react usestate spread operator

Do I need to use the spread operator when using useState hook on object when updating?

stackoverflow.com › questions › 60156883 › do-i-need-to-use-the-spread-operator-when-using-usestate-hook-on-object-when-upd

You still don't want to mutate state. So if your state is an object, you'll want to create a new object and set with that. This may involve spreading the old state. For example:

const [person, setPerson] = useState({ name: 'alice', age: 30 });

const onClick = () => {
  // Do this:
  setPerson(prevPerson => {
    return { 
      ...prevPerson, 
      age: prevPerson.age + 1 
    }
  })
  // Not this:
  //setPerson(prevPerson => {
  //  prevPerson.age++;
  //  return prevPerson;
  //});
}

That said, using hooks you often no longer need your state to be an object, and can instead use useState multiple times. If you're not using objects or arrays, then copying is not needed, so spreading is also not needed.

const [name, setName] = useState('alice');
const [age, setAge] = useState(30);

const onClick = () => {
  setAge(prevAge => prevAge + 1);
}

Answer from Nicholas Tower on Stack Overflow

Stack Overflow

stackoverflow.com › questions › 60156883 › do-i-need-to-use-the-spread-operator-when-using-usestate-hook-on-object-when-upd

Do I need to use the spread operator when using useState hook on object when updating?

Top answer

1 of 3

You still don't want to mutate state. So if your state is an object, you'll want to create a new object and set with that. This may involve spreading the old state. For example:

const [person, setPerson] = useState({ name: 'alice', age: 30 });

const onClick = () => {
  // Do this:
  setPerson(prevPerson => {
    return { 
      ...prevPerson, 
      age: prevPerson.age + 1 
    }
  })
  // Not this:
  //setPerson(prevPerson => {
  //  prevPerson.age++;
  //  return prevPerson;
  //});
}

const [name, setName] = useState('alice');
const [age, setAge] = useState(30);

const onClick = () => {
  setAge(prevAge => prevAge + 1);
}

2 of 3

What it means is that if you define a state variable like this:

const [myThings, changeMyThings] = useState({cats: 'yes', strings: 'yellow', pizza: true })

Then you do something like changeMyThings({ cats: 'no' }), the resulting state object will just be { cats: 'no' }. The new value is not merged into the old one, it is just replaced. If you want to maintain the whole state object, you would want to use the spread operator:

changeMyThings({ ...myThings, cats: 'no' })

This would give you your original state object and only update the one thing you changed.

Updating object with useState

React, how do I update just one property in an object that uses the useState hook?

React Hooks useState() with Object

How to update the object key's value in state using useState() hook?

How to update state with usestate in an array of objects?

More results from stackoverflow.com

reddit.com › r/reactjs › need help with usestate hook - can't update an array using the spread operator

r/reactjs on Reddit: Need help with useState hook - can't update an array using the spread operator

December 5, 2021 -

I am fairly new to React and I think I am missing an important concept about state.

I am trying to update a state variable, specifically an array. The following function successfully updates the array if I use Array.push(), but not if I use destructuring/the spread operator. What am I missing? Also, the template is completely non-reactive - even when the console.log statement displays the correct value, the HTML does not.

function MyComponent() {
    const [selection, setSelection] = useState([]);

    const handleSelection = (id) => {
        // doesn't work
        // let newSelection = [...selection, id];

        let newSelection = selection;
        newSelection.push(id);

        // Prints the correct value when using Array.push() but not destructuring
        console.log(selection);

        setSelection(newSelection);
    };

    return (
        {/* never changes */}
        <pre>{JSON.stringify(selection, null, 2)}</pre>
    )
}

EDIT: I thought this was a problem with how I was setting state on the selection variable, but it turns out the issue was with other parts of my code that are tangentially related to this. The problem was that I was trying to to loop over some data and generate a component for each datum, but I wasn't doing that correctly. I was setting a different state variable to hold these components, and the various pieces of state were out of sync. Here is a repro of the working code, where I am mapping the components correctly (in the template directly rather than setting state, which seems to be better practice). As you can see, spread/destructuring works perfectly as expected now.

Top answer

1 of 5

Both versions look like they should work -- maybe it's something in the surrounding context? Can you share more and/or repro in a codepen?

2 of 5

I think where your console log is reports stale data? I tried the code and it works. You don't need another array for the spread... This is my test, putting state result into a p tag on a button click. const MyComponent = () => { const [selection, setSelection] = useState([]); const handleSelection = (id) => { setSelection([...selection, id]) }; return (

{selection}

handleSelection("testId")}>CLICK ME ); };

Discussions

reactjs - Spread operator in React .setState() in the useState Hook - Stack Overflow

Why is the spread operator used in setName() function how does it work. What this code does is it takes the input from the user and displays it on the screen but before using spread operator the More on stackoverflow.com

stackoverflow.com

How do you update a value inside an array of objects with useState?

People already answered you here about how you use setState in your case. I would strongly suggest you to use useReducer instead of useState. It would allow you to have more complex operations tied to your state and thus, save you from writing too much code and increase readability. You could, for example, do this: dispatch({type: "update", obj: { id:3, newProperty: "hellowWorld" } }) or dispatch({type: "replace", obj: { id:3, ...properties }) Whenever your state gets too complex. Always consider using the useReducer hook . More on reddit.com

r/react

April 15, 2022

I think I have a fundamental misunderstanding of useState inside useEffect.

SetState is async, you need use useeffect for setSelectedItem ans setSize when the items array change. UseEffect(() => { SetSelectedItem(items[0]); SetSize(items[0].size); },[items]) More on reddit.com

r/react

April 11, 2022

Need help with useState hook - can't update an array using the spread operator

Both versions look like they should work -- maybe it's something in the surrounding context? Can you share more and/or repro in a codepen? More on reddit.com

r/reactjs

December 5, 2021

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