Modernizing Legacy Code with AI

Introduction

Every codebase ages. Implementations that were once best practices become "legacy" as frameworks evolve and ecosystems shift.

Modernizing legacy code is a persistent challenge for most teams. There is never enough time to pause feature work for a dedicated refactoring sprint, but ignoring the problem only lets technical debt compound. Many teams feel stuck in this cycle.

DevLoop Runner can dramatically accelerate legacy code modernization. By combining automated problem detection, AI-powered refactoring, and auto-generated tests, you can modernize incrementally while continuing regular development.

This article presents concrete approaches to legacy code modernization using AI.

What Is Legacy Code?

Defining Legacy Code

"Legacy code" does not just mean code written in an old language or framework. Code that fits any of the following descriptions qualifies:

• Code without tests - Impact of changes cannot be predicted
• Code using outdated patterns or libraries - High maintenance cost
• Code without documentation - No one fully understands it
• Code only one person can work on - Knowledge silo
• Code that breaks whenever it is changed - Fragile structure

Why Modernization Matters

Leaving legacy code untouched makes these problems worse over time:

Problem	Impact
Slower development	New features take longer to ship
Frequent bugs	Changes introduce unexpected regressions
Hiring difficulty	Outdated tech stacks repel talent
Security risk	End-of-life libraries with known vulnerabilities
Maintenance cost	Understanding old patterns consumes time

Big Bang vs. Incremental Modernization

There are two fundamental approaches to legacy code modernization.

Big Bang (Full Rewrite)

Replace all legacy code at once.

Pros:

• Completed in one effort
• No old/new code coexistence

Cons:

• Extremely high risk
• Regular development stops for an extended period
• Massive testing burden
• Rollback is painful if it fails

Incremental (Strangler Fig Pattern)

Gradually replace legacy code piece by piece.

Pros:

• Low risk
• Runs alongside normal development
• Each step can be verified independently
• Easy to roll back individual changes

Cons:

• Takes longer to complete
• Old and new code temporarily coexist

Loading diagram...

DevLoop Runner eliminates the main downside of incremental modernization -- the time it takes. You get the safety of incremental changes with speed that rivals a full rewrite. This is the core value proposition of AI-powered legacy modernization.

Detecting Legacy Code with Create Issue

DevLoop Runner's Issue Run includes a "Create Issue" feature that analyzes your repository and automatically creates Issues for detected problems.

How Create Issue Works

When you run Create Issue, the AI analyzes your codebase and detects issues such as:

• Usage of deprecated APIs and outdated patterns
• Modules with insufficient test coverage
• Duplicate code and unnecessary complexity
• Security concerns in the implementation
• Performance improvement opportunities

Prioritizing Detected Issues

Not every Issue generated by Create Issue needs immediate attention. Prioritize based on these criteria:

Priority	Criteria
Highest	Security risk present
High	Legacy code in frequently changed areas
Medium	Performance impact
Low	Code style consistency
Lowest	Working code with no planned changes

For more on Issue Run, see the Issue Run comprehensive guide and Auto Issue feature guide.

Automated Refactoring with Dev Run

Once Create Issue has identified problems, use Dev Run to execute refactoring automatically.

Writing Refactoring Issues

Refactoring Issues produce better results when they include specific details:

## Refactoring Target
- File: src/components/UserList.jsx
- Current: Class component with lifecycle methods
- Goal: Function component with Hooks

## Constraints
- Do not change the external interface (props, exports)
- All existing tests must pass
- No behavior changes (preserve existing behavior)

## Scope
- UserList component only
- Related child components are out of scope for this Issue

Key points:

• Be explicit about what to change and how
• Clearly state what must not change
• Limit scope to keep the PR manageable

See the Issue writing guide for more tips.

Choosing Execution Modes

Match execution modes to the nature of the refactoring:

Full phase works best for:

• Architecture-level changes
• Complex refactoring requiring design validation

Implementation only works best for:

• Well-defined pattern replacements (e.g., Class to Hooks)
• Code style standardization

Plan only works best for:

• Previewing the approach for large-scale refactoring

Concrete Examples: Common Modernization Patterns

jQuery to React Migration

## Issue Example

### Target
- File: src/pages/dashboard.js
- Current: jQuery DOM manipulation
- Goal: React component

### Migration Approach
- Replace jQuery DOM manipulation with React state management
- Replace AJAX calls with fetch API (or axios)
- Migrate event handlers to React's event system

### Constraints
- No visual or behavioral changes
- Keep existing CSS classes
- Do not change API endpoints

Running a Dev Run on this Issue produces a React component with equivalent behavior, complete with auto-generated tests for migration verification.

Class Components to Hooks

## Issue Example

### Target
- File: src/components/OrderForm.jsx
- Current: React Class Component (setState, componentDidMount, etc.)
- Goal: Function component + React Hooks

### Specific Conversions
- this.state → useState
- componentDidMount → useEffect
- this.handleChange → const handleChange
- componentWillUnmount cleanup → useEffect return

### Constraints
- Maintain the props interface
- All existing tests must pass

JavaScript to TypeScript

## Issue Example

### Target
- File: src/services/userService.js → src/services/userService.ts
- Current: JavaScript (no type definitions)
- Goal: TypeScript (strict mode)

### Typing Strategy
- Add types to all function parameters and return values
- Define data structures using interfaces
- Minimize use of any
- Use existing type definition files where available

### Constraints
- No behavior changes
- Minimize import path changes

Test-First Modernization Strategy

The biggest risk in legacy code modernization is breaking existing behavior. The key to minimizing this risk is test coverage.

The Test-First Approach

Loading diagram...

Step 1: Add Tests

Before any refactoring, create tests for the existing code. DevLoop Runner can auto-generate these tests.

## Issue Example: Add Tests

### Target
- File: src/services/paymentService.js

### Requirements
- Cover existing paymentService behavior with tests
- Include happy path, error cases, and boundary values
- Mock external API calls
- Target 80%+ coverage

### Note
- Do not modify the code (tests only)

Step 2: Verify Tests

Confirm all generated tests pass. If any fail, fix the tests first.

Step 3: Execute Refactoring

With tests in place, create a separate Issue for the refactoring and run Dev Run.

Step 4: Re-verify Tests

Confirm all tests still pass after refactoring. Failures indicate the refactoring may have changed existing behavior.

Why Separate Tests and Refactoring

Keeping test addition and refactoring in separate Issues is important:

• Separate PRs make review easier
• Tests merge first, establishing a safety net for the refactoring
• When problems arise, it is clear whether the issue is in the tests or the refactoring

Building a Migration Plan

Large-scale legacy modernization requires a structured plan.

Step 1: Assess the Current State

• Use Create Issue to auto-detect problem areas
• Measure current test coverage
• Map the impact radius of legacy code

Step 2: Set Priorities

• Prioritize by business impact
• Target frequently changed areas first
• Account for dependencies when ordering work

Step 3: Define Milestones

## Migration Plan Example: jQuery → React

### Phase 1 (Week 1-2): Foundation
- Set up React development environment
- Add tests for shared components
- Validate the migration pattern on one page

### Phase 2 (Week 3-6): Core Pages
- Dashboard (high traffic)
- User management screen
- Order list screen
- Add tests before each page migration

### Phase 3 (Week 7-8): Remaining Pages and Cleanup
- Migrate remaining pages
- Fully remove jQuery dependency
- Final testing and performance verification

Step 4: Accelerate with Parallel Processing

DevLoop Runner's parallel processing lets you migrate multiple pages simultaneously.

• Migrate pages with no interdependencies in parallel
• Create one Issue per page
• Run 3-5 Issues concurrently

Step 5: Visualize Progress

Share migration progress with the team to maintain momentum.

• Track migrated vs. remaining module counts
• Monitor test coverage trends
• Review progress in weekly retrospectives

Anti-Patterns to Avoid

Trying to Change Everything at Once

Large-scale changes in a single PR are hard to review and hard to debug when something goes wrong. Keep each PR focused on one change.

Refactoring Without Tests

Refactoring without tests means you cannot detect when existing behavior breaks. Always add tests first.

Ignoring the Old-New Interface

During incremental migration, old and new code coexist temporarily. Without carefully designing the interface between them, unexpected issues will arise.

Not Defining a Completion Criteria

Without clear milestones and a definition of "done," team motivation erodes. Set explicit targets and celebrate progress.

Conclusion

• Legacy code encompasses anything that is difficult to maintain: untested code, outdated patterns, knowledge silos, and fragile structures
• AI accelerates the incremental approach, combining safety with speed
• DevLoop Runner's Create Issue detects legacy code problems automatically, and Dev Run executes refactoring with auto-generated tests
• The test-first strategy -- adding tests before refactoring -- is essential for safe modernization
• Common migration patterns like jQuery to React, Class to Hooks, and JavaScript to TypeScript are well-suited for AI automation
• Build a migration plan with milestones, split work into focused Issues, and use parallel processing for speed
• Keep each PR to a single change and separate test additions from refactoring

Legacy code modernization may seem daunting, but with DevLoop Runner driving incremental progress, you can steadily improve your codebase without pausing regular development.