Purpose

Writes and modifies JavaScript and TypeScript code in a single file or tightly-coupled file pair (source + test), following the project's existing conventions for module format, linting, typing, and test patterns.

When to use

• A plan or ticket requires creating or modifying a .js, .ts, .jsx, or .tsx file.
• A bug fix or feature addition targets a specific Node.js/TypeScript module.
• A test file needs to be created or updated alongside a source file.
• A package.json script, dependency, or configuration needs modification.

Do NOT use when

• The change spans 3+ files across different modules — use implementer-hub for coordination.
• The target language is Python, Rust, Go, or another non-JS/TS language.
• The task is gathering context, not writing code — use implementer-context.
• The task is running or evaluating tests — use qa-validation.

Operating procedure

1. Read the context bundle (from implementer-context) or the ticket description to identify: target file path, required changes, and acceptance criteria.
2. Run cat tsconfig.json 2>/dev/null || echo "no tsconfig" to detect TypeScript mode. Record: strict mode (true/false), target (ES2020, ESNext, etc.), module format (ESM/CJS), and path aliases.
3. Run cat package.json | grep -A5 '"type"' to determine if the project uses "type": "module" (ESM) or defaults to CJS. Record the module format.
4. Run cat .eslintrc* .prettierrc* biome.json 2>/dev/null | head -40 to extract lint and format rules: quote style, semicolons, indent width, trailing commas.
5. If the target file exists, read it fully. Identify: existing imports, exported symbols, function signatures, and inline types or interfaces.
6. If the target file does not exist, examine 2 sibling files in the same directory to extract the file template pattern: header comments, import ordering, export style.
7. Write the implementation following these conventions exactly: match the import style (named vs default, path aliases vs relative), match the export style, preserve the existing indentation and formatting.
8. If a test file is required, locate the corresponding test directory or co-located test pattern. Run ls *test* *spec* __tests__/ 2>/dev/null in the target directory. Create the test file using the same runner (jest, vitest, mocha, node:test) and assertion style found in existing tests.
9. Run the project linter on the changed file: npx eslint <file> --fix 2>/dev/null || npx biome check <file> --fix 2>/dev/null to auto-fix formatting issues.
10. Run the specific test file: npx jest <test-file> 2>/dev/null || npx vitest run <test-file> 2>/dev/null || node --test <test-file> 2>/dev/null. Record pass/fail.
11. If tests fail, read the error output, identify the root cause, fix the code, and re-run. Repeat up to 3 times.
12. Run git --no-pager diff --stat to confirm only the expected files were modified.

Decision rules

• If the project uses ESM ("type": "module" or .mjs), never use require() — always use import.
• If tsconfig.json has "strict": true, ensure all variables have explicit types and no any is introduced.
• If the project has path aliases (e.g., @/utils), use them instead of relative paths that traverse >2 levels.
• If no test runner is detected, write tests using Node.js built-in node:test and node:assert.
• If a dependency is needed that is not in package.json, add it with npm install <pkg> before importing it.
• Never modify files outside the scope specified in the ticket or plan.

Output requirements

1. Files Changed — list of created or modified files with a one-line summary of each change.
2. Convention Compliance — confirmation that the implementation matches: module format, lint rules, type strictness, and naming conventions.
3. Test Results — test file path, pass/fail count, and any error output from failures.
4. Dependency Changes — any packages added or removed, with justification.
5. Code Diff — the full git diff output for review.

References

• Node.js ESM documentation: https://nodejs.org/api/esm.html
• TypeScript strict mode: https://www.typescriptlang.org/tsconfig#strict
• Project-local tsconfig.json, package.json, and linter configs

Related skills

• implementer-context — provides the context bundle this skill consumes
• implementer-hub — dispatches work to this skill for multi-file changes
• planner — produces the tickets and plans this skill implements
• qa-validation — validates the output of this skill against acceptance criteria

Failure handling

• If the linter produces errors that --fix cannot resolve, list the remaining errors and apply manual fixes following the reported rule names.
• If the test runner is unrecognized, fall back to node --test with node:assert/strict.
• If TypeScript compilation fails, run npx tsc --noEmit <file> to get the exact error and fix type mismatches.
• If the target file has merge conflict markers (<<<<<<<), resolve the conflict before making any changes and flag this in the output.
• If the implementation requires breaking a public API, document the breaking change and suggest a migration path.