JavaScript Thread

Also called JS thread

Part of
React Native Glossary
Also known as
JS thread
Related
UI Thread (Main Thread)JavaScript RuntimeWorkletReanimated
Mentioned in
Animated ComponentJavaScript RuntimeReanimatedUI Thread (Main Thread)Worklet
Updated

What is JavaScript Thread in React Native?

The JavaScript thread is where your React components, business logic and event handlers run. Blocking it makes the app feel frozen even though native rendering continues.

The JavaScript thread is the single thread where your application code executes: component renders, hooks, event handlers, network callbacks, and state updates all queue here.

It is separate from the UI thread, which draws the actual pixels. That separation is why a React Native app can stay visually responsive — a scroll can keep moving natively — while your JavaScript is stuck.

Because it is single-threaded, anything expensive blocks everything else:

  • Parsing a large JSON payload
  • A heavy map/filter over thousands of records on every render
  • Synchronous work inside a useEffect that runs on mount

Symptoms are a button that responds a second late, or a setState-driven animation that stutters. The fixes are to move work off the render path (useMemo, pagination, background native modules) or to move animation off this thread entirely with worklets.

Profiling the JS thread is the first step in almost any React Native performance investigation.