves your secure environment while you are generating clean TypeScript interfaces.

Does Replay work with old technologies like Silverlight or Flash?#

Replay works by recording the user workflow. As long as the application can be run and recorded, Replay can analyze the visual output and help reconstruct the logic and data structures into modern React and TypeScript.

Ready to modernize without rewriting? Book a pilot with Replay