# Refactor a Legacy Deno Module with Atlas in 2026

> Atlas helps Deno developers restructure old modules without changing behavior or breaking callers by leveraging `deno test` and precise code modifications.

Atlas empowers Deno developers in 2026 to refactor legacy modules safely by integrating directly with the Deno toolchain, including `deno test`, `deno fmt`, and `deno add (JSR)`. It maps public surfaces, pins behavior with existing tests, applies structural changes with precision, and re-verifies after each modification, ensuring no silent breakage.

## Key takeaways

- Atlas integrates directly with Deno's native toolchain, including `deno test`, `deno fmt`, and `deno add (JSR)`.
- Refactor Deno modules safely by mapping public APIs with `lsp documentSymbol` and tracking all callers with `lsp findReferences`.
- Atlas uses `apply_patch` for precise, context-aware code modifications in Deno files, preventing accidental drifts.
- Iterative testing with `deno test` after each change hunk prevents regressions and provides immediate feedback.
- Atlas's permission model aligns with Deno's, enhancing security for code changes and configuration management in `deno.json`.

## How to map a Deno module's public API and find callers?

Mapping a Deno module's public surface is the crucial first step in any refactor, ensuring you identify all external dependencies before making changes. Atlas uses its `lsp` tool's `documentSymbol` operation to enumerate exported symbols from your `src/legacy_module.ts` file, providing a clear overview of the module's public API in 2026.

Before restructuring any Deno module, Atlas first establishes a comprehensive understanding of its public interface and all its callers. Using the `lsp` tool, Atlas can perform a `documentSymbol` operation on your Deno source files, such as `src/legacy_module.ts`, to identify every exported function, class, or variable. This initial mapping provides a clear picture of the module's external contract. Once the public symbols are identified, Atlas then uses `lsp findReferences` on each of these exported symbols. This operation meticulously enumerates every single callsite across your Deno project, ensuring that no hidden dependencies are overlooked. Atlas tracks these identified callsites in a `todowrite` list, which serves as a living checklist, preventing a partially migrated module from being mistaken for a finished one. This systematic approach, deeply integrated with Deno's module resolution, minimizes the risk of silent breakage at unknown callsites.

## How to pin Deno module behavior before refactoring?

Pinning the existing behavior of a Deno module is essential to guarantee that refactoring efforts do not introduce regressions. Atlas achieves this by running your project's `deno test` suite via the `bash` tool, establishing a green baseline of expected behavior. This critical step, performed before any code modifications, ensures a verifiable starting point for your 2026 refactor.

A successful Deno refactor hinges on proving that the module's behavior remains unchanged. Atlas addresses this by first pinning the current behavior. It leverages the `bash` tool to execute your project's existing test suite using the native Deno test runner: `deno test`. This command is run behind a permission prompt, aligning with Deno's security model. The output of `deno test` establishes a 'green baseline' - a verified state where all existing tests pass. This baseline is recorded, providing a concrete reference point. Any subsequent changes introduced during the refactoring process must maintain this green baseline. If `deno test` reports failures after a modification, Atlas immediately flags a behavioral regression, allowing you to address it before proceeding. This rigorous, test-driven approach is fundamental to Atlas's safe refactoring workflow for Deno applications.

## How does Atlas apply structural changes to Deno code safely?

Applying structural changes to Deno code requires precision to avoid introducing errors or breaking existing functionality. Atlas uses its `apply_patch` tool, which anchors on context lines and old lines, to make targeted modifications. This method ensures that changes are applied only when the file's content matches the expected state, preventing accidental alterations to a drifted `src/legacy_module.ts` file in 2026.

When it's time to restructure the Deno module, Atlas employs the `apply_patch` tool for precise and safe code modifications. Unlike blind line-by-line edits, `apply_patch` operates by seeking each hunk's context and old_lines within the target file. This means that for a change to be applied, the surrounding code and the exact lines being replaced must match what Atlas expects. If the Deno file, such as `src/legacy_module.ts`, has drifted or been modified externally, `apply_patch` will fail with a "Failed to find context" error. This robust mechanism prevents Atlas from applying changes incorrectly to an outdated or unexpected file state, significantly reducing the risk of introducing subtle bugs. Every proposed edit is also presented as a unified diff for your approval before Atlas writes it, giving you full control over the structural changes to your Deno codebase.

## How to iteratively verify Deno refactors with `deno test`?

Iterative verification is a cornerstone of safe Deno refactoring, ensuring that each small change maintains module integrity. Atlas re-runs the `deno test` command using `bash` after every single hunk of code lands, not just once at the end. This granular approach provides immediate feedback, catching any behavioral regressions introduced by a specific modification in your 2026 Deno project.

The risk of silent breakage in a refactor is significantly mitigated by continuous verification. Atlas adopts an iterative testing strategy for Deno modules: after each individual hunk of code is successfully applied by `apply_patch`, Atlas immediately re-runs the `deno test` suite using the `bash` tool. This is a critical departure from a 'test once at the end' approach. By re-running `deno test` after every minor change, Atlas provides instant feedback on the impact of that specific modification. If a test fails, you know precisely which recent change caused the regression, making debugging and correction far more straightforward. This rapid feedback loop, powered by Deno's fast test runner, ensures that the module's behavior remains consistent throughout the entire refactoring process, building confidence with every step.

## How to track Deno call sites and refactoring progress?

Tracking remaining Deno call sites is vital to ensure a refactoring task is truly complete and no callers are left unmigrated. Atlas uses its `todowrite` tool to maintain a dynamic list of all identified call sites. This list prevents a partially migrated Deno module from being mistaken for a finished one, providing a clear roadmap for completing the refactor in 2026.

Refactoring a Deno module often involves updating numerous call sites across the codebase. Atlas helps manage this complexity by using its `todowrite` tool. After initially mapping all references with `lsp findReferences`, Atlas populates a `todowrite` list with every identified call site. As you or Atlas migrate each call site to the new module structure, the corresponding entry is removed from the list. This dynamic, persistent checklist ensures that no call site is forgotten or overlooked. The `todowrite` list acts as a clear indicator of progress, making it impossible to mistakenly consider a Deno module fully refactored when outstanding callers still exist. This systematic tracking provides transparency and accountability throughout the entire refactoring workflow.

## How does Atlas ensure secure Deno refactoring?

Atlas ensures secure Deno refactoring by aligning its permission-gated tool calls with Deno's native security model. Every Atlas tool execution, such as running `deno test` or `deno fmt`, is permission-gated against allow, ask, and deny rules. This mirrors Deno's `--allow-net` and `--allow-read` flags, providing granular control over agent actions in your 2026 Deno project.

Security is paramount when an AI agent modifies your codebase. Atlas is designed with a robust security model that perfectly complements Deno's built-in permission system. Every Atlas tool call, whether it's `lsp` for code analysis, `bash` for running `deno test`, or `apply_patch` for modifying files, is permission-gated. This means Atlas will ask for explicit permission before executing any action that could affect your system or code. For Deno projects, Atlas can even help tighten broad `--allow-all` tasks defined in your `deno.json` down to the specific `--allow-net` and `--allow-read` scopes that the code truly needs, enhancing your project's security posture. Furthermore, Atlas drafts a plan in a read-only agent and asks for approval before switching to a build agent, and computes a unified diff for every file edit, surfacing it for your approval before writing. This multi-layered approach ensures that all Deno code modifications are transparent, controlled, and secure.

## Steps

1. Ensure your Deno project contains a `deno.json` or `deno.jsonc` file for Atlas to read configuration and dependencies.
2. Ask Atlas to map the module's public surface using `lsp documentSymbol` on your target Deno file, e.g., `src/legacy_module.ts`.
3. For each exported symbol, instruct Atlas to find all references using `lsp findReferences` and add them to a `todowrite` list.
4. Run existing Deno tests with `atlas bash --allow-run "deno test"` to establish a green baseline of behavior.
5. Instruct Atlas to apply structural changes to the Deno module using `apply_patch`, reviewing each unified diff for approval.
6. After each `apply_patch` hunk lands, re-run `atlas bash --allow-run "deno test"` to verify that behavior remains unchanged.
7. As call sites are migrated to the new module structure, ask Atlas to update the `todowrite` list, marking them as complete.
8. Once the refactoring is complete, have Atlas run `deno fmt` and `deno lint` on the modified files, then stage and create a commit.

## FAQ

### How does Atlas ensure my Deno refactor doesn't break existing code?

Atlas uses `lsp findReferences` to map all callers before any changes, then verifies behavior by running `deno test` iteratively after each modification, ensuring no silent breakage.

### Can Atlas handle Deno's import maps and JSR dependencies?

Yes, Atlas reads your `deno.json` or `deno.jsonc` to understand import maps and JSR/npm specifiers, and can add dependencies with `deno add jsr:@std/...`.

### What Deno commands does Atlas use for testing and formatting?

Atlas directly invokes `deno test` for behavior verification and `deno fmt` for code formatting, behind permission prompts, ensuring native Deno toolchain integration.

### How does Atlas handle Deno's permission flags during refactoring?

Atlas can tighten broad `--allow-all` tasks to specific scopes like `--allow-net` and `--allow-read`, mirroring Deno's granular permission model and enhancing security.

### What if a Deno file drifts while Atlas is working on it?

Atlas's `apply_patch` tool anchors on context lines and will fail with "Failed to find context" if the target Deno file has drifted, preventing incorrect or unsafe applications.

### Can Atlas help me manage `deno.json` configuration?

Yes, Atlas reads your `deno.json` or `deno.jsonc` for tasks and dependencies, and can help manage them, including adding JSR packages with `deno add`.

### Does Atlas provide diffs for Deno code changes?

Yes, Atlas computes a unified diff for every file edit and surfaces it for your approval before writing, giving you full control over changes to your Deno codebase.

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Canonical HTML: https://runatlas.sh/resources/stacks/refactor-a-legacy-module-in-deno
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Licence: Atlas is proprietary with a free core. It is not open source and there is no public source repository.