React 19 Migration Safety Net: Upgrade Risk Before Runtime
A CLI that scans React codebases for removed APIs, unsafe hook patterns, migration blockers, and CI-enforceable readiness scores.
What this project is
React 19 Migration Safety Net is a CLI tool for scanning React applications before an upgrade. It looks for deprecated APIs, unsafe patterns, and migration blockers that can turn a React 19 upgrade into a runtime failure.
The project is aimed at frontend developers, tech leads, platform engineers, and QA engineers who need a clear migration readiness report instead of a manual search through a large codebase.
The problem
React 19 removes or changes APIs that worked in earlier versions. A common example is ReactDOM.render. Code that works in React 18 can fail after the upgrade because the API no longer exists.
In a small app, manual cleanup may be manageable. In a larger codebase, deprecated APIs can be scattered across hundreds of files. The risk is not only missing existing issues but also allowing new deprecated patterns to enter the codebase while the migration is in progress.
What the CLI detects
The tool scans for removed React DOM APIs, legacy lifecycle methods, string refs, old context patterns, PropTypes, React.createFactory, defaultProps on function components, and unsafe hook patterns.
Unsafe hook detection is important because migration work often touches component structure. Hooks called inside conditionals, loops, callbacks, or after early returns can create subtle bugs even if the app appears to compile.
Output model
The report includes file paths, line and column numbers, severity, category, rule IDs, human-readable explanations, suggested fixes, and a migration score from 0 to 100.
That structure gives the tool two useful modes. Developers can use it locally to find and fix issues. CI can use it as an enforcement gate that blocks critical problems from merging.
Why score and severity matter
Not every issue has the same urgency. A removed API such as ReactDOM.render is a critical blocker. A pattern such as defaultProps on function components may be a warning depending on the migration plan.
Severity helps teams prioritize. The score gives leads and reviewers a quick way to understand whether a codebase is close to migration-ready or still needs focused cleanup.
Demo workflow
The repo includes before and after demo source, a walkthrough, and comparison scripts. That makes the project easier to evaluate because someone reviewing it can see both the failure mode and the corrected state.
The CLI can analyze a target source directory and report deprecated APIs with exact locations. From there, the recommended fix text points toward changes such as switching to createRoot, replacing legacy lifecycle methods, or moving conditions inside hooks instead of around them.
What this demonstrates
This project demonstrates static analysis, migration tooling, CLI design, frontend platform thinking, and CI enforcement. It is not just a script that prints grep matches. It packages migration knowledge into a repeatable workflow that can support a team upgrade.
The main engineering lesson is that framework upgrades become safer when risk is measured before runtime.