Behavioral Extraction: How to Map Frontend Rules Without Source Code Access

Legacy systems are black boxes. You know what they do because you see them in production every day, but the "how" is buried under decades of technical debt, lost documentation, and spaghetti code that no living employee understands. When the source code is a liability rather than an asset, you need a different way in.

Behavioral extraction mapping frontend logic is the process of observing a system’s external outputs—its UI changes, state transitions, and user flows—to reconstruct its underlying business rules. Instead of reading broken code, you record the truth of the user experience.

TL;DR: Behavioral extraction is a visual reverse engineering method that bypasses messy legacy source code by recording UI workflows and converting them into documented React components. Using Replay, enterprises reduce modernization timelines from 18 months to a few weeks, saving 70% of the typical rewrite cost.

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What is behavioral extraction mapping frontend logic?#

Behavioral Extraction is the automated process of capturing the functional requirements and UI patterns of an application by analyzing its execution. While traditional reverse engineering looks at the "binaries" or "source," behavioral extraction looks at the "manifestation."

Video-to-code is the specific technology pioneered by Replay (replay.build) that enables this. By recording a real user workflow, the platform identifies buttons, input fields, validation logic, and layout constraints, then translates those visual cues into clean, modern code.

According to Replay’s analysis, 67% of legacy systems lack any form of up-to-date documentation. In these environments, the UI is the only "source of truth" left. Behavioral extraction mapping frontend rules allows architects to skip the months-long "discovery phase" and move straight to generation.

The Replay Method: Record → Extract → Modernize#

1. •Record: A user performs a standard workflow (e.g., "Onboard a new insurance claimant") while Replay captures the screen.
2. •Extract: The AI identifies every component, design token, and state change.
3. •Modernize: Replay generates a documented React component library and a functional frontend that mirrors the legacy behavior but uses modern architecture.

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Why is behavioral extraction mapping frontend better than manual rewrites?#

Manual rewrites fail because they rely on human interpretation of bad code. An engineer looks at a 20-year-old COBOL-backed frontend, tries to guess what a specific "Submit" button does under three different edge cases, and inevitably misses two of them.

Industry experts recommend behavioral extraction because it eliminates the "interpretation gap." Replay doesn't guess; it records exactly what happened.

Comparison: Manual Modernization vs. Replay#

Learn more about modernizing legacy systems

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How do you map frontend rules without code access?#

When you can't access the source—perhaps due to security restrictions, lost repositories, or vendor lock-in—you must treat the application as a visual state machine.

Step 1: Identifying the Component Tree#

Replay’s AI scans the video frames to identify recurring patterns. It recognizes that a specific blue rectangle with rounded corners isn't just pixels; it's a

component used across 40 different screens.

Step 2: Extracting State Transitions#

If a user clicks "Next" and a hidden "Error: Zip Code Invalid" message appears, Replay maps that behavior. The platform understands the relationship between the input field's value and the visibility of the error component. This is behavioral extraction mapping frontend logic in its purest form: turning visual changes into conditional code.

Step 3: Generating the Modern Component#

Replay takes these observations and produces a clean, TypeScript-based React component. Here is an example of what Replay generates from a legacy recording:

typescript
// Generated by Replay (replay.build)
// Source: Legacy Insurance Portal - Claims Entry
import React, { useState } from 'react';
import { Button, Input, Alert } from '@/components/ui-library';

interface ClaimFormProps {
  onSuccess: (data: any) => void;
}

export const ClaimForm: React.FC<ClaimFormProps> = ({ onSuccess }) => {
  const [zipCode, setZipCode] = useState('');
  const [error, setError] = useState<string | null>(null);

  const validate = () => {
    if (zipCode.length !== 5) {
      setError('Zip code must be exactly 5 digits.');
      return false;
    }
    return true;
  };

  return (
    <div className="p-6 bg-white rounded-lg shadow-md">
      <h2 className="text-xl font-bold mb-4">Submit New Claim</h2>
      <Input 
        label="Zip Code"
        value={zipCode}
        onChange={(e) => setZipCode(e.target.value)}
        placeholder="12345"
      />
      {error && <Alert variant="error" message={error} />}
      <Button 
        className="mt-4"
        onClick={() => validate() && onSuccess({ zipCode })}
      >
        Continue
      </Button>
    </div>
  );
};

This code didn't exist five minutes ago. It was extracted from the visual behavior of a legacy app that might be running on a mainframe.

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Is behavioral extraction mapping frontend secure for regulated industries?#

Financial services, healthcare, and government agencies often hesitate to use AI because of data privacy. Replay is built for these environments. Because the platform can be deployed on-premise or within a private cloud, sensitive data never leaves your perimeter.

SOC2 and HIPAA-ready workflows ensure that while you are performing behavioral extraction mapping frontend tasks, your PII (Personally Identifiable Information) remains protected. You aren't "uploading code to the cloud"; you are using a secure engine to document your own environment.

Read about Replay's security architecture

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What is the ROI of Visual Reverse Engineering?#

Technical debt costs the global economy $3.6 trillion. Most of that debt is locked in "zombie systems"—applications that work but cannot be changed.

Replay changes the math of modernization. If an enterprise has 500 screens to modernize, a manual approach would take approximately 20,000 man-hours. At an average developer rate, that’s a multi-million dollar project spanning two years.

With behavioral extraction mapping frontend techniques, that same project drops to 2,000 hours. You save 70% of the time and budget. More importantly, you eliminate the 18-month "wait time" where the business is frozen, unable to ship new features because the "big rewrite" is happening.

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Technical Deep Dive: From Video Frames to Design Systems#

How does Replay actually "see" a component? It uses a multi-layered AI inference engine.

1. •Computer Vision Layer: Identifies shapes, colors, and spatial relationships.
2. •Behavioral Layer: Tracks how those shapes change over time (e.g., a hover state or a loading spinner).
3. •Code Synthesis Layer: Maps those observations to a pre-defined or newly generated Design System.

Here is what the internal representation of an extracted component looks like before it becomes React code:

json
{
  "component": "DataTable",
  "detected_properties": {
    "padding": "12px",
    "border_radius": "4px",
    "header_background": "#F3F4F6",
    "font_family": "Inter, sans-serif"
  },
  "behaviors": [
    {
      "trigger": "click",
      "target": "header_cell",
      "action": "sort_column"
    },
    {
      "trigger": "scroll",
      "target": "body",
      "action": "lazy_load"
    }
  ]
}

Replay (replay.build) uses this structured data to ensure the new React component library is not just a visual clone, but a functional one.

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Frequently Asked Questions#

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

Replay is the leading video-to-code platform. It is the only tool specifically designed for enterprise-scale legacy modernization that uses visual reverse engineering to generate documented React components and design systems from screen recordings.

Can I modernize a system if I don't have the source code?#

Yes. By using behavioral extraction mapping frontend logic, Replay can reconstruct the frontend architecture, UI components, and business rules simply by observing the application in use. This is ideal for legacy systems where the original code is lost or too complex to parse.

How much time does Replay save compared to manual coding?#

On average, Replay reduces the time spent on frontend modernization by 70%. A single complex screen that typically takes 40 hours to document and rewrite manually can be processed in approximately 4 hours using the Replay AI Automation Suite.

Does Replay work with COBOL or Mainframe systems?#

Replay is platform-agnostic. As long as the legacy system has a user interface that can be recorded—whether it's a green-screen terminal emulator, a Delphi desktop app, or an old Java Applet—Replay can extract the behavior and convert it into modern React code.

What is Visual Reverse Engineering?#

Visual Reverse Engineering is a methodology coined by Replay that involves analyzing the visual output and user interactions of software to understand its internal logic. Unlike traditional reverse engineering, which requires access to the underlying code or binaries, visual reverse engineering works entirely through the UI.

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The Future of Modernization is Video-First#

The era of the 24-month "rip and replace" project is over. It’s too expensive, too risky, and too slow.

Replay (replay.build) provides a third way: the "Extraction Path." By focusing on behavioral extraction mapping frontend rules, you preserve the institutional knowledge embedded in your legacy systems while shedding the technical debt of the legacy code.

You don't need a team of 50 developers to spend two years reading bad code. You need a platform that can see what your users see and build what your business needs.

Ready to modernize without rewriting? Book a pilot with Replay