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Claude Code in Your Development Flow

Lesson 2: Refactoring Existing Code with Claude Code

Lesson Objectives

By the end of this lesson, students should be able to:

  • Frame a refactoring request that produces safe, targeted changes
  • Apply the plan-then-apply workflow for larger refactors
  • Validate that a refactored implementation preserves the original behavior
  • Identify when a refactor scope is too large for a single Claude Code task

Lesson Content

Framing refactoring requests.

Refactoring requests must specify: what to change, to what target state, and what must be preserved. Without all three, Claude makes judgment calls that may not match your intentions:

  • Weak: "Refactor this function"
  • Strong: "Refactor parseUserConfig() to use early returns instead of nested conditions. The public interface and all return values must remain unchanged. Tests in config.test.js must continue to pass."

The third component – what must be preserved – is the most commonly omitted. It is also the most important for safe refactoring.

The plan-then-apply workflow.

For any non-trivial refactor:

  1. Ask Claude to describe the refactoring plan: what it will change and how
  2. Review the plan against your intent
  3. Ask Claude to identify any tests that cover the code being refactored
  4. Apply the refactoring
  5. Run the tests
  6. Review the diff

This workflow prevents surprises and ensures you understand the change before it is applied.

Validating behavior preservation.

A refactored implementation that changes public behavior is not a refactoring – it is a change. Validate behavior preservation:

  • Existing tests pass (necessary but not sufficient – tests may not cover all behavior)
  • The public interface (function signatures, return types, error behavior) is unchanged
  • Edge cases behave identically (null inputs, empty collections, error conditions)

If the existing test coverage is weak, ask Claude to identify the edge cases that should be tested before refactoring – and add those tests first.

Recognizing scope creep in refactors.

Broad refactoring requests ("refactor this module") invite Claude to make many changes simultaneously – increasing the review surface and the risk of undetected behavior changes. Scope refactors to a single concern:

  • Naming only (rename variables/functions)
  • Structure only (early returns, extract functions)
  • Pattern migration only (async/await from callbacks)

Single-concern refactors produce clean, reviewable diffs. Mixed-concern refactors produce complex diffs that are hard to review confidently.

Practical Example

A developer needs to migrate a module from callbacks to async/await.

The module has 12 functions and reasonable test coverage.

Approach: (1) Ask Claude for a refactoring plan for one function – review the pattern it proposes. (2) Ask Claude to apply the plan to the first three functions only – run tests. (3) Review the diff carefully – three functions, clean diff, tests pass. (4) Repeat in batches of three until the full module is migrated.

This incremental approach keeps each review surface small and catches any behavior divergence after each batch.

A full-module refactor in one operation would produce a 200-line diff that is nearly impossible to review with confidence.

Safety Notes

Refactoring with Claude Code is faster than manual refactoring – which increases the risk that you review the diff less carefully. Resist the tendency to approve AI-generated refactors quickly because they look clean. Refactoring changes real behavior more often than it appears to. Run the full test suite and review the diff with the same rigor as any change.

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