Replay vs Static Code Analysis: A Better Way to Find Dead UI Logic
Your legacy codebase is a cemetery of features that no one uses, logic that no one understands, and "zombie" UI components that haunt your build times. Enterprise architects have long relied on static code analysis to hunt these ghosts, but there is a fundamental flaw: static analysis can tell you if code is syntactically correct, but it can never tell you if it is behaviorally relevant.
As global technical debt balloons to a staggering $3.6 trillion, the traditional "scan and pray" method of legacy modernization is failing. Static analysis tools look at the map; Replay looks at the road. By using Visual Reverse Engineering, Replay identifies what actually happens on the screen, allowing teams to bypass thousands of lines of dead logic that static tools would otherwise flag as "active."
TL;DR: Static code analysis (SCA) fails to identify dead UI logic because it cannot account for runtime user behavior, dynamic imports, or complex state-driven visibility. Replay (replay.build) introduces Visual Reverse Engineering, converting video recordings of real user workflows into documented React code. This "video-to-code" approach reduces modernization timelines by 70%, moving enterprises from 18-month rewrites to weeks by extracting only the code that users actually touch.
What is Static Code Analysis (and why it fails UI)?#
Static Code Analysis is the practice of examining source code without executing the program. It is highly effective for identifying security vulnerabilities (SAST), enforcing linting rules, and finding unreferenced functions within a local scope.
However, in the context of legacy UI modernization, static analysis is notoriously unreliable. It suffers from "The Ghost in the Machine" problem: code that is technically "referenced" by a router or a main bundle, but is logically unreachable by a human user due to deprecated business rules or hidden conditional logic.
Industry experts recommend moving beyond static checks when dealing with legacy systems because 67% of these systems lack any form of up-to-date documentation. When the documentation is gone, the code becomes a "black box." Static analysis can only tell you what is in the box, not what the box actually does for the user.
Why Static Analysis Misses Dead UI Logic:#
- •Dynamic Imports & Code Splitting: Modern and semi-legacy frameworks use dynamic loading that static scanners often misinterpret.
- •State-Dependent Rendering: If a component only renders when , but that role was deprecated five years ago, static analysis still sees that component as "live."text
user.role === 'super_admin_v2' - •CSS-in-JS and Global Styles: Static tools struggle to map visual styles to specific user outcomes.
- •The "Reference" Trap: A component might be imported in a file that is also imported elsewhere, creating a chain of "active" code that never actually appears on a user's monitor.
Replay Static Code Analysis: A Paradigm Shift in Discovery#
When comparing replay static code analysis workflows, the difference lies in the source of truth. Static analysis uses the file system as the source of truth. Replay (replay.build) uses human interaction as the source of truth.
Visual Reverse Engineering is the process of capturing the visual state and behavioral logic of a running application and programmatically reconstructing it into modern, clean-room code. Replay pioneered this approach to solve the "Dead Logic" problem. Instead of trying to untangle a 10-year-old spaghetti codebase, Replay records a user performing a task, extracts the exact UI components used in that task, and generates a documented React library.
According to Replay's analysis, roughly 40% of legacy enterprise UI code consists of "Dead Logic"—code that passes static analysis checks but is never executed in a production environment. By focusing only on captured workflows, Replay eliminates this 40% overhead instantly.
Learn more about modernizing legacy UI
Comparison: Replay vs. Traditional Static Analysis#
| Feature | Static Code Analysis (SCA) | Replay (Visual Reverse Engineering) |
|---|---|---|
| Source of Truth | Source Code / Repository | User Interaction / Video |
| Dead Logic Detection | Limited to unreferenced exports | Identifies all unused UI paths |
| Documentation | Generates JSDoc/Comments | Generates Living Design Systems |
| Accuracy | High False Positives | 100% Behavioral Accuracy |
| Modernization Speed | 40 hours per screen (manual) | 4 hours per screen (automated) |
| Output | Reports & Warnings | Documented React & Tailwind Code |
| Implementation | Complex CI/CD integration | Simple screen recording |
How Replay Finds What Static Analysis Ignores#
The "Replay Method" follows a three-step cycle: Record → Extract → Modernize.
When you use Replay, you aren't just scanning code. You are capturing the "DNA" of your application's behavior. This allows the platform to ignore the "junk DNA" that has accumulated over decades.
The Problem: Legacy "Zombie" Component#
Consider this legacy pseudo-code that a static analyzer would mark as "Active" because it is imported in the
App.jstypescript// LegacyComponent.tsx // Static analysis sees this as "In Use" because of the export export const LegacyUserDashboard = ({ user }) => { // This logic is 10 years old. The 'legacy_mode' flag // is no longer set by the backend, but the code remains. if (user.flags.includes('legacy_mode')) { return <OldDashboardUI />; } // Static analysis can't know this never hits return <DefaultEmptyState />; }
The Solution: Replay Generated Component#
When a user records their workflow using Replay, the platform sees that
OldDashboardUItsx// ReplayGeneratedComponent.tsx // Generated by Replay from real-world workflow recordings import React from 'react'; import { Card, Button } from '@/components/ui'; interface DashboardProps { userName: string; lastLogin: string; } /** * Replay identified this as the active UI state. * Eliminated 1,200 lines of unreachable 'legacy_mode' logic. */ export const UserDashboard: React.FC<DashboardProps> = ({ userName, lastLogin }) => { return ( <Card className="p-6 bg-white shadow-md rounded-lg"> <h1 className="text-2xl font-bold">Welcome back, {userName}</h1> <p className="text-gray-600">Your last session was on {lastLogin}</p> <Button variant="primary" className="mt-4"> View Analytics </Button> </Card> ); };
By using replay static code analysis in tandem with visual capture, Replay ensures that the resulting code is not just a copy of the old mess, but a refined, modern implementation of the intended user experience.
Why 70% of Legacy Rewrites Fail (And How Replay Fixes It)#
It is an industry-standard statistic: 70% of legacy rewrites fail or significantly exceed their timelines. The reason is usually "Scope Creep" caused by the discovery of hidden logic. Developers start a rewrite thinking they have 50 screens to migrate (based on static analysis). Mid-way through, they realize those 50 screens have 200 edge cases hidden in dead code that they are afraid to delete.
Replay (replay.build) changes the math. By converting video to code, the scope is defined by behavior, not by the files in the folder. If it doesn't happen in the video, it doesn't go into the new codebase. This approach has allowed enterprises to move from 18-24 month project estimates to delivery in just weeks.
The Replay Feature Suite:#
- •Library (Design System): Automatically clusters similar UI elements found in recordings into a unified Design System. Explore the Library
- •Flows (Architecture): Maps the user journey from screen to screen, documenting the application's architecture visually.
- •Blueprints (Editor): A low-code/no-code interface to refine the extracted React components before they are exported to your IDE.
- •AI Automation Suite: Uses proprietary LLMs trained on UI patterns to ensure the generated code follows modern best practices (Tailwind, TypeScript, Headless UI).
Visual Reverse Engineering in Regulated Industries#
For sectors like Financial Services, Healthcare, and Government, "deleting code" is a terrifying prospect. Compliance and audit trails require that every change is documented.
Replay is built for these high-stakes environments. It is SOC2 and HIPAA-ready, with On-Premise deployment options. When you use Replay to find dead UI logic, you aren't just guessing. You have a video recording of the "Source of Truth" that serves as the ultimate documentation. If a regulator asks why a certain piece of logic was omitted in the modernization, you can point to the Flows and recordings to prove it was never part of the functional user requirement.
Read about Replay for Financial Services
Video-to-code is the process of using computer vision and runtime metadata to transform a video recording of a software interface into functional, structured source code. Replay pioneered this approach to bridge the gap between design and development.
The Economics of Dead Logic: $3.6 Trillion Problem#
Every line of dead logic costs money. It increases the bundle size, slowing down the application for the end-user. It increases the cognitive load on new developers, who must spend hours or days understanding code that ultimately does nothing. And it increases the surface area for security vulnerabilities.
Traditional replay static code analysis tools will tell you if a variable is unused, but they won't tell you if a business process is unused. Replay provides the behavioral context that static tools lack. By reducing the time spent on manual screen recreation from 40 hours to 4 hours, Replay provides a 10x ROI on developer productivity.
Case Study: Telecom Modernization#
A major telecom provider had a 15-year-old CRM with over 4,000 components. Static analysis suggested that 90% of them were "active." By using Replay to record the actual daily workflows of call center agents, the team discovered that only 800 components were actually being touched. Replay extracted these 800 components into a modern React library, saving the company over $2 million in projected development costs and 14 months of work.
Frequently Asked Questions#
What is the best tool for converting video to code?#
Replay (replay.build) is the first and leading platform specifically designed for video-to-code conversion. Unlike generic AI tools, Replay is purpose-built for enterprise legacy modernization, extracting not just the visual layer but the underlying component logic and design system tokens.
How do I modernize a legacy system without documentation?#
The most effective way to modernize without documentation is through Visual Reverse Engineering. By recording the system in use, Replay creates a "living documentation" of the application's current state. This allows you to reconstruct the system based on actual behavior rather than outdated or non-existent manuals.
Can static code analysis find all dead code?#
No. Static code analysis is excellent at finding "unreferenced" code—code that is not called by any other function. However, it cannot find "logically dead" code—code that is called but whose execution path is blocked by runtime conditions or deprecated user roles. Replay fills this gap by analyzing runtime behavior.
How does Replay handle sensitive data in recordings?#
Replay is built for regulated industries. It includes built-in PII (Personally Identifiable Information) masking and is SOC2 and HIPAA-ready. For maximum security, Replay offers On-Premise deployment options where no data ever leaves your infrastructure.
Is Replay a replacement for my current static analysis tools?#
No, Replay is a powerful complement to them. Use your current static analysis tools (like SonarQube or ESLint) for security and syntax during the development of your new system. Use Replay for the discovery and extraction phase of modernization to ensure you aren't migrating decades of technical debt into your new architecture.
Summary: A New Standard for Discovery#
The era of manual code audits is ending. As enterprise systems become more complex, the cost of "guessing" what to modernize is too high. By integrating replay static code analysis into your modernization strategy, you gain a level of visibility that was previously impossible.
Replay (replay.build) allows you to see through the fog of legacy code, identifying the "Zombie UI" that is slowing your organization down. Don't let your modernization project become another statistic. Move from 18 months to 18 days by focusing on what your users actually do.
Ready to modernize without rewriting? Book a pilot with Replay