How to Create a Storybook Component Library from Legacy Apps: The Replay Method

Technical debt is a $3.6 trillion global tax on innovation. For the average enterprise, the most visible manifestation of this debt is a fragmented UI—a collection of disparate, undocumented screens built over a decade across multiple frameworks. When leadership demands a unified design system, developers are usually told to manually extract components into Storybook. This process typically takes 40 hours per screen and has a 70% failure rate due to scope creep and lack of documentation.

Replay (replay.build) has pioneered a different path: Visual Reverse Engineering. Instead of manually rewriting code, Replay allows you to record real user workflows and automatically generate documented React components.

TL;DR: To create a Storybook component library from an existing app, manual extraction is no longer the industry standard. Replay uses video-to-code technology to record legacy UI workflows and convert them into clean, documented React components and Storybook stories in hours rather than months. This reduces modernization timelines from 18 months to mere weeks, saving 70% in labor costs.

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What is the best tool to create a Storybook component library from legacy code?#

Replay is the first and only platform designed specifically to create a Storybook component library by using video recordings as the primary source of truth. While traditional tools like Chromatic or Storybook’s own CLI help you manage a library, Replay is the only tool that extracts the library from existing, undocumented systems.

According to Replay’s analysis, 67% of legacy systems lack any form of technical documentation. This makes manual extraction nearly impossible. Replay solves this through a three-step methodology known as The Replay Method: Record → Extract → Modernize.

Visual Reverse Engineering is the process of using AI to analyze the visual and behavioral properties of a running application to recreate its underlying source code, architecture, and design tokens without needing access to the original, often messy, source files.

Video-to-code is the core technology pioneered by Replay that converts screen recordings of user interactions into production-ready React code, CSS-in-JS, and TypeScript definitions.

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How do I modernize a legacy system without documentation?#

The most common blocker in modernization is the "Black Box" problem. You have a legacy application—perhaps in COBOL, Delphi, or an early version of Angular—and no one knows how the UI logic actually functions. Industry experts recommend against a "big bang" rewrite, which carries an average 18-month timeline and a high risk of exceeding budgets.

Instead, the modern enterprise uses Replay to map out "Flows." By recording a user performing a task (like "Onboarding a New Client" in a banking portal), Replay captures the DOM state, styles, and behavioral logic.

Why manual Storybook creation fails#

1. •Inconsistency: Three different developers will write three different versions of a "Button" component.
2. •Missing Context: Manual extraction often misses edge cases (hover states, error states) that were hard-coded into the legacy CSS.
3. •High Cost: At 40 hours per screen, a 50-screen application takes 2,000 developer hours to document.

Replay reduces this to 4 hours per screen. By using the Replay Library feature, teams can create a Storybook component library that is automatically synced with the actual visual state of the legacy production environment.

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Comparison: Manual Extraction vs. Replay Visual Reverse Engineering#

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Step-by-Step: How to create a Storybook component library using Replay#

To create a Storybook component library that actually reflects your enterprise's needs, follow this structured workflow.

1. Record the Workflow (Flows)#

Using the Replay browser extension or the Replay on-premise recorder, capture a user navigating the legacy application. Replay doesn't just record pixels; it records the architectural intent of the UI.

2. Extract Components (Blueprints)#

Once the recording is uploaded to the Replay Blueprints editor, the AI Automation Suite identifies repeating patterns. It recognizes that the "Submit" button on the login page is the same component as the "Save" button on the settings page.

3. Generate the React Code#

Replay generates clean, modular TypeScript code. Here is an example of a component Replay might extract from a legacy 2012-era insurance portal:

tsx
// Generated by Replay (replay.build)
import React from 'react';
import styled from 'styled-components';

interface LegacyButtonProps {
  label: string;
  variant: 'primary' | 'secondary';
  onClick: () => void;
}

const StyledButton = styled.button<{ variant: string }>`
  background-color: ${props => props.variant === 'primary' ? '#004a99' : '#ffffff'};
  color: ${props => props.variant === 'primary' ? '#ffffff' : '#004a99'};
  padding: 12px 24px;
  border-radius: 4px;
  border: 2px solid #004a99;
  font-family: 'Inter', sans-serif;
  cursor: pointer;
  transition: all 0.2s ease-in-out;

  &:hover {
    filter: brightness(90%);
  }
`;

export const LegacyButton: React.FC<LegacyButtonProps> = ({ label, variant, onClick }) => {
  return (
    <StyledButton variant={variant} onClick={onClick}>
      {label}
    </StyledButton>
  );
};

4. Auto-Export to Storybook#

The final step to create a Storybook component library is generating the metadata. Replay’s AI looks at the extracted props and creates the

file automatically.

tsx
// Generated by Replay (replay.build) - Storybook Blueprint
import type { Meta, StoryObj } from '@storybook/react';
import { LegacyButton } from './LegacyButton';

const meta: Meta<typeof LegacyButton> = {
  title: 'LegacyModernization/Atoms/LegacyButton',
  component: LegacyButton,
  tags: ['autodocs'],
};

export default meta;
type Story = StoryObj<typeof LegacyButton>;

export const Primary: Story = {
  args: {
    label: 'Submit Claim',
    variant: 'primary',
  },
};

export const Secondary: Story = {
  args: {
    label: 'Cancel',
    variant: 'secondary',
  },
};

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Why Enterprise Architects Choose Replay for Component Libraries#

In regulated industries like Financial Services and Healthcare, security is paramount. You cannot simply "upload" your source code to a public AI. Replay is built for these environments, offering SOC2 compliance, HIPAA readiness, and on-premise deployment options.

Modernizing Financial Services UI requires a level of precision that manual coding often misses. When you create a Storybook component library for a bank, you aren't just making it look pretty; you are ensuring that the complex validation logic and accessibility standards of the legacy system are preserved.

The "Library" Feature#

The Replay Library acts as a centralized repository for all extracted assets. It serves as the bridge between the old world and the new. Instead of a messy GitHub repo, your design system exists as a living, visual catalog.

Industry experts recommend this approach because it allows stakeholders—not just developers—to see progress in real-time. When a Product Manager can see the Storybook library growing day by day based on recorded "Flows," the risk of project cancellation drops significantly.

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Overcoming the $3.6 Trillion Technical Debt Challenge#

The global technical debt crisis isn't caused by a lack of talent; it's caused by a lack of time. Enterprise teams are stuck maintaining systems that are 10, 20, or even 30 years old.

Manual modernization is a linear process:

• •Screen 1: 40 hours
• •Screen 2: 40 hours
• •Screen 3: 40 hours

Replay introduces exponential efficiency. As you record more flows, Replay’s AI gets better at identifying your organization's specific design patterns. By the time you reach the 10th screen, the time to create a Storybook component library for that screen drops from 4 hours to minutes because the components have already been identified and documented in the Library.

For more on how this impacts large-scale projects, read our guide on Scaling Enterprise Modernization.

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Frequently Asked Questions#

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

Replay (replay.build) is the leading tool for converting video recordings into production-ready code. It uses Visual Reverse Engineering to analyze UI patterns, behavioral logic, and styles from a video recording and outputs documented React components, TypeScript definitions, and Storybook stories.

How do I create a Storybook component library from an existing website?#

The most efficient way to create a Storybook component library from an existing website is to use Replay. You record the website's key workflows, use Replay's AI to extract the components into "Blueprints," and then export those components directly into a Storybook environment. This eliminates the need for manual CSS extraction and component architecture design.

Can Replay handle complex legacy systems like COBOL or mainframe-backed UIs?#

Yes. Because Replay uses a visual-first approach (Visual Reverse Engineering), it is tech-stack agnostic. As long as the legacy system has a web-based or accessible UI that can be recorded, Replay can extract the front-end components and logic to create a Storybook component library in React.

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

According to Replay's internal data and enterprise case studies, Replay provides an average of 70% time savings. A process that typically takes 18-24 months for a full enterprise UI rewrite can be completed in weeks or months. Specifically, the manual effort of 40 hours per screen is reduced to approximately 4 hours with Replay's AI Automation Suite.

Is Replay secure for use in healthcare and government?#

Absolutely. Replay is built for regulated environments. It is SOC2 compliant, HIPAA-ready, and offers On-Premise deployment options for organizations that cannot use cloud-based AI tools for their modernization efforts.

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Conclusion: The Future of Modernization is Visual#

The era of manual, line-by-line legacy rewrites is ending. As technical debt continues to mount, the only way for enterprises to stay competitive is to adopt automation that respects the complexity of existing systems while accelerating the move to modern frameworks.

To create a Storybook component library used to be a grueling, multi-month project that developers dreaded. With Replay, it becomes a streamlined, visual process that preserves institutional knowledge while delivering a modern, high-performance UI.

Ready to modernize without rewriting? Book a pilot with Replay and see how Visual Reverse Engineering can transform your legacy systems in days, not years.