React Re-render Performance

When invoked, you measure how every React component in a project behaves under parent re-renders and render a light-theme, Lighthouse-style website that scores each component 0-100. The engine bundles a harness with the project's own React, mounts each component in jsdom inside a <Profiler>, re-renders its parent a fixed number of times with referentially-stable props, and uses React's onRender callback to count the re-renders that did real work despite unchanged props — the wasted renders that React.memo would skip — plus mount and per-render durations.

Global Context

• User request / scope: $ARGUMENTS — empty, or a path to the React project root
• Engine: scripts/rerender.mjs (Node; only dependencies are esbuild and jsdom, installed beside the skill)
• Template: assets/template.html
• Output: rerender-report/index.html and rerender-report/data.json in the current directory

Rules

• Every number is measured at runtime, never estimated. If the engine cannot mount a component, it records that component as not-measured rather than inventing numbers.
• Read-only against the project. The skill only writes inside rerender-report/ (and a .rerender-tmp/ it deletes when done).
• The engine needs the project's node_modules (it mounts the project's real React and components). If they are missing, it cannot run.
• Do not add comments to any command you run.

Step 1 — Find the project

Decide the target from $ARGUMENTS:

• A path to a directory → that is the project root.
• Empty → use the current working directory.

Confirm it is a React project: a package.json exists and react is installed under node_modules. The engine scans src/ if present, otherwise the project root, for .jsx/.tsx (and React-flavored .js/.ts) component files, skipping node_modules, build output, tests, stories, and the entry file that calls createRoot/hydrateRoot.

Step 2 — Make sure dependencies are installed

The harness mounts the project's real React, so its dependencies must resolve:

test -d <project>/node_modules/react && echo present || echo missing

If missing, tell the user the engine needs an install first and offer to run it:

npm --prefix <project> install

Step 3 — Run the benchmark engine

Invoke the engine by its installed absolute path. It always writes rerender-report/ into the current working directory, so run it from where you want the report. Pass the project root:

cd <project> && node "$HOME/.claude/skills/rerender/scripts/rerender.mjs" .

The engine discovers the components, statically analyses each (memoization, hooks, inline props, array work in render), then bundles and runs the measurement harness once. It prints a per-component summary to stdout and writes the full report. If it cannot bundle the project (path aliases, exotic loaders) it explains why — relay that instead of producing a partial report.

Step 4 — Report back

Relay the stdout summary in your own words: the overall score, how many components are clean vs. wasting renders, the total wasted renders out of stressed updates, and the slowest component. Name the components that scored poorly and the one-line fix for each (wrap in React.memo, move heavy work into useMemo, stop passing inline object/array/function props). Point the user at the generated site:

rerender-report/index.html

Offer to open it. The page is self-contained — an overall score gauge, animated metric counters, a wasted-renders bar chart, a score distribution, an average-render-cost chart, and a sortable/filterable card per component with its score, metrics, a per-render sparkline, and concrete fixes — no server or network needed.

What the numbers mean

• Wasted renders — under N parent re-renders with unchanged props, the count of re-renders that still did measurable work. A correctly memoized component bails out and scores 0; an un-memoized one re-renders every time. This is the headline re-render lens.
• Avg / peak ms — actualDuration from React's Profiler. Measured in a React development profiling build (required for the Profiler to report timings), so absolute values run a little high but are honest and comparable across components.
• Score — 0-100, Lighthouse-style: green ≥90, orange 50-89, red <50. Driven mainly by the wasted-render ratio, plus render cost and static anti-patterns.
• Root component — the one rendered by createRoot has no real parent, so it is measured for mount cost only and exempt from the wasted-render test.