API-First Transformation: Wrapping Legacy UI Logic in React Hooks Automatically

Technical debt is not a theoretical problem; it is a $3.6 trillion tax on global innovation that forces enterprise architects to choose between maintaining "zombie" systems or risking high-stakes rewrites. Most legacy modernization projects fail because teams try to rebuild the engine while the car is moving. Specifically, 70% of legacy rewrites fail or significantly exceed their timelines because the business logic is inextricably tangled within the UI layer.

The traditional path—manually auditing thousands of lines of undocumented code—is a death march. API-first transformation wrapping legacy logic into modern React hooks offers a third way: an automated bridge that extracts the functional intent of a system without needing to decipher the spaghetti code underneath.

By leveraging Replay, organizations can move from a monolithic "black box" to a modular, React-based architecture in weeks rather than years.

TL;DR: Manual legacy modernization takes 40+ hours per screen and has a 70% failure rate. By using Replay’s visual reverse engineering, enterprises can achieve an API-first transformation wrapping legacy UI logic into clean, documented React hooks automatically. This reduces modernization timelines from 18 months to mere weeks while providing 70% average time savings.

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The Strategic Necessity of API-First Transformation Wrapping Legacy Systems#

Legacy systems are often "undocumented by design." According to Replay's analysis, 67% of legacy systems lack up-to-date documentation, leaving developers to guess how complex business rules are triggered. When you attempt an api-first transformation wrapping legacy logic, you aren't just changing the UI; you are defining the contract between the user's intent and the system's data.

Industry experts recommend a "Sidecar" or "Strangler Fig" pattern for modernization. However, the bottleneck is always the same: identifying the state transitions. In a 20-year-old PowerBuilder or JSP application, the logic for "Calculate Insurance Premium" isn't in a clean API—it’s buried in a

event that performs 50 database lookups and 100 conditional checks.

The Cost of Manual Extraction#

When an engineer manually attempts to wrap legacy logic into a modern hook, they must:

1. •Trace the network requests (if they exist).
2. •Map the DOM elements to state variables.
3. •Reverse engineer the validation logic.
4. •Manually write the TypeScript interfaces.

This process takes an average of 40 hours per screen. With Replay, this is reduced to 4 hours.

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Defining the Modernization Stack#

Before diving into the implementation, we must define the core technology driving this shift.

Video-to-code is the process of converting high-fidelity screen recordings of legacy software interactions into functional, documented React components, state management logic, and API contracts.

Visual Reverse Engineering is the methodology of using the UI as the "source of truth" to reconstruct the underlying business logic and data flows, bypassing the need for original source code access.

For teams in Financial Services or Healthcare, the apifirst transformation wrapping legacy systems must also happen within a secure perimeter. This is why Replay offers SOC2 and HIPAA-ready environments, including On-Premise deployment options for highly regulated sectors like Government and Telecom.

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Automating the API-First Transformation Wrapping Legacy Code with Replay#

The core of Replay's value proposition lies in its ability to observe a user workflow and generate the "Flows" and "Blueprints" necessary for a modern React application. Instead of writing a

hook from scratch, Replay observes the inputs and outputs of the legacy UI and generates a functional wrapper.

Step 1: Recording the Workflow#

A subject matter expert (SME) records a standard workflow—for example, processing a claims adjustment. Replay captures the DOM mutations, the network traffic, and the state changes.

Step 2: Extracting the "Ghost API"#

Most legacy systems don't have RESTful APIs. They have "Ghost APIs"—form submissions or SOAP calls that act as the data layer. Replay's AI Automation Suite identifies these patterns and suggests a modern API contract.

Step 3: Generating the React Hooks#

Replay generates TypeScript hooks that encapsulate the logic. This is where the apifirst transformation wrapping legacy becomes tangible. You move from a "Page-based" mindset to a "Hook-based" mindset.

Learn more about generating Design Systems from legacy recordings

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Technical Implementation: From Legacy Event to React Hook#

Let’s look at a concrete example. Imagine a legacy insurance portal where the "Calculate Rate" logic is trapped in a monolithic script.

The Legacy Mess (Conceptual)#

In the old system, you might have something like this buried in a 5,000-line file:

javascript
// Legacy JSP/jQuery snippet
function onCalculateClick() {
    var age = $('#input_age').val();
    var history = $('#select_history').val();
    
    // Hidden business logic
    if (age > 25 && history === 'clean') {
        $.ajax({
            url: '/cgi-bin/calc_premium.pl',
            data: { a: age, h: history },
            success: function(res) {
                $('#result_display').text(res.split('|')[1]);
            }
        });
    }
}

The Replay-Generated Transformation#

After recording this interaction, Replay generates a clean, type-safe React hook and component structure. The apifirst transformation wrapping legacy logic ensures that the new React UI doesn't care how the legacy backend works—it only cares about the hook's interface.

typescript
/**
 * Generated by Replay AI Automation Suite
 * Source: Insurance_Portal_Workflow_04.mp4
 * Description: Encapsulates the premium calculation logic from the legacy CGI-BIN endpoint.
 */

import { useState, useCallback } from 'react';

interface UsePremiumCalculator {
  calculate: (age: number, history: string) => Promise<void>;
  premium: string | null;
  isLoading: boolean;
  error: string | null;
}

export const usePremiumCalculator = (): UsePremiumCalculator => {
  const [premium, setPremium] = useState<string | null>(null);
  const [isLoading, setIsLoading] = useState(false);
  const [error, setError] = useState<string | null>(null);

  const calculate = useCallback(async (age: number, history: string) => {
    setIsLoading(true);
    try {
      // Replay maps the legacy 'cgi-bin' to a modern proxy or direct call
      const response = await fetch('/api/v1/modernized/calculate-premium', {
        method: 'POST',
        body: JSON.stringify({ age, history }),
      });
      
      const data = await response.json();
      setPremium(data.formattedPremium);
    } catch (err) {
      setError('Failed to calculate premium. Please check legacy connectivity.');
    } finally {
      setIsLoading(false);
    }
  }, []);

  return { calculate, premium, isLoading, error };
};

By using this hook, the frontend team can build a modern, responsive UI in React while the backend team works on eventually replacing the Perl script with a microservice. The apifirst transformation wrapping legacy logic provides the abstraction layer needed for parallel development.

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Why "Visual First" Beats "Code First" for Modernization#

Industry experts recommend visual reverse engineering because code lies, but workflows don't. A legacy codebase often contains "dead" logic—code that is never executed but remains in the repository. If you modernize by reading the code, you waste 30% of your time porting features that nobody uses.

According to Replay's analysis, recording a workflow ensures that you only modernize the "happy paths" and "edge cases" that actually occur in production. This is the essence of the Replay Library and Flows features.

The Replay Library (Design System)#

As you record workflows, Replay identifies recurring UI patterns (buttons, inputs, tables) and automatically populates a Design System. This ensures that your apifirst transformation wrapping legacy systems results in a consistent, themed UI across the entire enterprise.

Replay Flows (Architecture)#

Flows provide a bird's-eye view of how data moves through the application. It visualizes the state machine of your legacy system, allowing architects to see exactly where a React hook needs to be injected to replace a legacy transition.

Read about mapping complex enterprise flows

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Implementation Detail: Handling Complex State Transitions#

One of the hardest parts of apifirst transformation wrapping legacy logic is managing multi-step forms where the state is stored in the session or a hidden global variable. Replay’s Blueprints editor allows developers to "stitch" together multiple recordings into a single cohesive React hook.

Consider a multi-step mortgage application. The legacy system might refresh the page between every step. A modern React app needs to handle this as a single SPA (Single Page Application) flow.

typescript
// Example of a Replay-generated multi-step state hook
import { create } from 'zustand';

interface MortgageState {
  step: number;
  formData: Record<string, any>;
  setStep: (step: number) => void;
  updateData: (data: Record<string, any>) => void;
  submitToLegacy: () => Promise<void>;
}

export const useMortgageFlow = create<MortgageState>((set, get) => ({
  step: 1,
  formData: {},
  setStep: (step) => set({ step }),
  updateData: (data) => set((state) => ({ 
    formData: { ...state.formData, ...data } 
  })),
  submitToLegacy: async () => {
    const { formData } = get();
    // Replay identifies the sequence of legacy POST calls required
    await ReplayLegacyBridge.executeSequence('MORTGAGE_SUBMIT', formData);
  }
}));

This level of abstraction is what allows the 18-month average enterprise rewrite timeline to shrink into weeks. You aren't rewriting the database; you are rewriting the interface to the database.

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Security and Compliance in Regulated Industries#

For industries like Healthcare and Insurance, an apifirst transformation wrapping legacy logic cannot happen in a public cloud without extreme scrutiny. Replay is built for these environments:

1. •SOC2 Type II & HIPAA: Data privacy is baked into the platform.
2. •On-Premise Availability: Keep your legacy recordings and generated code within your own VPC.
3. •PII Masking: Replay’s recording engine can automatically mask sensitive user data during the capture phase, ensuring that developers only see the functional logic, not the private data.

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The Roadmap to a Modernized Enterprise#

How do you begin the apifirst transformation wrapping legacy systems? We recommend a four-phase approach:

1. •Discovery (Week 1): Record the top 20 most critical user workflows using Replay.
2. •Extraction (Week 2): Use the Replay AI Automation Suite to generate the initial Component Library and React Hooks.
3. •Refinement (Week 3-4): Use the Blueprints editor to refine the API contracts and connect them to your modern backend or legacy proxy.
4. •Deployment (Week 5+): Roll out the new React frontend, powered by the encapsulated legacy logic.

This "Visual-to-code" pipeline is the only way to combat the $3.6 trillion technical debt bubble. By focusing on the UI as the contract, you bypass the friction of legacy code analysis.

Explore the Replay Product Suite

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Frequently Asked Questions#

How does Replay handle undocumented logic during an API-first transformation wrapping legacy?#

Replay uses visual reverse engineering to observe the actual behavior of the application in real-time. By capturing DOM changes and network requests, it creates a functional map of the business logic that is independent of the original source code. This allows for the creation of accurate React hooks even when the underlying code is a "black box."

Can we use the generated hooks with our existing REST or GraphQL infrastructure?#

Yes. While Replay generates hooks that can interface with legacy endpoints, the "Blueprints" editor allows you to easily remap those hooks to modern REST or GraphQL APIs. This makes Replay an ideal tool for the transition phase of a "Strangler Fig" modernization strategy.

What is the security posture for regulated industries like Healthcare or Finance?#

Replay is built for high-compliance environments. It is SOC2 and HIPAA-ready, and we offer On-Premise deployment options. Additionally, our AI Automation Suite includes features for PII masking to ensure that sensitive data is never exposed during the reverse engineering process.

How much faster is Replay compared to manual modernization?#

According to Replay's analysis, manual screen-to-code conversion takes an average of 40 hours per screen. With Replay’s automated pipeline, that time is reduced to 4 hours per screen, representing a 70% to 90% time saving across the lifecycle of an enterprise project.

Does Replay require access to our legacy source code?#

No. Replay operates on the "Visual Layer." By recording the UI and the browser/client interactions, it can reconstruct the necessary React components and logic hooks without ever needing to read a single line of your legacy COBOL, Java, or .NET source code.

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Ready to modernize without rewriting from scratch? Book a pilot with Replay and see how visual reverse engineering can accelerate your transformation.