Codemod React Pipeline

A safe, repeatable workflow for applying JSX / TSX / React codemods to large legacy codebases — from a 50-file rename to a 100k-file API migration. It composes the Codemod CLI (JSSG, ast-grep, workflows) into an inner/outer loop and gates every mass change behind a dry-run plus validation. For the why behind each rule it cites, see the sibling codemod best-practices reference.

When to Apply

Use this skill when:

• You're migrating a pattern across a large React/TS codebase (component/prop rename, deprecated API, import swap, hook migration) and a hand-run codemod jssg run ./t.ts ./src is too risky.
• You need the change to land incrementally and reviewably — per-batch commits, resumable on failure, with the build/typecheck/tests green at every checkpoint.
• You want to see and validate the findings (what would change, and whether it's safe) before touching files at scale.
• The transform must be idempotent and proven so before mass apply.

Don't use it for a one-file edit, or a change better done by hand in a single PR.

Workflow Overview

00 plan ─▶ 01 scaffold ─▶ 02 inner loop ─▶ 03 dry-run ─▶ 04 validate ─▶ 05 batched apply ─▶ 06 verify
   goal       templates      fixture tests     preview       gates          per-batch            final
   + blast    (transform/    + 1-file trial    + findings     (idempotency,  apply+verify+commit  assertions
   radius     rule/wf/tests)                    report        type/lint/test) (resumable)
                                                  │                                  │
                                                  └── sentinel ──── required by ──────┘
                              (on any failure: working tree reverted; fix and re-run — nothing committed yet)

A PreToolUse hook (hooks/hooks.json) blocks a broad codemod jssg run that lacks --dry-run when no dry-run sentinel exists — so step 5 (or an ad-hoc apply) cannot skip steps 3–4.

Requirements

• Codemod CLI — npm i -g codemod (or the scripts fall back to npx codemod). Provides jssg run/test and workflow run/validate/resume/status.
• git — the pipeline requires a clean tree and makes per-batch checkpoint commits.
• jq — the scripts read config.json with it.
• Your project's gate tooling — TypeScript, ESLint, Prettier, a test runner (whichever gates you enable in config.json).

Setup

Edit config.json for the target repo (see each field's _setup_instructions):

• src_globs: the files the codemod may touch (scopes blast radius + gates)
• language: tsx | jsx | typescript | javascript
• typecheck_cmd / lint_cmd / format_cmd / test_cmd: gate commands (empty disables a gate)
• batch_size: files per checkpoint in the outer loop
• gates: per-gate on/off toggles

Add your state_dir (default .codemod-pipeline) to the target repo's .gitignore.

Quick Reference

How to Use

1. From the target repo root, run 00-plan.sh "<goal>" and read the generated plan.md.
2. 01-scaffold.sh <name>, then implement codemods/<name>/transform.ts and its fixtures.
3. Iterate with 02-inner-loop.sh <name> --watch until fixtures pass; try --file <path> on a real file.
4. 03-dry-run.sh <name> and read the findings report; then 04-validate-findings.sh <name>.
5. Only after gates pass: 05-run-batched.sh <name> (resume by re-running it after any fix).
6. verify.sh <name> to confirm the migration is complete and idempotent.

See references/workflow.md for per-step inputs, outputs, failure handling, and rollback; references/inner-outer-loop.md for the mental model; and references/safety-and-scale.md for batching and rollback at 100k-file scale.

Gotchas

See gotchas.md. The big ones: workflow commit: checkpoints are cloud-only (the pipeline commits locally instead); always exclude generated/vendored trees; and design for idempotency from the first line.

Related Skills

• codemod — the best-practices reference (48 rules) this pipeline operationalizes. Steps here cite specific rules (e.g. test-run-on-subset-first, state-use-for-resumability, pattern-ensure-idempotency, security-minimize-capabilities).