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Use AI to help map a tangled JavaScript file and propose small extractions—but decide the module boundaries yourself and verify behavior after every change. The goal is not simply to create more files; it is to make responsibilities easier to understand while keeping observable behavior intact.
Start by defining what must not change
Refactoring changes a program’s internal structure without changing its observable behavior. Martin Fowler defines it as “a change made to the internal structure of software to make it easier to understand and cheaper to modify without changing its observable behavior” (Martin Fowler’s book page).
Before moving code, write down the behavior that matters: inputs and outputs, user-visible results, network requests, storage changes, events, and error handling. Use the project’s existing tests or add checks for important behavior if practical. Treat these as the boundary conditions for the cleanup—not as a promise that every behavior is already covered.
Use AI to understand the code, not to choose the architecture
An assistant can explain a selected section, identify likely dependencies, and suggest candidate refactorings. GitHub Docs says Copilot Chat can suggest ways to make code easier to understand and maintain; that describes the feature, not a guarantee that a generated change preserves behavior (GitHub Docs: Refactor code).
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Ask for an inventory before asking for edits
Give the assistant a manageable section and ask it to list the section’s purpose, inputs, outputs, side effects, and dependencies on other code. Then check that explanation against the implementation. An AI-generated summary can miss implicit coupling—for example, code that relies on a global variable, execution order, or a side effect elsewhere in the file.
Request a small, reviewable proposal
Once you understand the section, ask for one cohesive extraction at a time. Request the proposed new module, its exports, the import changes, and any assumptions. Review the diff yourself before accepting it. Avoid asking for a whole-file rewrite: a large, simultaneous change makes it harder to identify which edit altered behavior.
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Choose module boundaries around responsibilities
Group code by what it does and by the features that genuinely need to reuse it. A module might own a focused responsibility such as formatting, validation, or a particular data-access task. Keep related logic together when separating it would create more cross-module coordination than clarity.
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Do not treat file count or export count as a measure of modularity. A study by Paltoglou, Zafeiris, Diamantidis, and Giakoumakis examined an automated migration method on 19 open-source projects. For that method and sample, the authors reported that 78.6% of extracted features corresponded to reusable module-scope elements and a fourfold increase in reusable elements per project. They validated the results through code inspection and project test suites; these findings do not measure generative AI or predict what a different project will gain (2021 study: Automated Refactoring of Legacy JavaScript Code to ES6 Modules).
Extract one unit and check it before moving on
- Pick a cohesive section. Choose code with a clear responsibility and identifiable inputs and outputs. Note any globals, shared state, callbacks, or ordering assumptions.
- Create the module. Move the smallest useful unit and expose only the interface the rest of the application needs.
- Update its callers. Replace the old access path with imports, preserving call order and existing behavior.
- Review the diff. Look for accidental changes to conditions, defaults, error handling, side effects, and initialization order.
- Run relevant checks. Use the project’s tests and any behavior-focused checks available. Do not infer success from code that merely looks plausible.
- Repeat only after the change is understood. Keep each extraction small enough that a failure can be traced to a limited set of edits.
For a general treatment of behavior-preserving refactoring, Martin Fowler’s Refactoring: Improving the Design of Existing Code, second edition, includes JavaScript examples according to its publisher (InformIT: Refactoring, 2nd edition).
Check how your project interprets modules
Moving code into files does not by itself make those files compatible with the runtime. The correct setup depends on whether the project runs in a browser, Node.js, or a build system that transforms modules.
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For Node.js, identify ESM or CommonJS first
Node.js supports ECMAScript modules and CommonJS. Check the project’s existing conventions and its package.json before changing syntax. Node’s interpretation can depend on file extensions and the package-level type field: .mjs explicitly denotes ESM, while .cjs denotes CommonJS. In ESM, relative import specifiers need their full file extensions, such as ./utils.js (Node.js v24.x: ECMAScript modules).
Do not convert an entire project’s module system just to extract one responsibility unless the application’s runtime, dependencies, and deployment setup support that change. When the project already uses a bundler or framework, follow its established conventions.
For browser ESM, use a server and a module script
A browser entry point typically declares a module with <script type="module">. Serve the page through a local web server and confirm JavaScript files are served with an appropriate JavaScript MIME type. Opening a module page directly through file:// can trigger CORS errors, so it is not a reliable test setup. Browser modules also have module scope and run in strict mode, which can expose assumptions that worked in older script-based code (MDN: JavaScript modules).
Judge the cleanup by clarity and verified behavior
A useful result is a structure where each module has a comprehensible responsibility and the application still behaves as intended under the checks you actually ran. Record which boundaries you chose, what behavior you checked, and any remaining coupling or migration constraint. AI can help explain code and draft candidate edits, but the project’s architecture and the evidence that behavior was preserved remain the developer’s responsibility.
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