# javascript – Understanding unique keys for array children in React.js

## The Question :

751 people think this question is useful

I’m building a React component that accepts a JSON data source and creates a sortable table.
Each of the dynamic data rows has a unique key assigned to it but I’m still getting an error of:

Each child in an array should have a unique “key” prop.
Check the render method of TableComponent.

My TableComponent render method returns:

<table>
<tbody key="tbody">
{ rows }
</tbody>
</table>



The TableHeader component is a single row and also has a unique key assigned to it.

Each row in rows is built from a component with a unique key:

<TableRowItem key={item.id} data={item} columns={columnNames}/>



And the TableRowItem looks like this:

var TableRowItem = React.createClass({
render: function() {

var td = function() {
return this.props.columns.map(function(c) {
return <td key={this.props.data}>{this.props.data}</td>;
}, this);
}.bind(this);

return (
<tr>{ td(this.props.item) }</tr>
)
}
});



What is causing the unique key prop error?

• Your rows in JS array should have unique key property. It’ll help ReactJS to find references to the appropriate DOM nodes and update only content inside mark-up but not re-render the whole table/row.
• Can you also share rows array or more preferably a jsfiddle? You dont need a key property on thead and tbody by the way.
• I added the row component to the original question @nilgun.
• Is it possible that some items do not have an id or have same id?

708 people think this answer is useful

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. Please take a look at https://stackoverflow.com/a/43892905/2325522

React documentation on the importance of keys in reconciliation: Keys

605 people think this answer is useful

Be careful when iterating over arrays!!

It is a common misconception that using the index of the element in the array is an acceptable way of suppressing the error you are probably familiar with:

Each child in an array should have a unique "key" prop.



However, in many cases it is not! This is anti-pattern that can in some situations lead to unwanted behavior.

# Understanding the key prop

React uses the key prop to understand the component-to-DOM Element relation, which is then used for the reconciliation process. It is therefore very important that the key always remains unique, otherwise there is a good chance React will mix up the elements and mutate the incorrect one. It is also important that these keys remain static throughout all re-renders in order to maintain best performance.

That being said, one does not always need to apply the above, provided it is known that the array is completely static. However, applying best practices is encouraged whenever possible.

A React developer said in this GitHub issue:

• key is not really about performance, it’s more about identity (which in turn leads to better performance). randomly assigned and changing values are not identity
• We can’t realistically provide keys [automatically] without knowing how your data is modeled. I would suggest maybe using some sort of hashing function if you don’t have ids
• We already have internal keys when we use arrays, but they are the index in the array. When you insert a new element, those keys are wrong.

In short, a key should be:

• Unique – A key cannot be identical to that of a sibling component.
• Static – A key should not ever change between renders.

# Using the key prop

As per the explanation above, carefully study the following samples and try to implement, when possible, the recommended approach.

<tbody>
{rows.map((row, i) => {
return <ObjectRow key={i} />;
})}
</tbody>



This is arguably the most common mistake seen when iterating over an array in React. This approach isn’t technically “wrong”, it’s just… “dangerous” if you don’t know what you are doing. If you are iterating through a static array then this is a perfectly valid approach (e.g. an array of links in your navigation menu). However, if you are adding, removing, reordering or filtering items, then you need to be careful. Take a look at this detailed explanation in the official documentation.

class MyApp extends React.Component {
constructor() {
super();
this.state = {
arr: ["Item 1"]
}
}

click = () => {
this.setState({
arr: ['Item ' + (this.state.arr.length+1)].concat(this.state.arr),
});
}

render() {
return(
<div>
<ul>
{this.state.arr.map(
(item, i) => <Item key={i} text={"Item " + i}>{item + " "}</Item>
)}
</ul>
</div>
);
}
}

const Item = (props) => {
return (
<li>
<label>{props.children}</label>
<input value={props.text} />
</li>
);
}

ReactDOM.render(<MyApp />, document.getElementById("app"));

<script src="https://cdnjs.cloudflare.com/ajax/libs/react/15.1.0/react.min.js"></script>
<script src="https://cdnjs.cloudflare.com/ajax/libs/react/15.1.0/react-dom.min.js"></script>
<div id="app"></div>


In this snippet we are using a non-static array and we are not restricting ourselves to using it as a stack. This is an unsafe approach (you’ll see why). Note how as we add items to the beginning of the array (basically unshift), the value for each <input> remains in place. Why? Because the key doesn’t uniquely identify each item.

In other words, at first Item 1 has key={0}. When we add the second item, the top item becomes Item 2, followed by Item 1 as the second item. However, now Item 1 has key={1} and not key={0} anymore. Instead, Item 2 now has key={0}!!

As such, React thinks the <input> elements have not changed, because the Item with key 0 is always at the top!

So why is this approach only sometimes bad?

This approach is only risky if the array is somehow filtered, rearranged, or items are added/removed. If it is always static, then it’s perfectly safe to use. For example, a navigation menu like ["Home", "Products", "Contact us"] can safely be iterated through with this method because you’ll probably never add new links or rearrange them.

In short, here’s when you can safely use the index as key:

• The array is static and will never change.
• The array is never filtered (display a subset of the array).
• The array is never reordered.
• The array is used as a stack or LIFO (last in, first out). In other words, adding can only be done at the end of the array (i.e push), and only the last item can ever be removed (i.e pop).

Had we instead, in the snippet above, pushed the added item to the end of the array, the order for each existing item would always be correct.

<tbody>
{rows.map((row) => {
return <ObjectRow key={Math.random()} />;
})}
</tbody>



While this approach will probably guarantee uniqueness of the keys, it will always force react to re-render each item in the list, even when this is not required. This a very bad solution as it greatly impacts performance. Not to mention that one cannot exclude the possibility of a key collision in the event that Math.random() produces the same number twice.

Unstable keys (like those produced by Math.random()) will cause many component instances and DOM nodes to be unnecessarily recreated, which can cause performance degradation and lost state in child components.

### Very good

<tbody>
{rows.map((row) => {
return <ObjectRow key={row.uniqueId} />;
})}
</tbody>



This is arguably the best approach because it uses a property that is unique for each item in the dataset. For example, if rows contains data fetched from a database, one could use the table’s Primary Key (which typically is an auto-incrementing number).

The best way to pick a key is to use a string that uniquely identifies a list item among its siblings. Most often you would use IDs from your data as keys

### Good

componentWillMount() {
let rows = this.props.rows.map(item => {
return {uid: SomeLibrary.generateUniqueID(), value: item};
});
}

...

<tbody>
{rows.map((row) => {
return <ObjectRow key={row.uid} />;
})}
</tbody>



This is also a good approach. If your dataset does not contain any data that guarantees uniqueness (e.g. an array of arbitrary numbers), there is a chance of a key collision. In such cases, it is best to manually generate a unique identifier for each item in the dataset before iterating over it. Preferably when mounting the component or when the dataset is received (e.g. from props or from an async API call), in order to do this only once, and not each time the component re-renders. There are already a handful of libraries out there that can provide you such keys. Here is one example: react-key-index.

18 people think this answer is useful

This may or not help someone, but it might be a quick reference. This is also similar to all the answers presented above.

I have a lot of locations that generate list using the structure below:

return (
{myList.map(item => (
<>
<div class="some class">
{item.someProperty}
....
</div>
</>
)}
)



After a little trial and error (and some frustrations), adding a key property to the outermost block resolved it. Also, note that the <> tag is now replaced with the <div> tag now.

return (

{myList.map((item, index) => (
<div key={index}>
<div class="some class">
{item.someProperty}
....
</div>
</div>
)}
)



Of course, I’ve been naively using the iterating index (index) to populate the key value in the above example. Ideally, you’d use something which is unique to the list item.

8 people think this answer is useful

Check: key = undef !!!

You got also the warn message:

Each child in a list should have a unique "key" prop.



if your code is complete right, but if on

<ObjectRow key={someValue} />



someValue is undefined!!! Please check this first. You can save hours.

6 people think this answer is useful

data.map((marker)=>{
return(
<YourComponents
key={data.id}     // <----- unique key
/>
);
})



5 people think this answer is useful

Warning: Each child in an array or iterator should have a unique “key” prop.

This is a warning as for array items which we are going to iterate over will need a unique resemblance.

React handles iterating component rendering as arrays.

Better way to resolve this is provide index on the array items you are going to iterate over.for example:

class UsersState extends Component
{
state = {
users: [
{name:"shashank", age:20},
{name:"vardan", age:30},
{name:"somya", age:40}
]
}
render()
{
return(
<div>
{
this.state.users.map((user, index)=>{
return <UserState key={index} age={user.age}>{user.name}</UserState>
})
}
</div>
)
}



index is React built-in props.

2 people think this answer is useful

In ReactJS if you are rendering an array of elements you should have a unique key for each those elements. Normally those kinda situations are creating a list.

Example:

function List() {
const numbers = [0,1,2,3];

return (
<ul>{numbers.map((n) => <li> {n} </li>)}</ul>
);
}

ReactDOM.render(
<List />,
document.getElementById('root')
);



In the above example, it creates a dynamic list using li tag, so since li tag does not have a unique key it shows an error.

After fixed:

function List() {
const numbers = [0,1,2,3];

return (
<ul>{numbers.map((n) => <li key={n}> {n} </li>)}</ul>
);
}

ReactDOM.render(
<List />,
document.getElementById('root')
);



Alternative solution when use map when you don’t have a unique key (this is not recommended by react eslint ):

function List() {
const numbers = [0,1,2,3,4,4];

return (
<ul>{numbers.map((n,i) => <li key={i}> {n} </li>)}</ul>
);
}

ReactDOM.render(
<List />,
document.getElementById('root')
);



Live example: https://codepen.io/spmsupun/pen/wvWdGwG

1 people think this answer is useful

Best solution of define unique key in react: inside the map you initialized the name post then key define by key={post.id} or in my code you see i define the name item then i define key by key={item.id}:

<div className="container">
{posts.map(item =>(

<div className="card border-primary mb-3" key={item.id}>
<div className="card-body" >
<p className="card-text">{item.email}</p>
</div>
</div>
))}
</div>


1 people think this answer is useful

I was running into this error message because of <></> being returned for some items in the array when instead null needs to be returned.

1 people think this answer is useful

I had a unique key, just had to pass it as a prop like this:

<CompName key={msg._id} message={msg} />



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

0 people think this answer is useful

This is a warning, But addressing this will make Reacts rendering much FASTER,

This is because React needs to uniquely identify each items in the list. Lets say if the state of an element of that list changes in Reacts Virtual DOM then React needs to figure out which element got changed and where in the DOM it needs to change so that browser DOM will be in sync with the Reacts Virtual DOM.

As a solution just introduce a key attribute to each li tag. This key should be a unique value to each element.

0 people think this answer is useful
var TableRowItem = React.createClass({
render: function() {

var td = function() {
return this.props.columns.map(function(c, i) {
return <td key={i}>{this.props.data}</td>;
}, this);
}.bind(this);

return (
<tr>{ td(this.props.item) }</tr>
)
}
});



This will sove the problem.

-1 people think this answer is useful

I fixed this using Guid for each key like this: Generating Guid:

guid() {
return this.s4() + this.s4() + '-' + this.s4() + '-' + this.s4() + '-' +
this.s4() + '-' + this.s4() + this.s4() + this.s4();
}

s4() {
return Math.floor((1 + Math.random()) * 0x10000)
.toString(16)
.substring(1);
}



And then assigning this value to markers:

{this.state.markers.map(marker => (
<MapView.Marker
key={this.guid()}
coordinate={marker.coordinates}
title={marker.title}
/>
))}