By the numbers

Data collected on 2026-04-30 from the main branch at commit f4e0d4ed0c (latest at time of generation: [enableInfiniteRenderLoopDetection] Add a flag to force throwing (#36357)).

Size

The runtime side of the monorepo (packages/) is around 539,000 lines of source across roughly 1,870 source files (mostly Flow-typed JavaScript). The React Compiler workspace (compiler/packages/) adds another 69,000 lines of TypeScript on top.

xychart-beta horizontal
  title "Source files by package area"
  x-axis ["react-devtools-shared", "react-reconciler", "react-dom-bindings", "compiler", "react-server", "react-native-renderer", "shared", "react-server-dom-webpack", "react-dom", "react"]
  y-axis "Files" 0 --> 700
  bar [620, 78, 145, 380, 60, 90, 110, 70, 80, 35]

(Counts include source, test, fixture, and golden files for the compiler — the compiler's snapshot fixtures dominate its file count.)

The biggest single source files are all in react-reconciler:

File	Lines
packages/react-reconciler/src/ReactFiberWorkLoop.js	~5,100
packages/react-reconciler/src/ReactFiberCommitWork.js	~5,000
packages/react-reconciler/src/ReactFiberHooks.js	~4,500
packages/react-reconciler/src/ReactFiberBeginWork.js	~4,500
packages/react-reconciler/src/ReactFiberCompleteWork.js	~2,000
packages/react-reconciler/src/ReactChildFiber.js	~1,900
packages/react-reconciler/src/ReactFiberApplyGesture.js	~1,400
packages/react-reconciler/src/ReactFiberLane.js	~1,200

These eight files together are about 5% of the runtime, and they account for the bulk of the work React does at runtime. See packages/react-reconciler for a tour.

Activity

Repository age:       12+ years (first commit May 29, 2013)
Total commits:        ~21,500 on main
Distinct authors:     ~1,970 over the project's lifetime
Commits in last 12 mo:~1,400
Commits in last 90 d: ~160
Commits in last 30 d: ~30

The project has averaged a steady ~1,500 commits/year for several years, and growth is roughly linear, not exponential — the codebase has matured into incremental refinement rather than periodic rewrites.

Churn hotspots (last 90 days)

File / area	Commits in last 90 d
packages/react-devtools-shared/src/backend/fiber/renderer.js	14
packages/shared/forks/ReactFeatureFlags.*.js (all variants combined)	50+
packages/shared/ReactFeatureFlags.js	10
compiler/fault-tolerance-overview.md	10
packages/react-reconciler/src/ReactFiberWorkLoop.js	7
compiler/packages/babel-plugin-react-compiler/src/Entrypoint/Pipeline.ts	7
packages/react-devtools-shared/src/devtools/store.js	6

The dominant editing pattern in any 90-day window is feature-flag flipping in packages/shared/forks/ReactFeatureFlags.*.js. This is by design: the team builds new features behind flags, ships them down release-channel-by-release-channel, and finally deletes the flag once it's everywhere. See reference/configuration for the channel layout.

DevTools is the second-most-active area, mostly because its renderer-walking code (packages/react-devtools-shared/src/backend/fiber/renderer.js) has to keep up with new fiber tags, hook implementations, and Activity/View Transition primitives.

Bot-attributed commits

Approximately 600 commits (≈ 3% of all commits) carry an explicit bot attribution — either a Co-authored-by: trailer pointing at a [bot] account, or a direct authoring [bot] email. The breakdown:

Bot	Approximate count
dependabot[bot] (direct authoring)	~96
Various bots in trailers (copilot[bot], github-actions[bot], dependabot[bot])	~500
Factory / Droid bot trailers	0 (this codebase)

This is a lower bound on AI-assisted work — inline IDE assistants leave no trace in git history.

Complexity

Some quick proxies for complexity, restricted to source files (excluding tests and fixtures):

• The react-reconciler package has the deepest call graph: ReactFiberWorkLoop.js reaches roughly 30 sibling files in packages/react-reconciler/src/, plus the host config, plus scheduler, plus shared.
• The most-imported single file in the runtime is packages/shared/ReactSymbols.js — every React-element-handling code path uses the REACT_ELEMENT_TYPE and friends defined there.
• The most-imported file in the compiler is compiler/packages/babel-plugin-react-compiler/src/HIR/HIR.ts (the central HIR types).

Tests

The test runner (scripts/jest/jest-cli.js) ran the runtime through approximately:

• 5 release channels (stable, experimental, www-modern, www-classic, next-major) on every push for the most heavily covered packages.
• A second axis: --build mode, which runs the same tests against the produced bundle artifacts in build/ rather than against source.

Internal test fixtures live next to the code they test under __tests__/ directories. The compiler additionally maintains thousands of golden-file fixtures under compiler/packages/babel-plugin-react-compiler/src/__tests__/fixtures/.

TODO/FIXME density

Source files in packages/ have on the order of a few hundred TODO/FIXME/XXX comments. They are not concentrated in any one package — most are short, file-local notes about tightening a check, removing a workaround once a feature flag flips, or revisiting a corner of the API. There is no central tracker for them.

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The numbers above are a snapshot, not a trend dashboard. For long-running history (eras, milestones, deprecations, rewrites), see lore.