how to set state array using react hooks

Question

Thanks in advance. I have a state array as below.

I need to add an item to state array, I came across that we need not do state mutation. How do i set state with prevState.

const [messages, setMessages] = React.useState(
        [
            {
                _id: 12,
                createdAt: new Date(),
                text: 'All good',
                user: {
                    _id: 1,
                    name: 'Sian Pol',
                }
            },
            {
                _id: 21,
                createdAt: "2019-11-10 22:21",
                text: 'Hello user',
                user: {
                    _id: 2,
                    name: 'User New'
                }
            }]
    )

How to to i call the set State to append this state array.

Something like this?

setMessages(previousState => ({...stat

Can anyone help me in getting the above line code.

Does this answer your question? [Push method in React Hooks (useState)?](https://stackoverflow.com/questions/54676966/push-method-in-react-hooks-usestate) — Tolotra Raharison, Nov 30 '19 at 16:15

Tolotra Raharison · Accepted Answer · 2022-09-27T17:04:12.750

42

To insert new element at the end of the list

const addMessage = (newMessage) => setMessages(oldMessages => [...oldMessages, newMessage])

To insert new element at the begining of the list

const addMessage = (newMessage) => setMessages(oldMessages => [newMessage, ...oldMessages])

edited Sep 27 '22 at 17:04

answered Nov 29 '19 at 08:03

Tolotra Raharison

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1

In my case, using push was an issue because I was returning the result from the push itself, which returns the new length of the array, not the array itself. `setSelectedList(prevState => prevState.push(val))` will set the state object to the length of the array, in other words. – Nate Kindrew May 05 '21 at 15:13

Astrit Spanca · Answer 2 · 2021-02-05T09:33:39.903

15

More readable and cleaner solution it would be:

Create a variable that holds a copy of the actual state:

If state is an array and you need to add an element in it

const newState = [...messages, 'Hi buddy'];
setMessages(newState);
 
or

setMessages(prevState => [...prevState, "Hi Buddy"]);

If state is an object and you need to update a property in it

const newState = Object.assign({}, message, {name: 'Michael Scott'});
setMessages(newState);

or

setMessages(prevState => {...prevState, name: "Michael Scott" });

edited Feb 05 '21 at 09:33

answered Nov 29 '19 at 09:10

Astrit Spanca

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I think this will mutate the state. – Ankit Jayaprakash Nov 29 '19 at 09:20
2

It will not for sure – Astrit Spanca Nov 29 '19 at 22:15
2

@AnkitJayaprakash That is what the ...spread operator does, make a copy, thus no mutation happens – CatalinBerta Jun 05 '20 at 05:35
What are the consequences of one on another? Are both same performance wise? – Priyansh Gaharana May 22 '23 at 07:30

score 1 · Answer 3 · answered Nov 29 '19 at 08:00

Your state is an array so you will need to spread your previous state into a new array and add the new message using [...prevState, newMessage]

What you try to do is return an object, because {} is a code block so you need to wrap it inside () if you return an object which is what you are trying to do

setMessages(prevState => [...prevState, newMessage])

score 1 · Answer 4 · edited Dec 24 '22 at 18:59

1

setMessages(prevState => [...prevState, newMessage])

edited Dec 24 '22 at 18:59

Pedram

15,766
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answered Nov 29 '19 at 09:06

Ankit Jayaprakash

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score 0 · Answer 5 · answered Nov 29 '19 at 08:15

0

There is no real need to use the prevState, you could just do:

setMessages([...messages, newMessage])

answered Nov 29 '19 at 08:15

Konstantin

1,390
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how to set state array using react hooks

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