Understanding Unique Keys For Array Children in React.Js

Understanding unique keys for array children in React.js

You should add a key to each child as well as each element inside children.

This way React can handle the minimal DOM change.

In your code, each <TableRowItem key={item.id} data={item} columns={columnNames}/> is trying to render some children inside them without a key.

Check this example.

Try removing the key={i} from the <b></b> element inside the div's (and check the console).

In the sample, if we don't give a key to the <b> element and we want to update only the object.city, React needs to re-render the whole row vs just the element.

Here is the code:

var data = [{name:'Jhon', age:28, city:'HO'},
            {name:'Onhj', age:82, city:'HN'},
            {name:'Nohj', age:41, city:'IT'}
           ];

var Hello = React.createClass({
    
    render: function() {
            
      var _data = this.props.info;
      console.log(_data);
      return(
        <div>
            {_data.map(function(object, i){
               return <div className={"row"} key={i}> 
                          {[ object.name ,
                             // remove the key
                             <b className="fosfo" key={i}> {object.city} </b> , 
                             object.age
                          ]}
                      </div>; 
             })}
        </div>
       );
    }
});
 
React.render(<Hello info={data} />, document.body);

The answer posted by @Chris at the bottom goes into much more detail than this answer.

React documentation on the importance of keys in reconciliation: Keys

React.js, Each child in a list should have a unique key prop. how to fix if key is already given

Try passing the key here in the code

{props.data.UserGroup.map((person) => (
                    OwnerName = person.Name,
                    person.Animals.map((Animal) => ( 
                        Animalname = Animal.AnimalName,
                        breed = Animal.Breed,
                        Animal.Vists.map((vist, index) => (
                         // or visit.id if available
                            <tr key={index}>
                                <td>  <i class="bi bi-person-fill"></i> {OwnerName} </td>
                                <td> {Animalname}</td>
                                <td> {breed} </td>
                                <td> {vist.time} </td>
                            </tr>
                        )) 
                    )) 
                ))}

It's recommended to use keys coming from the data source such as visit.id. Last resort should be using index. For more information you can read here.

Understanding unique key prop in React.js

The createdAt values are not unique for them. The quick fix is to use index instead, but if you plan to do any removing or sorting, you should rather use some sort of unique identifier for the items (phone, email, user ID, etc). I only skimmed your code, but you could use phone instead.

The keys are important because it hints to React when it needs to update the list. The docs are helpful https://reactjs.org/docs/lists-and-keys.html#keys for more on this.

Anyway, the quick fix is shown here. Read https://stackoverflow.com/a/43642421 as to why you shouldn't do this. I'll leave a better implementation up to you.

...
{ cartItems.map((item, index) => {
    return (
        <View key={index}>
            <PreOrderItem item={item} />
        </View> );
    }) 
}
...

Each child unique key prop in React Native

To avoid refresh all components when data change, you should use "key" attribute for your component.

if (x == 0){
        loadingView.push(<View key={`view_${i}`} style={[styles.loading,{borderBottomColor:'transparent'}]}><Spinner type="Radial" color="#fff" size={40} /></View>);

The best practice is to use a unique id and not an index as in this case.

Warning: Each child in a list should have a unique key prop. in react native

It is similar to passing an id to objects to recognise them. Similarly here in react-native you need to provide a key which can be a number or anything but different for each component to recognise that and to keep that seperate from others.

In your example you can pass a key at ;

<TouchableOpacity key={index} onPress={() => setIndexSelect(index)}> // If index is different for each component.

React - Each child in a list should have a unique 'key' prop

Checking your PokemonItem it reveals that the reason may be laying in this piece of code:

{pokemon.types.map((type) => {
    return <div className="type">{type}</div>;
})}

This is easily fixed by adding the key attribute:

{pokemon.types.map((type) => {
    return <div className="type" key={type.id}>{type}</div>;
})}

You need to add a key in every item returned from a map in order to avoid this error. Also I advice you to add the console output related to your question in the body so it's easier to pinpoint the errors.

Warning: Each child in a list should have a unique key prop. with input - react

The fragment element (<></>) is the top-level child element here, and it needs a key prop. The short syntax being used doesn't support adding an attribute, but the longer syntax does:

<React.Fragment key={item.id}>
  <input type="checkbox" />
  <h3>{item.value}</h3>
</React.Fragment>

React Component Function with Each child in a list should have a unique key prop. Check the render method

I believe you problem lies in the component

in this code block you are building up a your time left object, using a forEach, just append a key to the first span to solve the warning.

  Object.keys(timeLeft).forEach((interval) => {
    // adding 00 to null values
    if (!timeLeft[interval]) {
      timerComponents.push(
          <span>{"00"}</span>
      );
    }
    // adding 0 to less than 10 values
     else if(timeLeft[interval] < 10){
      timerComponents.push(
          <span>{'0'}{timeLeft[interval]}</span>
      );
    }
     // adding other values
    else {
      timerComponents.push(
          <span>{timeLeft[interval]}</span>
      );
    }
    // adding colon
      timerComponents.push(
          <span>{":"}</span>
      );
  });

<span key={index}>{"00"}</span>

React Warning: Each child in a list should have a unique key prop. in render() function

If you use map in your JSX return then you need to provide the parent element with the key prop so that it's uniquely identified.

https://reactjs.org/docs/lists-and-keys.html

You would preferably use an object ID but if you know a field (or combinations of fields) that would constitute a unique key then you could use that instead:

{profiles.map((profile) => (
  <div 
    key={'profileList_'+profile.first_name+profile.last_name} 
    className="card"
  >
    ...
  </div>
)};

NOTE: In the example I used profileList_ as a prefix just in case you need to use the same unique identifier (object ID or in this case profile.list_name+profile.last_name) as a key somewhere else in a different context.

Warning: Each child in a list should have a unique key prop but i have key props

you have to pass key props to fragment(<>), not to the PostBasic.

Use like this,

Change this

(
  <>
    <PostBasic post={post} postBefore={postBefore} key={post.id} />
    <RandomMessage />
  </>
);

(<React.Fragment key={post.id}>
  <PostBasic post={post} postBefore={postBefore} />
  <RandomMessage />
</React.Fragment>
)

Understanding Unique Keys For Array Children in React.Js