How does React's act work?

How does React's `act` work?

tl;dr: It wraps some work you'd like to do and waits for that work to complete before returning

An update to Component inside a test was not wrapped in act(...).

tl;dr: A React component is updating (probably as a result of changing state, props, or hooks) but that update is not happening inside of something wrapped in act.

Implemented in ReactFiberWorkLoop
That error is produced when running react in a test environment (not sure how it knows this tbh) and actQueue is null (actQueue is set when act is called)
Produced while calling scheduleUpdateOnFiber, which is ?probably? called as a result of changing state or props

Here is where act is implemented. React Testing Library calls act for you when calling render as well as when calling fireEvent (this and this)

FYI - act is only callable in developer mode - otherwise it will throw an exception

act creates a new actQueue or uses the current one if it exists already
then it calls the block passed in (the code you're wrapping in act)
- that passed in block does the work you specified in your test
- that work causes state and props to update, and eventually ReactFiberRootScheduler.scheduleCallback is called by react, which pushes work onto the act actQueue
then it flushes the current actQueue by executing every work item in it
then it sets actQueue = null (here for synchronous callbacks, here for async callbacks)