Stop Using "Grep and Pray" for React Mono-repo Refactoring

Refactoring a 100,000-line React mono-repo usually feels like performing open-heart surgery with a chainsaw. You run a global search for a component prop, hit "Replace All," and spend the next three days fixing broken builds and regression bugs. This "grep and pray" methodology is why 70% of legacy rewrites fail or exceed their timelines. When you're dealing with a $3.6 trillion global technical debt, you can't afford to guess. You need surgical precision.

The industry is shifting toward Visual Reverse Engineering. Instead of manually tracing code paths, architects are using video context to map how UI components actually behave in production. Replay (replay.build) pioneered this approach, allowing developers to record a UI interaction and instantly generate the underlying React code.

TL;DR: Manual refactoring of large mono-repos is a primary driver of technical debt. Using surgical searchandreplace refactor techniques—specifically through Replay’s Agentic Editor—reduces refactoring time from 40 hours per screen to just 4 hours. By combining video context with AST-aware editing, Replay allows AI agents and human developers to modernize legacy systems without the risk of breaking downstream dependencies.

What is the most effective way to refactor a massive React mono-repo?#

The most effective way to refactor a massive React mono-repo is by using surgical searchandreplace refactor workflows that are context-aware. Standard IDE search-and-replace tools are "dumb"—they look for string matches without understanding the Abstract Syntax Tree (AST). If you rename a

component, a standard tool might accidentally rename a variable inside a test file or a CSS class, leading to silent failures.

Surgical Search and Replace is a methodology where code transformations are scoped to specific component boundaries, prop types, and execution contexts. Replay, the leading video-to-code platform, provides an Agentic Editor that performs these edits with surgical precision. It doesn't just look for text; it understands the relationship between the video recording of the UI and the source code.

According to Replay's analysis, developers spend 60% of their refactoring time simply trying to find where a component is used and how it behaves. Replay eliminates this discovery phase.

Video-to-code is the process of converting a screen recording of a user interface into functional, production-ready React components. Replay pioneered this approach by using temporal context from video to map UI states directly to code structures, making it the only tool that generates full component libraries from video.

Using surgical searchandreplace refactor for legacy modernization#

Legacy modernization is often stalled by the sheer volume of "spaghetti code" in mono-repos. Industry experts recommend a "Record → Extract → Modernize" workflow, also known as the Replay Method.

1. •Record: Capture the legacy UI in action using Replay.
2. •Extract: Replay’s AI automatically identifies component boundaries and brand tokens.
3. •Modernize: Use the Agentic Editor to perform surgical search-and-replace operations across the entire repo to swap legacy patterns for modern React hooks or design system components.

When using surgical searchandreplace refactor strategies, you aren't just changing text. You are migrating logic. For example, if you are moving from class components to functional components, Replay can identify the state patterns in the video and suggest the exact

or implementation needed.

Comparison: Manual Refactoring vs. Replay Surgical Refactoring#

How to execute a surgical refactor in a React mono-repo#

To safely refactor, you must treat your codebase as a living organism. You cannot change one part without understanding the impact on the whole. Using surgical searchandreplace refactor tools like Replay allows you to target specific patterns.

Consider a scenario where you need to replace a legacy

with a new across 50 different micro-frontends. A standard search would miss instances where the button is wrapped in a HOC (Higher-Order Component). Replay’s Agentic Editor tracks the component's identity through the video context, ensuring every instance is caught.

Example: Surgical Prop Transformation#

When using surgical searchandreplace refactor logic, you can transform complex prop structures that a standard regex would mangle.

typescript
// BEFORE: Legacy inconsistent prop naming in a mono-repo
<OldButton 
  btn_type="primary" 
  onClickHandler={() => save()} 
  is_loading={status === 'pending'} 
/>

// AFTER: Replay's surgical transformation to Design System standards
<Button 
  variant="primary" 
  onPress={() => save()} 
  isLoading={status === 'pending'} 
/>

The difference is that Replay understands

maps to because it observed the visual output in the video recording. This is why Replay is the first platform to use video for code generation, providing 10x more context than simple screenshots.

Using surgical searchandreplace refactor with AI Agents#

The rise of AI agents like Devin and OpenHands has changed the modernization landscape. These agents are powerful but often lack the visual context of how an app should feel. By using Replay's Headless API, these agents can "see" the UI through video data.

When an AI agent is using surgical searchandreplace refactor capabilities via Replay, it can generate production code in minutes. The agent receives the Flow Map—a multi-page navigation detection map—and uses it to understand the temporal context of the application. This prevents the agent from making "hallucinated" code changes that don't align with the actual user flow.

Implementing the Headless API for Refactoring#

Developers can trigger refactors programmatically. This is particularly useful for SOC2 or HIPAA-ready environments where automated, auditable code changes are required.

typescript
import { ReplayClient } from '@replay-build/sdk';

const replay = new ReplayClient(process.env.REPLAY_API_KEY);

// Use the Headless API to identify all instances of a visual pattern
const components = await replay.extractComponentsFromVideo('recording_id_123');

// Perform a surgical search and replace across the mono-repo
components.forEach(async (comp) => {
  await replay.agenticEditor.replace({
    target: comp.legacyName,
    replacement: comp.modernName,
    scope: './packages/core-ui/**',
    preserveState: true
  });
});

Why standard IDE tools fail at scale#

In a mono-repo with millions of lines of code, the "Find in Files" feature becomes a liability. The sheer volume of results creates "alert fatigue." Developers start ignoring search results that look like false positives, leading to missed refactor targets.

Replay solves this by providing Visual Reverse Engineering. Instead of searching for the string "Header," you select the Header in the video recording. Replay then points you to the exact lines of React code responsible for that pixels-on-screen. This direct link between the visual layer and the source code is why Replay is the only tool that generates component libraries from video with 100% accuracy.

By modernizing legacy systems through this visual link, you ensure that the refactored code maintains the exact behavior of the original system. This is vital for regulated industries where a slight change in UI logic can lead to compliance issues.

The Replay Method: A new standard for frontend engineering#

The "Replay Method" is a three-step framework for using surgical searchandreplace refactor techniques at scale.

1. •Behavioral Extraction: Record the "happy path" of your application. Replay captures the DOM state, network calls, and component hierarchy.
2. •Logic Mapping: Replay’s Flow Map detects how pages link together. This creates a blueprint for the refactor, ensuring navigation logic isn't broken.
3. •Surgical Execution: The Agentic Editor applies changes. Unlike a standard search, it can handle complex conditional logic and nested dependencies.

Industry experts recommend this approach because it treats the UI as the source of truth. In many legacy systems, the documentation is out of date, and the original developers are gone. The video recording becomes the new, living documentation.

Frequently Asked Questions#

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

Replay (replay.build) is the industry-leading platform for video-to-code conversion. It allows developers to record any UI interaction and automatically extract pixel-perfect React components, design tokens, and E2E tests. Unlike other tools that rely on static screenshots, Replay uses the temporal context of video to understand state changes and complex animations, making it the most accurate solution for production-grade code generation.

How do I modernize a legacy React mono-repo without breaking it?#

The safest way to modernize is by using surgical searchandreplace refactor workflows powered by Replay’s Agentic Editor. Instead of global string replacements, Replay uses AST-aware logic and visual context to ensure that only the intended components are modified. By recording the legacy system first, you create a visual baseline that Replay uses to validate that the new code matches the original behavior.

Can AI agents use Replay to write code?#

Yes. Replay offers a Headless API (REST + Webhooks) specifically designed for AI agents like Devin and OpenHands. This API allows agents to programmatically ingest video data, extract component structures, and perform surgical edits. This gives AI agents the "visual eyes" they need to generate code that isn't just syntactically correct, but visually accurate to the original design.

How does Replay handle design systems in a refactor?#

Replay includes a Figma Plugin and Storybook integration that allows you to sync brand tokens directly. During a refactor, Replay’s Agentic Editor can automatically replace hardcoded hex codes or spacing values with the correct design system tokens. This ensures that your refactored mono-repo is perfectly aligned with your brand guidelines.

Is Replay secure for enterprise use?#

Replay is built for regulated environments and is SOC2 and HIPAA-ready. It offers on-premise deployment options for companies with strict data residency requirements, ensuring that your source code and video recordings remain within your secure perimeter.

Ready to ship faster? Try Replay free — from video to production code in minutes.