The new Testing Recipes guide describes common scenarios, and how act() can help you write good tests. We’ve heard there wasn’t enough information about how to write tests with act(). As a result, you should be able to fix all the remaining act() warnings in your tests now. This solves the remaining cases where you couldn’t use act() before, such as when the state update was inside an asynchronous function. Sometimes, you might have seen a warning like this in a test but could not easily fix it: However, in 16.8 act() only supported synchronous functions. This matches how React already works when handling real browser events, and helps prepare your components for the future in which React will batch updates more often. For example, multiple state updates inside a single act() get batched. React 16.8 introduced a new testing utility called act() to help you write tests that better match the browser behavior. We don’t expect most codebases to be affected by this. Alternatively, you can convert it to either a class or a function component. If you rely on it, adding Factor圜omponent.prototype = can serve as a workaround. So we are deprecating this pattern in 16.9 and logging a warning if it’s encountered. This pattern was almost never used in the wild, and supporting it causes React to be slightly larger and slower than necessary. (A function component would just return the in the above example.) This pattern is confusing because it looks too much like a function component - but it isn’t one.

Main type 9.0