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> <thead key="thead"> <TableHeader columns={columnNames}/> </thead> <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[c]}>{this.props.data[c]}</td>; }, this); }.bind(this); return ( <tr>{ td(this.props.item) }</tr> ) } });
What is causing the unique key prop error?
⚠️ Warning: Each child in a list should have a unique “key” prop. This is because React uses a unique “key” prop on each child of the list to create a relationship between the component and the DOM. This is to ensure that react re-renders the child correctly next time.
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.
The main purpose of keys is to help React differentiate and distinguish elements from each other, increasing its performance when diffing between the virtual and real DOM. To use keys, simply add the prop inside an element such as <li> . Unique IDs are the best value to assign to keys.
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);
React documentation on the importance of keys in reconciliation: Keys
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.
key
propReact 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:
key
propAs 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> <button onClick={this.click}>Add</button> <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
:
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.
<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
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.
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