Unexpected Initial State Jump in 'useEffect" with 'setTimeout' and State Dependencies

[locus032]

Description:
When using useEffect in React with a state variable in the dependency array and a setTimeout function inside the effect, the initial state update causes an unexpected jump from 0 to 2 within the first interval period. This behavior deviates from the expected incremental update and can lead to confusion and potential bugs.

Steps to Reproduce:

1. Create a component with the following code:

   import { useState, useEffect } from "react";
   
   function Temp() {
     const [count, setCount] = useState(0);
   
     useEffect(() => {
       setTimeout(() => {
         setCount((count) => count + 1);
       }, 5000);
     }, [count]);
   
     return <h1>I've rendered {count} times!</h1>;
   }
   
   export default Temp;

2. Render the Temp component.
3. Observe the count value after the first timeout period (5 seconds).

Expected Behavior:
The count should increment from 0 to 1 after the first 5-second interval and continue incrementing by 1 every subsequent 5-second interval.

Actual Behavior:
The count jumps from 0 to 2 within the first 5-second interval, and then increments correctly by 1 every subsequent 5-second interval.

Explanation:
This unexpected jump occurs due to the way React handles state updates and effect executions. Here’s a detailed breakdown:

1. Initial Render:

   • count is initialized to 0.
   • useEffect schedules a setTimeout to increment count after 5 seconds.

2. First Timeout Execution (after 5 seconds):

   • The setTimeout callback executes and increments count from 0 to 1.
   • This state change triggers a re-render of the component.
   • useEffect runs again because count is in the dependency array, scheduling another setTimeout for 5 seconds.

3. State Update and Re-render:

   • React processes the state update, causing the component to re-render with count set to 1.
   • The re-render triggers useEffect again, scheduling another setTimeout almost immediately.

React batches state updates and effect re-runs to optimize performance. This batching can result in both the initial state update (from 0 to 1) and the next setTimeout scheduling occurring within the same event loop tick. Consequently, the setTimeout callback executes twice in quick succession:

1. The first setTimeout increments count from 0 to 1 after 5 seconds.
2. The state update triggers a re-render, and useEffect schedules another setTimeout immediately due to the new count value (1).
3. The second setTimeout increments count from 1 to 2 almost immediately.

Impact:

• Developer Confusion: The unexpected behavior can lead to confusion, especially for developers new to React or those expecting consistent interval updates.
• Potential Bugs: Misunderstanding this behavior can lead to bugs in applications, particularly in scenarios requiring precise timing or consistent state updates.
• Increased Complexity: Developers need to add additional logic to handle this case, increasing the complexity of the code.

Suggested Improvements:

1. Documentation: Enhance the React documentation to explicitly mention this behavior and provide guidelines on how to handle it.
2. Development Warnings: Introduce development-time warnings for common patterns that might lead to this behavior, suggesting alternative patterns or solutions.
3. API Enhancements: Consider API enhancements that provide a more intuitive handling of such scenarios, potentially through built-in mechanisms for managing intervals and timeouts predictably.

I hope that by addressing this issue, you can make state updates and effect handling more intuitive and predictable, thereby enhancing the overall developer experience.

[eps1lon]

Is that Component wrapped in React.StrictMode? React will double invoke Effects in React.StrictMode on mount to flush out missing cleanups like in your example:

   useEffect(() => {
-    setTimeout(() => {
+    const timeoutID = setTimeout(() => {
       setCount((count) => count + 1);
     }, 5000);
+   return () => clearTimeout(timeoudID)
   }, [count]);

[locus032]

Thank you for the clarification regarding React.StrictMode and the double invocation of effects.

Cleaning up the side effects by adding cleanup functions is understandable and a good practice. However, if this behavior causes noticeable changes in the UI or affects the development experience, it can be problematic.

In my case, when I remove React.StrictMode, the component works as expected without the initial jump in the state value. This suggests that the double invocation in strict mode is leading to unintended UI changes.

Could you please provide more guidance or recommendations on how to handle such cases to ensure a smooth development experience without unintended UI behavior?

Thank you!

[eps1lon]

StrictMode was added for this reason and the recommendation is to either clean up Effects or not using Effects at all.