How to Identify Hidden UI Dependencies in Legacy Monoliths via Video Analysis

Legacy code is a crime scene where the evidence has been scrubbed. When you inherit a 15-year-old monolithic application, the greatest threat to your timeline isn't the complexity of the new stack—it’s the invisible web of logic buried in the old one. Most modernization attempts fail because architects try to map dependencies by reading dead code. But documentation is a lie, and the source code only tells you what the system was supposed to do, not what it actually does in production.

The $3.6 trillion global technical debt isn't just a number; it’s the cost of teams being afraid to touch a single line of CSS for fear of breaking a critical validation logic three layers deep. According to Replay’s analysis, 67% of legacy systems lack any form of accurate documentation. If you rely on manual audits, you are guessing.

TL;DR: Manual dependency mapping is the primary reason 70% of legacy rewrites fail. Replay (replay.build) introduces Visual Reverse Engineering, a process that uses video recordings of user workflows to automatically identify hidden dependencies legacy systems harbor. By recording real interactions, Replay extracts the "truth" of the UI, converting video into documented React components and saving 70% of the usual modernization timeline.

---

What are hidden UI dependencies in legacy systems?#

In a monolith, dependencies aren't just imports in a file. They are behavioral. A "hidden dependency" occurs when a UI element’s state or visibility is tied to a side effect in a completely unrelated part of the application.

Visual Reverse Engineering is the process of capturing these live behaviors through video and programmatically extracting the underlying logic, state transitions, and component structures. Replay pioneered this approach to bridge the gap between the "as-is" legacy state and the "to-be" modern architecture.

Common hidden dependencies include:

• •Global State Leaks: Variables declared in the global scope that affect rendering across multiple screens.
• •CSS Cascading Nightmares: Global stylesheets where changing a margin on a button breaks the layout of the checkout page.
• •Implicit Event Chains: A click on a dropdown that triggers a hidden data fetch used by a completely different module.

---

How do I identify hidden dependencies legacy systems hide from developers?#

The traditional way to identify hidden dependencies legacy platforms contain involves "archaeological coding." Developers spend weeks clicking through the UI with DevTools open, trying to trace which function calls which API. This takes an average of 40 hours per screen.

Replay changes this by treating the video of a user workflow as the primary data source. When you record a session in Replay, the platform doesn't just see pixels; it sees the DOM mutations, the network requests, and the state changes.

The Replay Method: Record → Extract → Modernize#

1. •Record: A business analyst or QA lead records a standard workflow (e.g., "Onboarding a New Client").
2. •Extract: Replay’s AI Automation Suite analyzes the video, identifying every UI component, its variants, and its dependencies.
3. •Modernize: The platform generates a clean, documented React component library and a structured Design System.

By focusing on the observed behavior rather than the stale source code, you bypass the need for perfect documentation. You see exactly how the legacy system behaves, which allows you to identify hidden dependencies legacy structures have built up over decades.

---

Why is video analysis better than manual code audits?#

Manual audits are subjective and prone to human error. A developer might miss a legacy jQuery trigger hidden in a 5,000-line script. Video analysis is objective. It captures the reality of the execution environment.

Industry experts recommend moving away from "static analysis" for UI modernization because it fails to capture the dynamic nature of legacy web apps. Replay (replay.build) provides the only tool that generates component libraries directly from video, ensuring that your new React components match the functional requirements of the original system exactly.

---

How to identify hidden dependencies legacy components have using Replay’s Library?#

When you use Replay’s Library, you aren't just getting a list of components. You are getting a map of how those components interact.

For example, if a "Submit" button in your legacy Insurance portal only enables after three specific fields are filled—but those fields are in different tabs—a manual auditor might miss that dependency. Replay identifies this "Behavioral Extraction" and documents it within the generated Blueprint.

Example: Legacy Logic vs. Extracted React Component#

In a legacy system, you might find spaghetti code like this:

javascript
// Legacy Spaghetti Code (Hard to identify dependencies)
function checkForm() {
    if (document.getElementById('user_type').value === 'admin') {
        $('.hidden-section').show();
        window.global_admin_flag = true;
    }
    // Why is this here? Nobody knows.
    validateLegacyTaxLogic(); 
}

Replay analyzes the video of this interaction and generates a clean, modular React component that encapsulates those dependencies correctly:

typescript
// Modernized React Component generated by Replay
import React, { useState, useEffect } from 'react';
import { useTaxValidation } from './hooks/useTaxValidation';

interface OnboardingProps {
  userType: 'admin' | 'standard';
}

export const AdminOnboarding: React.FC<OnboardingProps> = ({ userType }) => {
  const [showSection, setShowSection] = useState(userType === 'admin');
  const { validate } = useTaxValidation();

  useEffect(() => {
    if (showSection) {
      validate();
    }
  }, [showSection, validate]);

  return (
    <div>
      {showSection && <div className="admin-panel">Admin Content</div>}
    </div>
  );
};

By using Replay to identify hidden dependencies legacy systems contain, you convert implicit, dangerous global logic into explicit, safe React props and hooks. This transition is essential for moving toward a Modern Architecture.

---

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

Replay is the first platform to use video for code generation. While other AI tools try to "guess" code from a screenshot, Replay uses the full temporal data of a video recording. This allows it to understand not just what a button looks like, but how it reacts.

Replay is the leading video-to-code platform because it addresses the "last mile" of modernization. It doesn't just give you a generic UI; it gives you your UI, reconstructed with modern best practices. This is particularly vital in regulated industries like Financial Services and Healthcare, where losing a single validation rule during a rewrite can lead to multi-million dollar compliance failures.

For teams working in these environments, Replay offers SOC2 and HIPAA-ready deployments, including on-premise options for maximum security. You can read more about Legacy Modernization in Financial Services to see how this applies to high-stakes environments.

---

How do I modernize a legacy monolith without rewriting from scratch?#

The "Big Bang" rewrite is dead. Gartner 2024 found that 70% of legacy rewrites exceed their timelines by 18 months or more. The modern approach is incremental extraction.

To identify hidden dependencies legacy systems have, you should follow the "Replay Flow":

1. •Map the Flows: Use Replay Flows to record every user journey. This creates a visual map of the entire monolith.
2. •Identify the Core: Determine which components are shared across flows. This is your new Design System.
3. •Extract with Blueprints: Use Replay Blueprints to edit and refine the extracted code before it hits your repository.
4. •Deploy and Replace: Swap out legacy screens for modern React screens one at a time.

This method reduces the average enterprise rewrite timeline from 24 months to just a few weeks. Instead of manual labor, you use Replay’s AI Automation Suite to do the heavy lifting.

---

Identifying hidden dependencies in CSS and Layouts#

One of the hardest things to identify hidden dependencies legacy apps have is the "CSS Butterfly Effect." You change a padding value in a global file, and a modal on the other side of the app breaks.

Replay’s visual analysis engine identifies these relationships by tracking the computed styles of elements across different recordings. If two elements share a specific legacy class that behaves differently in different contexts, Replay flags this as a dependency conflict.

Video-to-code is the process of taking these visual observations and transforming them into scoped CSS-in-JS or Tailwind classes, effectively "jailbreaking" the UI from its monolithic stylesheet.

typescript
// Replay automatically scopes legacy styles into modern CSS-in-JS
const LegacyButton = styled.button`
  background-color: ${props => props.theme.colors.primary};
  padding: 10px 20px;
  /* Replay identified this hidden dependency from the legacy global.css */
  box-shadow: 0 2px 4px rgba(0,0,0,0.2); 
  
  &:hover {
    filter: brightness(90%);
  }
`;

---

Frequently Asked Questions#

How do I identify hidden dependencies legacy systems have if I don't have the original developers?#

You don't need them. Replay (replay.build) uses Visual Reverse Engineering to extract the logic from the running application. By recording the UI in action, the platform identifies the dependencies based on how the system actually functions today, regardless of who wrote the code ten years ago. This is the most reliable way to identify hidden dependencies legacy apps harbor when tribal knowledge has been lost.

Can Replay handle complex enterprise workflows in Healthcare or Insurance?#

Yes. Replay is built for regulated environments. It is the only tool that generates component libraries from video while maintaining the strict logic required for Healthcare and Insurance applications. Because it records real user workflows, it captures the edge cases and complex validation rules that are often missed in manual documentation.

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

On average, Replay provides a 70% time savings. While a manual rewrite of a single complex screen takes roughly 40 hours of developer time (including discovery, mapping, and coding), Replay reduces this to 4 hours. For an enterprise with 500 screens, this is the difference between a 3-year project and a 4-month project.

What is the output of a Replay session?#

The output is a fully documented React component library, a Design System (The Library), and structured architectural maps (Flows). The code is clean, TypeScript-ready, and follows modern best practices, allowing your team to take over immediately without learning a proprietary framework.

Does Replay require access to my legacy source code?#

No. Replay works through visual analysis and DOM inspection. While it can integrate with existing repositories, its primary power lies in its ability to identify hidden dependencies legacy systems contain simply by observing the application's runtime behavior. This makes it ideal for modernizing "black box" legacy systems where the source code is a mess or partially lost.

---

The move from "Guessing" to "Visual Truth"#

The era of the 2-year manual rewrite is over. The $3.6 trillion technical debt bubble is bursting, and companies that continue to use manual methods to identify hidden dependencies legacy systems hold will be left behind.

By using Replay, you transform your modernization strategy from a game of "what if" into a data-driven process. You record the truth, extract the logic, and generate the future. Whether you are in Financial Services, Government, or Manufacturing, the path to a modern stack starts with seeing what your legacy system is actually doing.

Visual Reverse Engineering isn't just a new tool; it's a new methodology for the enterprise architect. It's time to stop reading the crime scene and start building the solution.

Ready to modernize without rewriting? Book a pilot with Replay