Every React Native developer has watched JS FPS tank and asked: what is using it all? JS Top answers live:

#  TASK ORIGIN                        5S MS   %BUSY  CALLS  AVG   MAX
1  setInterval ← startPolling         1240    41.2   50     24.8  31.0
2  Promise.then ← processTodos         610    20.3   10     61.0  75.2
3  requestAnimationFrame ← rafLoop     402    13.4   300    1.3   4.1
4  unattributed (events / React)       255     8.5   –      –     –

• Pure JavaScript. No native module, no debugger, no dev client — works in Expo Go and even release builds.
• Zero overhead when closed. The engine only runs while the tool is open (or a Buoy Desktop dashboard is watching).
• Honest accounting. Exclusive-time bookkeeping (nested callbacks never double-count), a calibrated busy probe for true thread occupancy, and an explicit unattributed row for work no JS wrapper can see.
• Blocking-task attribution. Every 50ms+ freeze (via React Native's longtask observer) is matched to the callback that overlapped it.

Install

npm install @buoy-gg/js-top

With Buoy auto-discovery, that's it — JS TOP appears in your floating menu. Explicit registration:

import { FloatingDevTools } from "@buoy-gg/core";
import { jsTopPreset, jsTopModalPreset } from "@buoy-gg/js-top";

<FloatingDevTools apps={[jsTopModalPreset, jsTopPreset]} />

How it works

You can't sample a blocked JS thread from JS — so JS Top doesn't sample. It wraps every entry point work can take onto the thread (setTimeout / setInterval / setImmediate / requestAnimationFrame / queueMicrotask / Promise reactions / legacy-bridge call-ins) and deterministically times each callback, attributing cost to where the callback was scheduled from — with lazy stack capture so hot origins get named setInterval ← startPolling without per-call overhead.

On the New Architecture, touch/React-commit work enters through paths pure JS can't wrap; that time shows as unattributed (the in-tool banner explains). Timers, rAF, microtasks and Promise chains are always fully attributed.

Buoy Desktop + AI

• The desktop dashboard mirrors the device's task table live, and adds a dev-only per-function Hermes sampler (attaches to Metro's inspector).
• Via the Buoy MCP server, your AI agent can call get_js_thread_top — "why is JS FPS low?" becomes one tool call.

License

Part of React Buoy. See buoy.gg for licensing.