Stephane Rangaya

Posted on Mar 15, 2020

How to make sure useEffect catches array changes

#react #useeffect #array

There is a way to use useEffect to only run when variables provided in a second argument are updated, like this:

const [count, setCount] = useState(0);

useEffect(() => {
  console.log('Something happened')
}, [count]); // Only re-run the effect if count changes

Unfortunately, this doesn't always work with arrays:

const [data, setData] = useState([
    {id: 1, name: 'Alexander'},
    {id: 2, name: 'Fabien'},
    {id: 3, name: 'Yuki'},
  ]);


useEffect(() => {
  console.log('Something happened')
}, [data]); // Changes won't be caught :( !

const onCapitalizeClick = () => {
  setData(capitalizeAllRows());
};

// in another hook:
function capitalizeAllRows() {
    setDataRows(props);
    dataRows.forEach(row => {
      row.name = Capitalize(row.name);
      setDataRows([...data]);
    });
    return dataRows;
  }

In this case, the effect won't run again because it is comparing the array at the highest level and it is therefore not picking up the change in capitalization of names: the array has the same amount of rows, nothing structurally changed.

But if you were to add a row, the changes will be picked up and the effect would be executed.

In order to have the effect always run when any changes happen to the array, stringifying the array works well!

Instead of using data in the second argument array of useEffect(), use [JSON.stringify(data)]:

const [data, setData] = useState([
    {id: 1, name: 'Alexander'},
    {id: 2, name: 'Fabien'},
    {id: 3, name: 'Yuki'},
  ]);


useEffect(() => {
  console.log('Something happened')
}, [JSON.stringify(data)]); // Changes will be caught :) !

const onCapitalizeClick = () => {
  setData(capitalizeAllRows());
};

// in another hook:
function capitalizeAllRows() {
    setDataRows(props);
    dataRows.forEach(row => {
      row.name = Capitalize(row.name);
      setDataRows([...data]);
    });
    return dataRows;
  }

Top comments (14)

Igor Quirino • May 18 '20

JSON.stringify(data) will add extra processing...
Why not to just add a counter (setState(0)) and then increment it when object get changed?

Stephane Rangaya • Jul 1 '20

How would you do this?

Nicklas Pouey-Winger • Jul 3 '20 • Edited on

I assume by calling setLoadCount((prevState) => prevState + 1) or something like that :)

Igor Quirino • Jul 20 '20

Maybe: sung.codes/blog/2018/11/08/emulate...

NeehaChoudhary • Apr 25

Created an account to actually thank you for this! :D

Tobias Jacob • Jan 4 '21

I don't think this is a good example. The fault is not, that react cannot look for array changes, but instead that you mutated your state.

NEVER MUTATE THE STATE

In the example above, this means, that you have to create a whole new array with whole new objects for react to pick up the changes. For example

const [data, setData] = useState([])

In another hook:
function capitalizeAllRows() {
setData(data.map((row) => ({...row, name: Capitalize(row.name)})))
}

note that with the spread operator, we created a new object.

I still like the idea of using hashs (for example json.stringify, even tough there are more performant ways) for applying effects if an array changes. Just the example is not well chosen

mvozaar • Feb 11 '21

example is good but not for solving the problem. but for detection and verification of root cause is good :). it saves your time if you want to find problem. right solution for state changes is as you mentioned "NEVER MUTATE THE STATE".

Eirik Årdal • Jun 21 • Edited on

I ended up with wrapping the data in another object with a key field I can update on each change and listen to changes on:

// Key.ts
interface Key<T> { key: string, data: T }

// hookUtils.ts
import { Key } from '@app/models'
import { v4 as uuid } from 'uuid'

export function genId(): string {
  return uuid()
}

export function newKey<TData>(data: TData): Key<TData> {
  return { key: genId(), data }
}

// SomeComponent.tsx
import { newKey } from '@app/utils/hookUtils'

const [foos, setFoos] = useState<Key<Foo[]>>(newKey([]))

...

useEffect(() => {
    // react to foos having changed
}, [foos.key])

...

// update list somewhere
const updatedFoos = [ ...foos, { whatever: "something" } ]
setFoos(newKey(updatedFoos))

return <Foos items={foos.data} />

Magnus Ringkjøb • Nov 3

Thanks! Your suggestion nudged me in the right direction on how to solve this. As other people has pointed out JSON.stringify can add a lot of unnecessary processing. Perhaps there's some other state you can listen to? In my case I realized the data is only updated after the loading state changed.

So for me, listening to the loading state helped me use the array of objects in useEffect as intended.

Harshan Morawaka • Oct 14 '21

just use data.length as the dependency

martin2844 • Jan 4 '21

Thanks man. Could'nt figure out why react wasnt catching changes in my array

mvozaar • Feb 11 '21

I have spotted similar behavior, when i missed object changes. changes was done by delete_by_key. this is "mutating the state".

in original code was used spread operator to clone new object, but in removal was used delete. additions was detected, but removal not.

Erfan Azary • Nov 17 '21

Of course the worst solution for Object/Array comparison is JSON.stringify
especially for big Objects/Arrays, it will have massive impact on performance