Big useEffect mistake

March 7, 2021

In Reactjs, we encountered infinite loops or useless re-rendering a lot. It's something unavoidable, but we can be more attentive about it sometimes. One of the reasons that cause us this kind of problems is useEffect; I'm going to talk about one of the mistakes we can make with it and learn how to be aware of it.

The mistake

Check this out:

const [state, setState] = React.useState(0);
React.useEffect(() => {
}, [{ ...someData }])

Now if we do setStatemultiple times, we're going to see this in console:


Wait; what? We just passed someData as a dependency list; why it logs that? That's not even related to state. Yes, it's not related, but pay more attention to the dependency list; We passed an inline object, which means it's always different from its previous version; we create it every time we cause a re-render to the component. Check this:

{ ...someData } === { ...someData } // false
{} === {} // false
[] === [] // false
// all are inline and have different references

useEffect somehow cache the dependency list and check if it's equal to the next value. That's why inline non-primitive values ([], {}, {...data}, ...) are always different in this tool's eyes.

Symbol() is an exception here, it's different every time we create it, but it's a primitive value.

For example, check this, I saw many developers that they check part of an array like this:

const [state, setState] = React.useState([1, 2, 3, 4, 5]);
React.useEffect(() => {
}, [state.slice(0, 2)]); 
	prevVal = state.slice(0, 2) // first render
	nextVal = state.slice(0, 2) // second render
	prevVal === nextVal // false

The expected behaviour is checking 1 to 3 values, but it doesn't work like that because the slice method will always return a new array (reference).

I hope you enjoyed this article. Don't forget to share and send reactions to my article. If you wanted to tell me something, tell me on Twitter or mention me anywhere else, You can even subscribe to my newsletter and follow me on Github.

Join the Newsletter🐞

Subscribe to get our latest content by email.

    We won't send you spam. Unsubscribe at any time.


    Written by M. Bagher Abiat, the one Who loves solving stuff and writing about them.
    Follow him on Twitter, Github and Dev.to. Check out his newsletter.