react-scan

React Scan

Use targeted captures to optimize measured Studio interactions. Do not infer a performance problem from source alone when the workflow can be reproduced.

Record a capture

Start from the repository root:

bun run react-scan:capture -- --label <short-kebab-case-description>

Wait for Studio to finish building and settle. Perform one named interaction three times, then stop the command with Ctrl+C. Keep captures short so startup, HMR, and unrelated interactions do not dominate the data.

The command writes an ignored directory under out/react-scan/ containing:

• metadata.json: capture context, git state, browser, and tested URL.
• summary.json: ranked component costs, render causes, slow commits, and

profiling-hook status.

• events.ndjson: raw React Scan Lite events for deeper inspection.

out/react-scan/latest.json points to the newest capture.

React Scan replaces the React DevTools profiling-hook channel while active. Do not record with the React DevTools Timeline Profiler at the same time.

Analyze the evidence

Read metadata.json and summary.json first. Confirm the capture contains commit events and inspect profilingHooksStatuses; do not interpret a missing profiling channel as an idle application.

Prioritize components with high total self duration, repeated expensive renders, or a clear user-visible slow commit. Inclusive duration contains child work and can double-count a subtree, so use it to locate an expensive boundary, then use self duration and the raw tree to identify the actual work.

Use render causes as evidence, not automatic prescriptions:

• High parentRenderCount with no relevant prop, state, context, or hook change

suggests a stable memoization boundary.

• Repeated changed props suggest checking referential stability at the source

location before adding memoization.

• Hook, state, or context changes require following the owning update; wrapping

the component in memo() will not block them.

• Source locations identify JSX call sites. Search for the component definition

before editing.

For timeline-position findings, also read ../studio-perf/SKILL.md before changing subscriptions or memoization boundaries.

Verify an optimization

Preserve the baseline capture. After changing code, record the same interaction the same number of times with the same viewport and compare a new capture. Require an improvement in the relevant commit/component metrics without a regression in behavior. Do not optimize every re-render: cheap necessary renders are often preferable to added memoization complexity.

Report both capture directories, the measured before/after change, and the focused tests used to protect behavior.