codemod

# Codemod Best Practices Comprehensive best practices guide for Codemod (JSSG, ast-grep, workflows), designed for AI agents and LLMs. Contains 48 rules across 11 categories, prioritized by impact to guide automated refactoring and code generation. ## When to Apply Reference these guidelines when: - Writing new codemods with JSSG or ast-grep - Designing workflow configurations for migrations - Debugging pattern matching or AST traversal issues - Reviewing codemod code for performance and safety - Setting up test fixtures for transform validation ## Rule Categories by Priority | Priority | Category | Impact | Prefix | |----------|----------|--------|--------| | 1 | AST Understanding | CRITICAL | `ast-` | | 2 | Pattern Efficiency | CRITICAL | `pattern-` | | 3 | Parsing Strategy | CRITICAL | `parse-` | | 4 | Node Traversal | HIGH | `traverse-` | | 5 | Semantic Analysis | HIGH | `semantic-` | | 6 | Edit Operations | MEDIUM-HIGH | `edit-` | | 7 | Workflow Design | MEDIUM-HIGH | `workflow-` | | 8 | Testing Strategy | MEDIUM | `test-` | | 9 | State Management | MEDIUM | `state-` | | 10 | Security and Capabilities | LOW-MEDIUM | `security-` | | 11 | Package Structure | LOW | `pkg-` | ## Quick Reference ### 1. AST Understanding (CRITICAL) - [`ast-explore-before-writing`](references/ast-explore-before-writing.md) - Use AST Explorer before writing patterns - [`ast-understand-named-vs-anonymous`](references/ast-understand-named-vs-anonymous.md) - Understand named vs anonymous nodes - [`ast-use-kind-for-precision`](references/ast-use-kind-for-precision.md) - Use kind constraint for precision - [`ast-field-access-for-structure`](references/ast-field-access-for-structure.md) - Use field access for structural queries - [`ast-check-null-before-access`](references/ast-check-null-before-access.md) - Check null before property access ### 2. Pattern Efficiency (CRITICAL) - [`pattern-use-meta-variables`](references/pattern-use-meta-variables.md) - Use meta variables for flexible matching - [`pattern-avoid-overly-generic`](references/pattern-avoid-overly-generic.md) - Avoid overly generic patterns - [`pattern-combine-with-rules`](references/pattern-combine-with-rules.md) - Combine patterns with rule operators - [`pattern-use-constraints`](references/pattern-use-constraints.md) - Use constraints for reusable matching logic - [`pattern-use-relational-patterns`](references/pattern-use-relational-patterns.md) - Use relational patterns for context - [`pattern-ensure-idempotency`](references/pattern-ensure-idempotency.md) - Ensure patterns are idempotent ### 3. Parsing Strategy (CRITICAL) - [`parse-select-correct-parser`](references/parse-select-correct-parser.md) - Select the correct parser for file type - [`parse-handle-embedded-languages`](references/parse-handle-embedded-languages.md) - Handle embedded languages with parseAsync - [`parse-provide-pattern-context`](references/parse-provide-pattern-context.md) - Provide context for ambiguous patterns - [`parse-early-return-non-applicable`](references/parse-early-return-non-applicable.md) - Early return for non-applicable files ### 4. Node Traversal (HIGH) - [`traverse-use-find-vs-findall`](references/traverse-use-find-vs-findall.md) - Use find() for single match, findAll() for multiple - [`traverse-single-pass-collection`](references/traverse-single-pass-collection.md) - Collect multiple patterns in single traversal - [`traverse-use-stopby-for-depth`](references/traverse-use-stopby-for-depth.md) - Use stopBy to control traversal depth - [`traverse-use-siblings-efficiently`](references/traverse-use-siblings-efficiently.md) - Use sibling navigation efficiently - [`traverse-cache-repeated-lookups`](references/traverse-cache-repeated-lookups.md) - Cache repeated node lookups ### 5. Semantic Analysis (HIGH) - [`semantic-use-file-scope-first`](references/semantic-use-file-scope-first.md) - Use file scope semantic analysis first - [`semantic-check-null-results`](references/semantic-check-null-results.md) - Handle null semantic analysis results - [`semantic-verify-file-ownership`](references/semantic-verify-file-ownership.md) - Verify file ownership before cross-file edits - [`semantic-cache-cross-file-results`](references/semantic-cache-cross-file-results.md) - Cache semantic analysis results ### 6. Edit Operations (MEDIUM-HIGH) - [`edit-batch-before-commit`](references/edit-batch-before-commit.md) - Batch edits before committing - [`edit-preserve-formatting`](references/edit-preserve-formatting.md) - Preserve surrounding formatting in edits - [`edit-handle-overlapping-ranges`](references/edit-handle-overlapping-ranges.md) - Handle overlapping edit ranges - [`edit-use-flatmap-for-conditional`](references/edit-use-flatmap-for-conditional.md) - Use flatMap for conditional edits - [`edit-add-imports-correctly`](references/edit-add-imports-correctly.md) - Add imports at correct position ### 7. Workflow Design (MEDIUM-HIGH) - [`workflow-order-nodes-by-dependency`](references/workflow-order-nodes-by-dependency.md) - Order nodes by dependency - [`workflow-use-matrix-for-parallelism`](references/workflow-use-matrix-for-parallelism.md) - Use matrix strategy for parallelism - [`workflow-use-manual-gates`](references/workflow-use-manual-gates.md) - Use manual gates for critical steps - [`workflow-validate-before-run`](references/workflow-validate-before-run.md) - Validate workflows before running - [`workflow-use-conditional-steps`](references/workflow-use-conditional-steps.md) - Use conditional steps for dynamic workflows ### 8. Testing Strategy (MEDIUM) - [`test-use-fixture-pairs`](references/test-use-fixture-pairs.md) - Use input/expected fixture pairs - [`test-cover-edge-cases`](references/test-cover-edge-cases.md) - Cover edge cases in test fixtures - [`test-use-strictness-levels`](references/test-use-strictness-levels.md) - Choose appropriate test strictness level - [`test-update-fixtures-intentionally`](references/test-update-fixtures-intentionally.md) - Update test fixtures intentionally - [`test-run-on-subset-first`](references/test-run-on-subset-first.md) - Test on file subset before full run ### 9. State Management (MEDIUM) - [`state-use-for-resumability`](references/state-use-for-resumability.md) - Use state for resumable migrations - [`state-make-transforms-idempotent`](references/state-make-transforms-idempotent.md) - Make transforms idempotent for safe reruns - [`state-log-progress-for-observability`](references/state-log-progress-for-observability.md) - Log progress for long-running migrations ### 10. Security and Capabilities (LOW-MEDIUM) - [`security-minimize-capabilities`](references/security-minimize-capabilities.md) - Minimize requested capabilities - [`security-validate-external-inputs`](references/security-validate-external-inputs.md) - Validate external inputs before use - [`security-review-before-running-third-party`](references/security-review-before-running-third-party.md) - Review third-party codemods before running ### 11. Package Structure (LOW) - [`pkg-use-semantic-versioning`](references/pkg-use-semantic-versioning.md) - Use semantic versioning for packages - [`pkg-write-descriptive-metadata`](references/pkg-write-descriptive-metadata.md) - Write descriptive package metadata - [`pkg-organize-by-convention`](references/pkg-organize-by-convention.md) - Organize package by convention ## How to Use Read individual reference files for detailed explanations and code examples: - [Section definitions](references/_sections.md) - Category structure and impact levels - [Rule template](assets/templates/_template.md) - Template for adding new rules ## Full Compiled Document For a complete guide with all rules expanded, see [AGENTS.md](AGENTS.md).