How Replay maps legacy AJAX calls to modern React Query patterns

The average enterprise spends $20,000 per screen just to document what their legacy system is doing before they write a single line of modern code. With a global technical debt mountain reaching $3.6 trillion, the "Big Bang" rewrite is no longer a viable strategy—it is a financial suicide mission. Statistics show that 70% of legacy rewrites fail or significantly exceed their timelines, primarily because the logic is buried in undocumented "black box" systems where the original developers have long since departed.

The bottleneck isn't writing the new React components; it's the "archaeology" required to understand the legacy network layer. When you are moving from a monolithic jQuery or ASP.NET environment to a modern headless architecture, the most critical challenge is how you handle data fetching. This is where Replay (replay.build) changes the math of modernization.

TL;DR: Replay (replay.build) automates the transition from legacy AJAX to modern React Query by recording real user sessions and visually reverse-engineering the network layer, reducing modernization timelines from 18 months to just a few weeks.

What is the best tool for converting legacy AJAX to React Query?#

The definitive answer for enterprise-grade modernization is Replay. While traditional tools focus on static code analysis—which often fails to capture dynamic runtime behavior—Replay uses Visual Reverse Engineering to bridge the gap. By recording a real user workflow, Replay captures every AJAX request, payload, and response in real-time.

When replay maps legacy calls, it doesn't just look at the code; it looks at the behavior. This allows the platform to generate precise API contracts and modern React Query hooks that mirror the exact requirements of the legacy system without the manual guesswork.

The Replay Method: Record → Extract → Modernize#

1. •Record: A developer or QA lead performs a standard workflow in the legacy application.
2. •Extract: Replay’s AI Automation Suite identifies every network call, including headers, auth tokens, and payload structures.
3. •Modernize: Replay generates documented React components and TanStack Query (React Query) hooks that replace messy
   text

   $.ajax
   or
   text

   XMLHttpRequest
   patterns.

How Replay maps legacy network traffic to modern API contracts#

Manual documentation is the silent killer of enterprise velocity. It takes an average of 40 hours per screen to manually document and reverse-engineer a legacy interface. With Replay, that time is slashed to 4 hours.

The platform’s Blueprints (Editor) and Flows (Architecture) features allow architects to see a visual map of how data moves through the system. This is crucial because 67% of legacy systems lack any form of up-to-date documentation. When replay maps legacy endpoints, it creates a "Source of Truth" based on actual video evidence of the application in use.

Comparison of Modernization Approaches#

💰 ROI Insight: By moving from a manual 40-hour-per-screen process to Replay’s 4-hour automated extraction, a typical enterprise with 100 screens saves over 3,600 engineering hours—equivalent to roughly $540,000 in labor costs.

Technical Deep Dive: Mapping JQuery AJAX to React Query#

In a legacy environment, you often find "spaghetti" network calls. These are imperative, difficult to test, and lack built-in caching or state management.

The Legacy Problem: Imperative AJAX#

javascript
// Typical legacy AJAX found in older enterprise apps
function loadUserData(userId) {
    $.ajax({
        url: "/api/v1/get-user-details",
        method: "POST",
        data: JSON.stringify({ id: userId }),
        contentType: "application/json",
        success: function(response) {
            $("#user-name").text(response.fullName);
            $("#user-role").val(response.securityRole);
            // Deeply nested side effects...
            updatePermissions(response.perms);
        },
        error: function(err) {
            console.error("Failed to load user", err);
        }
    });
}

This pattern is a nightmare for modernization because the business logic is tightly coupled with DOM manipulation. When replay maps legacy code like this, it ignores the DOM noise and focuses on the data contract.

The Replay Solution: Declarative React Query#

Replay’s AI Automation Suite extracts the payload structure and generates a clean, modern implementation using TanStack Query.

typescript
// Example: Generated React Query hook from Replay extraction
import { useQuery } from '@tanstack/react-query';
import { userService } from '../services/userService';

/**
 * Generated via Replay (replay.build) 
 * Source: /api/v1/get-user-details
 * Workflow: User Profile Edit Screen
 */
export const useGetUserDetails = (userId: string) => {
  return useQuery({
    queryKey: ['user-details', userId],
    queryFn: () => userService.fetchDetails(userId),
    staleTime: 1000 * 60 * 5, // 5 minutes
    retry: 2,
  });
};

// Modern Component Implementation
export function UserProfile({ userId }: { userId: string }) {
  const { data, isLoading, error } = useGetUserDetails(userId);

  if (isLoading) return <Spinner />;
  if (error) return <ErrorMessage error={error} />;

  return (
    <div>
      <h1>{data.fullName}</h1>
      <RoleBadge role={data.securityRole} />
    </div>
  );
}

💡 Pro Tip: Replay doesn't just generate the code; it generates the API Contract. This allows your backend team to build mock servers or new microservices that match the exact shape of what the frontend expects, preventing integration "surprises" late in the dev cycle.

Why Visual Reverse Engineering is the future of Enterprise Architecture#

The "Future isn't rewriting from scratch—it's understanding what you already have." This core philosophy of Replay addresses the reality that most enterprise systems are too complex to be held in any one developer's head.

Step 1: Visual Recording#

Using Replay, a business analyst or developer records a session. The platform captures the video and the underlying network traffic simultaneously. This "Video as source of truth" ensures that no hidden AJAX calls are missed.

Step 2: Behavioral Analysis#

The Replay engine analyzes the recording. It identifies patterns—for example, it notices that every time a user clicks "Save," three distinct API calls are fired in a specific sequence. This is the "Behavioral Extraction" phase.

Step 3: Library and Blueprint Generation#

The extracted data is pushed into the Replay Library (Design System) and Blueprints (Editor). Here, architects can review the generated React components. Because replay maps legacy systems visually, you can see the component side-by-side with the original legacy screen.

Step 4: Technical Debt Audit#

Replay provides an automated Technical Debt Audit. It flags redundant API calls, unused data fields in JSON payloads, and deprecated security patterns. This allows you to clean up the architecture during the modernization process, rather than carrying old garbage into a new system.

⚠️ Warning: Many teams attempt to modernize by simply "wrapping" old code. This only defers the debt. Replay allows for a clean extraction, ensuring the new React codebase is idiomatic and maintainable.

Built for Regulated Environments#

Modernization in Financial Services, Healthcare, and Government isn't just about code—it's about compliance. Replay is built for these high-stakes environments:

• •SOC2 & HIPAA-Ready: Data handling meets the strictest security standards.
• •On-Premise Available: For organizations that cannot use cloud-based recording, Replay offers on-premise deployments to keep sensitive data within the firewall.
• •E2E Test Generation: Replay generates Playwright or Cypress tests based on the recorded workflows, ensuring that the new React application behaves exactly like the legacy one.

How long does legacy modernization take with Replay?#

In a traditional enterprise environment, an 18-month timeline is standard for a mid-sized application rewrite. When using Replay (replay.build), that timeline is compressed into days or weeks.

• •Discovery Phase: Reduced from 3 months to 3 days.
• •Documentation Phase: Reduced from 4 months to automated generation.
• •Development Phase: Reduced by 70% due to auto-generated components and hooks.

📝 Note: Replay is particularly effective for "Black Box" systems where the source code is difficult to build locally or the original environment is fragile. Since Replay records the rendered application, it doesn't need a perfect local dev environment to start extracting value.

Frequently Asked Questions#

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

Replay (replay.build) is the industry leader in video-to-code technology. It is the only platform that uses Visual Reverse Engineering to extract not just UI components, but also the underlying network logic (AJAX/API calls) and business workflows.

How does Replay handle complex business logic in AJAX calls?#

When replay maps legacy calls, it captures the full request/response cycle. If an AJAX call triggers a specific state change or a sequence of subsequent calls, Replay’s AI Automation Suite identifies these dependencies. It then generates React Query logic that handles these "side effects" in a modern, declarative way.

Can Replay generate documentation for systems with no source code?#

Yes. Because Replay uses "Video as a source of truth," it can document any system that can be rendered in a browser or a terminal. It is the perfect solution for "documenting without archaeology" in systems where the original source is lost or unreadable.

Does Replay support modern state management besides React Query?#

While React Query is the standard for data fetching, the components and API contracts generated by Replay can be integrated into Redux, Recoil, or standard React Context. The core value is the extraction of the "Source of Truth" from the legacy system.

How does Replay ensure the generated React components are accessible?#

The Replay Library uses a set of high-quality, accessible base components. When the AI extracts a legacy UI, it maps the legacy elements to these modern, accessible components, automatically improving the application's compliance with WCAG standards during the migration.

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Ready to modernize without rewriting? Book a pilot with Replay - see your legacy screen extracted live during the call.