We Keep Coding

How to update a nested react state

I'm trying to update a react state that holds nested values. I want to update data that is 3 levels deep.
Here is the state that holds the data:
const [companies, setCompanies] = useState(companies)
Here is the data for the first company (the companies array holds many companies):
const companies = [
{
companyId: 100,
transactions: [
{
id: "10421A",
amount: "850",
}
{
id: "1893B",
amount: "357",
}
}
]
Here is the code for the table component:
function DataTable({ editCell, vendors, accounts }) {
const columns = useMemo(() => table.columns, [table]);
const data = useMemo(() => table.rows, [table]);
const tableInstance = useTable({ columns, data, initialState: { pageIndex: 0 } }, useGlobalFilter, useSortBy, usePagination);
const {
getTableProps,
getTableBodyProps,
headerGroups,
prepareRow,
rows,
page,
state: { pageIndex, pageSize, globalFilter },
} = tableInstance;
return (
<Table {...getTableProps()}>
<MDBox component="thead">
{headerGroups.map((headerGroup) => (
<TableRow {...headerGroup.getHeaderGroupProps()}>
{headerGroup.headers.map((column) => (
<DataTableHeadCell
{...column.getHeaderProps(isSorted && column.getSortByToggleProps())}
width={column.width ? column.width : "auto"}
align={column.align ? column.align : "left"}
sorted={setSortedValue(column)}
>
{column.render("Header")}
</DataTableHeadCell>
))}
</TableRow>
))}
</MDBox>
<TableBody {...getTableBodyProps()}>
{page.map((row, key) => {
prepareRow(row);
return (
<TableRow {...row.getRowProps()}>
{row.cells.map((cell) => {
cell.itemsSelected = itemsSelected;
cell.editCell = editCell;
cell.vendors = vendors;
cell.accounts = accounts;
return (
<DataTableBodyCell
noBorder={noEndBorder && rows.length - 1 === key}
align={cell.column.align ? cell.column.align : "left"}
{...cell.getCellProps()}
>
{cell.render("Cell")}
</DataTableBodyCell>
);
})}
</TableRow>
);
})}
</TableBody>
</Table>
)
}
For example, I want to update the amount in the first object inside the transactions array. What I'm doing now is update the entire companies array, but doing this rerenders the whole table and creates problems. Is there a way I can only update the specific value in a manner that rerenders just the updated field in the table without rerendering the whole table? I've seen other answers but they assume that all values are named object properties.
FYI, I'm not using any state management and would prefer not to use one for now.

You have to copy data (at least shallow copy) to update state:
const nextCompanies = { ...companies };
nextCompanies.transactions[3].amount = 357;
setState(nextCompanies);
Otherwise react won't see changes to the original object. Sure thing you can use memoization to the child component to skip useless rerenders. But I strongly recommend to provide an optimisation only when it is needed to optimise. You will make the code overcomplicated without real profit.

When updating state based on the previous state, you probably want to pass a callback to setCompanies(). For example:
setCompanies((currCompanies) => {
const nextCompanies = [...currCompanies];
// modify nextCompanies
return nextCompanies;
})
Then, in order for React to only re-render the elements that changed in the DOM, you should make sure to set the key prop in each of those elements. This way, React will know which element changed.
// inside your component code
return (
<div>
companies.map(company => (
<Company key={company.id} data={company} />
))
</div>
)
Does this solve the problem? If not, it may be helpful to add some more details so we can understand it fully.

What I'm doing now is update the entire companies array, but doing
this rerenders the whole table and creates problems.
When you say it creates problems what type of problems exactly? How does re-rendering create problems? This is expected behavior. When state or props change, by default a component will re-render.
You seem to be asking two questions. The first, how to update state when only modifying a subset of state (an amount of a transaction). The second, how to prevent unnecessary re-rendering when render relies on state or props that hasn't changed. I've listed some strategies for each below.
1. What is a good strategy to update state when we only need to modify a small subset of it?
Using your example, you need to modify some data specific to a company in a list of companies. We can use map to iterate over each company and and conditionally update the data for the company that needs updating. Since map returns a new array, we can map over state directly without worrying about mutating state.
We need to know a couple things first.
What transaction are we updating?
What is the new amount?
We will assume we also want the company ID to identify the correct company that performed the transaction.
We could pass these as args to our function that will ultimately update the state.
the ID of the company
the ID of the transaction
the new amount
Any companies that don't match the company ID, we just return the previous value.
When we find a match for the company ID, we want to modify one of the transactions, but return a copy of all the other previous values. The spread operator is a convenient way to do this. The ...company below will merge a copy of the previous company object along with our updated transaction.
Transactions is another array, so we can use the same strategy with map() as we did before.
const handleChangeAmount = ({ companyId, transactionId, newAmount }) => {
setCompanies(() => {
return companies.map((company) => {
return company.id === companyId
? {
...company,
transactions: company.transactions.map((currTransaction) => {
return currTransaction.id === transactionId
? {
id: currTransaction.id,
amount: newAmount
}
: currTransaction;
})
}
: company;
});
});
};
2. How can we tell React to skip re-rendering if state or props hasn't changed?
If we are tasked with skipping rendering for parts of the table that use state that didn't change, we need a way of making that comparison within our component(s) for each individual company. A reasonable approach would be to have a reusable child component <Company /> that renders for each company, getting passed props specific to that company only.
Despite our child company only being concerned with its props (rather than all of state), React will still render the component whenever state is updated since React uses referential equality (whether something refers to the same object in memory) whenever it receives new props or state, rather than the values they hold.
If we want to create a stable reference, which helps React's rendering engine understand if the value of the object itself hasn't changed, the React hooks for this are useCallback() and useMemo()
With these hooks we can essentially say:
if we get new values from props, we re-render the component
if the values of props didn't change, skip re-rendering and just use the values from before.
You haven't listed a specific problem in your question, so it's unclear if these hooks are what you need, but below is a short summary and example solution.
From the docs on useCallback()
This is useful when passing callbacks to optimized child components that rely on reference equality to prevent unnecessary renders
From the docs on useMemo()
This optimization helps to avoid expensive calculations on every render.
Demo/Solution
https://codesandbox.io/s/use-memo-skip-child-update-amount-vvonum
import { useState, useMemo } from "react";
const companiesData = [
{
id: 1,
transactions: [
{
id: "10421A",
amount: "850"
},
{
id: "1893B",
amount: "357"
}
]
},
{
id: 2,
transactions: [
{
id: "3532C",
amount: "562"
},
{
id: "2959D",
amount: "347"
}
]
}
];
const Company = ({ company, onChangeAmount }) => {
const memoizedCompany = useMemo(() => {
console.log(
`AFTER MEMOIZED CHECK COMPANY ${company.id} CHILD COMPONENT RENDERED`
);
return (
<div>
<p>Company ID: {company.id}</p>
{company.transactions.map((t, i) => {
return (
<div key={i}>
<span>id: {t.id}</span>
<span>amount: {t.amount}</span>
</div>
);
})}
<button onClick={onChangeAmount}> Change Amount </button>
</div>
);
}, [company]);
return <div>{memoizedCompany}</div>;
};
export default function App() {
const [companies, setCompanies] = useState(companiesData);
console.log("<App /> rendered");
const handleChangeAmount = ({ companyId, transactionId, newAmount }) => {
setCompanies(() => {
return companies.map((company) => {
return company.id === companyId
? {
...company,
transactions: company.transactions.map((currTransaction) => {
return currTransaction.id === transactionId
? {
id: currTransaction.id,
amount: newAmount
}
: currTransaction;
})
}
: company;
});
});
};
return (
<div className="App">
{companies.map((company) => {
return (
<Company
key={company.id}
company={company}
onChangeAmount={() =>
handleChangeAmount({
companyId: company.id,
transactionId: company.transactions[0].id,
newAmount: Math.floor(Math.random() * 1000)
})
}
/>
);
})}
</div>
);
}
Explanation
On mount, the child component renders twice, once for each company.
The button will update the amount on the first transaction just for that company.
When the button is clicked, only one <Company /> component will render while the other one will skip rendering and use the memoized value.
You can inspect the console to see this in action. Extending this scenario, if you had 100 companies, updating the amount for one company would result in 99 skipped re-renders with only one new component rendering for the updated company.

React key in map not working. Map within a map also concatenating unique keys [duplicate]

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?

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:
const data = [
{ name: "Nuri", age: 28, city: "HO" },
{ name: "Talib", age: 82, city: "HN" },
{ name: "Jenny", age: 41, city: "IT" },
];
const ExampleComponent = React.createClass({
render: function () {
const infoData = this.props.info;
return (
<div>
{infoData.map((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(<ExampleComponent 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

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.
Bad (Potentially)
<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:
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.
Very bad
<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.

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.

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.

Just add the unique key to the your Components
data.map((marker)=>{
return(
<YourComponents
key={data.id} // <----- unique key
/>
);
})

You should use a unique value for each children key of tbody where
the value cannot not be identical (same) to its sibling
should not change between renders
For example, the key value can be database id or UUID (Universal Unique Identifier).
Here the keys are handling manually:
<tbody>
{rows.map((row) => <ObjectRow key={row.uuid} />)}
</tbody>
You can also let React handle the keys using React.Children.toArray
<tbody>
{React.Children.toArray(rows.map((row) => <ObjectRow />))}
</tbody>

Here are the React docs that explain well using the Key property, the key should be defined at the parent component it should not be used inside the child component.React Docs

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.

When you don’t have stable IDs for rendered items, you may use the item index as a key as a last resort:
const todoItems = todos.map((todo, index) =>
// Only do this if items have no stable IDs
<li key={index}>
{todo.text}
</li>
);
Please refer to List and Keys - React

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

If we have array object data . then we map to show the data . and pass the unique id (key = {product.id} ) because browser can select the unique data.
example : [
{
"id": "1",
"name": "walton glass door",
"suplier": "walton group",
"price": "50000",
"quantity": "25",
"description":"Walton Refrigerator is the Best Refrigerator brand in bv
Bangladesh "
},
{
"id": "2",
"name": "walton glass door",
"suplier": "walton group",
"price": "40000",
"quantity": "5",
"description":"Walton Refrigerator is the Best Refrigerator brand in
Bangladesh "
},
}
now we are mapping the data and pass the unique id:
{
products.map(product => <product product={product} key={product.id}
</product>)
}

According to React docs, each row/item should have a unique key.
Keys help React identify which items have changed, are added, or are removed.
Personally, I prefer using the crypto interface to generate a random UUID:
(crypto is built-in in vanilla-js)
const numbers = [1, 2, 3, 4, 5];
const listItems = numbers.map((number) =>
<li key={crypto.randomUUID()}>item {number}
</li>
);

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-header">{item.name}</div>
<div className="card-body" >
<h4 className="card-title">{item.username}</h4>
<p className="card-text">{item.email}</p>
</div>
</div>
))}
</div>

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

I had a unique key, just had to pass it as a prop like this:
<CompName key={msg._id} message={msg} />
This page was helpful:
https://reactjs.org/docs/lists-and-keys.html#keys

In my case, set id to tag
<tbody key={i}>
The problem is solved.

A visual explanation.
The incorrect way key=index (of an array)
As you can see, label 3, label 2, and label 1 ALL got re-rendered (flashing in the Elements panel).
The correct way key=uniqueId
Only the top new element flashes (gets re-rendered).

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.

var TableRowItem = React.createClass({
render: function() {
var td = function() {
return this.props.columns.map(function(c, i) {
return <td key={i}>{this.props.data[c]}</td>;
}, this);
}.bind(this);
return (
<tr>{ td(this.props.item) }</tr>
)
}
});
This will sove the problem.

If you are getting error like :
> index.js:1 Warning: Each child in a list should have a unique "key" prop.
Check the render method of `Home`. See https://reactjs.org/link/warning-keys for more information.
Then Use inside map function like:
{classes.map((user, index) => (
<Card **key={user.id}**></Card>
))}`enter code here`

This is a simple example,I have used a react condition with && first then map, in the I have added the key the user id to be sure that it's unique
<tbody>
{users &&
users.map((user) => {
return <tr key={user._id}>
<td>{user.username}</td>
<td><input
name="isGoing"
type="checkbox"
checked={user.isEnabled}
onChange={handleInputChangeNew}/></td>
<td>{user.role.roleTitle} - {user.role.department.departmentName}</td>
{/*<td className="text-right">
<Button>
ACTION
</Button>
</td>*/}
</tr>
})
}
</tbody>

your key should be unique.like an unique id.And your code should be like this
<div>
{products.map(product => (
<Product key={product.id}>
</Product>
))}
</div>

I don't go with the detail explanation but key to this answer is "key"
just put the key attribute in your tag and ensure that every-time you iterate you give unique value to it
#ensure that key's value is not clashing with others
Example
<div>
{conversation.map(item => (
<div key={item.id } id={item.id}>
</div>
))}
</div>
where conversation is an array something like below :
const conversation = [{id:"unique"+0,label:"OPEN"},{id:"unique"+1,label:"RESOLVED"},{id:"unique"+2,label:"ARCHIVED"},
]

I think when working with tables (or in similar examples), creating a unique key should be passed to child component from the parent component for the sake of REUSABILITY.
Because if you are creating a table, that means you are passing data from the parent. If you assign key={row.name} maybe currently data has name property but if you want to use this table component somewhere else you assume that in each row of data that you have passed, you have name property.
Since the engineer will be preparing the data in the parent component, the engineer should create a key function based on the data.
const keyFunc = (student) => {
return student.id;
};
In this case engineer knows what data it is sending, it knows that each row has id property which is unique. Maybe in the different data set, the data set is stock prices and it does not have "id" property but "symbol"
const keyFunc = (stock) => {
return stock.symbol;
};
this keyFunc should be passed to the child component as a prop to guarantee the reusability and uniqueness.

The "Each child in a list should have a unique "key" prop." warning happens in React when you create a list of elements without the special key attribute. Keys must be assigned to each element in a loop to give stable identity to elements in React.
We can set the id property of the object as a unique key.
export default function App() {
const posts = [
{ id: 1, title: "First "},
{ id: 2, title: "Second" },
{ id: 3, title: "Third" }
];
return (
<div>
<ul>
{posts.map(value =>
<li key={value.id}>{value.title}</li>
)}
</ul>
</div>
);}
//simple way
//if u using ant design remove the empty fragment...
//worng ans---> change to following crt ans
export default function App() {
const posts = [
{ id: 1, title: "First "},
{ id: 2, title: "Second" },
{ id: 3, title: "Third" }
];
{fields.map((field,index)=>{
return(
<> //empty fragment
<Row key={index}>
<Col span={6}>hello</Col>
</Row>
</>
)
})}
//correct ans
//remove the empty fragments after solve this key.prop warning problem
export default function App() {
const posts = [
{ id: 1, title: "First "},
{ id: 2, title: "Second" },
{ id: 3, title: "Third" }
];
{fields.map((field,index)=>{
return(
<> //empty fragment
<Row key={index}>
<Col span={6}>hello</Col>
</Row>
</>
)
})}

I faced a similar problem but not exact. Tried every possible solution and couldn't get rid of that error
Each child in an array should have a unique "key" prop.
Then I tried opening it in a different local host. I don't know how, but it worked!

If you are struggling with this error Each child in a list should have a unique "key" prop.
Solve by declaring index value to the key attribute inside the rendering element.
App.js component
import Map1 from './Map1';
const arr = [1,2,3,4,5];
const App = () => {
return (
<>
<Map1 numb={arr} />
</>
)
}
export default App
Map.js component
const Map1 = (props) => {
let itemTwo = props.numb;
let itemlist = itemTwo.map((item,index) => <li key={index}>{item}</li>)
return (
<>
<ul>
<li style={liStyle}>{itemlist}</li>
</ul>
</>
)
}
export default Map1

Render a unique key [duplicate]

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?

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:
const data = [
{ name: "Nuri", age: 28, city: "HO" },
{ name: "Talib", age: 82, city: "HN" },
{ name: "Jenny", age: 41, city: "IT" },
];
const ExampleComponent = React.createClass({
render: function () {
const infoData = this.props.info;
return (
<div>
{infoData.map((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(<ExampleComponent 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

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.
Bad (Potentially)
<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:
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.
Very bad
<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.

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.

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.

Just add the unique key to the your Components
data.map((marker)=>{
return(
<YourComponents
key={data.id} // <----- unique key
/>
);
})

You should use a unique value for each children key of tbody where
the value cannot not be identical (same) to its sibling
should not change between renders
For example, the key value can be database id or UUID (Universal Unique Identifier).
Here the keys are handling manually:
<tbody>
{rows.map((row) => <ObjectRow key={row.uuid} />)}
</tbody>
You can also let React handle the keys using React.Children.toArray
<tbody>
{React.Children.toArray(rows.map((row) => <ObjectRow />))}
</tbody>

Here are the React docs that explain well using the Key property, the key should be defined at the parent component it should not be used inside the child component.React Docs

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.

When you don’t have stable IDs for rendered items, you may use the item index as a key as a last resort:
const todoItems = todos.map((todo, index) =>
// Only do this if items have no stable IDs
<li key={index}>
{todo.text}
</li>
);
Please refer to List and Keys - React

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

If we have array object data . then we map to show the data . and pass the unique id (key = {product.id} ) because browser can select the unique data.
example : [
{
"id": "1",
"name": "walton glass door",
"suplier": "walton group",
"price": "50000",
"quantity": "25",
"description":"Walton Refrigerator is the Best Refrigerator brand in bv
Bangladesh "
},
{
"id": "2",
"name": "walton glass door",
"suplier": "walton group",
"price": "40000",
"quantity": "5",
"description":"Walton Refrigerator is the Best Refrigerator brand in
Bangladesh "
},
}
now we are mapping the data and pass the unique id:
{
products.map(product => <product product={product} key={product.id}
</product>)
}

According to React docs, each row/item should have a unique key.
Keys help React identify which items have changed, are added, or are removed.
Personally, I prefer using the crypto interface to generate a random UUID:
(crypto is built-in in vanilla-js)
const numbers = [1, 2, 3, 4, 5];
const listItems = numbers.map((number) =>
<li key={crypto.randomUUID()}>item {number}
</li>
);

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-header">{item.name}</div>
<div className="card-body" >
<h4 className="card-title">{item.username}</h4>
<p className="card-text">{item.email}</p>
</div>
</div>
))}
</div>

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

I had a unique key, just had to pass it as a prop like this:
<CompName key={msg._id} message={msg} />
This page was helpful:
https://reactjs.org/docs/lists-and-keys.html#keys

In my case, set id to tag
<tbody key={i}>
The problem is solved.

A visual explanation.
The incorrect way key=index (of an array)
As you can see, label 3, label 2, and label 1 ALL got re-rendered (flashing in the Elements panel).
The correct way key=uniqueId
Only the top new element flashes (gets re-rendered).

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.

var TableRowItem = React.createClass({
render: function() {
var td = function() {
return this.props.columns.map(function(c, i) {
return <td key={i}>{this.props.data[c]}</td>;
}, this);
}.bind(this);
return (
<tr>{ td(this.props.item) }</tr>
)
}
});
This will sove the problem.

If you are getting error like :
> index.js:1 Warning: Each child in a list should have a unique "key" prop.
Check the render method of `Home`. See https://reactjs.org/link/warning-keys for more information.
Then Use inside map function like:
{classes.map((user, index) => (
<Card **key={user.id}**></Card>
))}`enter code here`

This is a simple example,I have used a react condition with && first then map, in the I have added the key the user id to be sure that it's unique
<tbody>
{users &&
users.map((user) => {
return <tr key={user._id}>
<td>{user.username}</td>
<td><input
name="isGoing"
type="checkbox"
checked={user.isEnabled}
onChange={handleInputChangeNew}/></td>
<td>{user.role.roleTitle} - {user.role.department.departmentName}</td>
{/*<td className="text-right">
<Button>
ACTION
</Button>
</td>*/}
</tr>
})
}
</tbody>

your key should be unique.like an unique id.And your code should be like this
<div>
{products.map(product => (
<Product key={product.id}>
</Product>
))}
</div>

I don't go with the detail explanation but key to this answer is "key"
just put the key attribute in your tag and ensure that every-time you iterate you give unique value to it
#ensure that key's value is not clashing with others
Example
<div>
{conversation.map(item => (
<div key={item.id } id={item.id}>
</div>
))}
</div>
where conversation is an array something like below :
const conversation = [{id:"unique"+0,label:"OPEN"},{id:"unique"+1,label:"RESOLVED"},{id:"unique"+2,label:"ARCHIVED"},
]

I think when working with tables (or in similar examples), creating a unique key should be passed to child component from the parent component for the sake of REUSABILITY.
Because if you are creating a table, that means you are passing data from the parent. If you assign key={row.name} maybe currently data has name property but if you want to use this table component somewhere else you assume that in each row of data that you have passed, you have name property.
Since the engineer will be preparing the data in the parent component, the engineer should create a key function based on the data.
const keyFunc = (student) => {
return student.id;
};
In this case engineer knows what data it is sending, it knows that each row has id property which is unique. Maybe in the different data set, the data set is stock prices and it does not have "id" property but "symbol"
const keyFunc = (stock) => {
return stock.symbol;
};
this keyFunc should be passed to the child component as a prop to guarantee the reusability and uniqueness.

The "Each child in a list should have a unique "key" prop." warning happens in React when you create a list of elements without the special key attribute. Keys must be assigned to each element in a loop to give stable identity to elements in React.
We can set the id property of the object as a unique key.
export default function App() {
const posts = [
{ id: 1, title: "First "},
{ id: 2, title: "Second" },
{ id: 3, title: "Third" }
];
return (
<div>
<ul>
{posts.map(value =>
<li key={value.id}>{value.title}</li>
)}
</ul>
</div>
);}
//simple way
//if u using ant design remove the empty fragment...
//worng ans---> change to following crt ans
export default function App() {
const posts = [
{ id: 1, title: "First "},
{ id: 2, title: "Second" },
{ id: 3, title: "Third" }
];
{fields.map((field,index)=>{
return(
<> //empty fragment
<Row key={index}>
<Col span={6}>hello</Col>
</Row>
</>
)
})}
//correct ans
//remove the empty fragments after solve this key.prop warning problem
export default function App() {
const posts = [
{ id: 1, title: "First "},
{ id: 2, title: "Second" },
{ id: 3, title: "Third" }
];
{fields.map((field,index)=>{
return(
<> //empty fragment
<Row key={index}>
<Col span={6}>hello</Col>
</Row>
</>
)
})}

I faced a similar problem but not exact. Tried every possible solution and couldn't get rid of that error
Each child in an array should have a unique "key" prop.
Then I tried opening it in a different local host. I don't know how, but it worked!

If you are struggling with this error Each child in a list should have a unique "key" prop.
Solve by declaring index value to the key attribute inside the rendering element.
App.js component
import Map1 from './Map1';
const arr = [1,2,3,4,5];
const App = () => {
return (
<>
<Map1 numb={arr} />
</>
)
}
export default App
Map.js component
const Map1 = (props) => {
let itemTwo = props.numb;
let itemlist = itemTwo.map((item,index) => <li key={index}>{item}</li>)
return (
<>
<ul>
<li style={liStyle}>{itemlist}</li>
</ul>
</>
)
}
export default Map1

Flat List not maintaining unique List?

I wanted to know if KeyExtrator contributes to maintain the unique list ?
if yes then its not helping me at all.
I am trying to display elements on flat list when a new Item comes in, the list just addes it without checking for uniqueness of the list
Main goal is to make my list unique, I had tried ListView It was working
due to logic implemented using rowHasChanged method.trying to see if there is anything for FlatList.
constructor(props) {
super(props);
this.state = {
TESTDATA: [{
BarCode: '1',
StopAddress: 'Label 1'
}]
};
};
onReceivedMessage(messages) {
var jsondata = JSON.parse(messages)
var dataTest = this.state.TESTDATA;
var Data = addressToDataMap.get(dataTest.BarCodes);
dataTest.push(jsondata);
this.setState({
TESTDATA: dataTest
});
}
<FlatList
extraData={this.state}
data={this.state.TESTDATA}
keyExtractor={(item, index) => item.BarCode.toString()}
renderItem={({ item}) => (
<Text style={styles.baseText}>
Stop Address: {item.StopAddress}{'\n'}
Barcodes: {item.BarCode}
</Text>
)}
/>

I wanted to know if KeyExtrator contributes to maintain the unique list ? NO is the Answer
Explanation : keyExtractor tells the list to use the ids for the react keys instead of the default key property. Keys help React identify which items have changed, are added, or are removed. Key also used for caching and as the react key to tracking item re-ordering. These unique keys are what allow the VirtualizedList (which is what FlatList is built on) to track items and are really important in terms of efficiency. The keyExtractor falls back to using the index by default. For example
<FlatList
style={{}}
data={this.state.FeedDataCollection}
keyExtractor={(item, index) => item.key}
renderItem={(rowData) =>this.RenderFeedCard(rowData)}
/>
Here item.key is is key.So if you want display elements based on the uniqueness, you have write a logic for that. keyExtractor cant do that. Learn more about React Keys here https://reactjs.org/docs/lists-and-keys.html and KeyExtractor here https://facebook.github.io/react-native/docs/flatlist#keyextractor

React props.children is not array

According to the react docs, if a component has multiple children, this.props.children should be an array.
I have the following component:
export class Two extends React.Component {
componentDidMount() {
console.log(Array.isArray(this.props.children)); // false
}
render() {
return(
<div>
{this.props.children}
</div>
);
}
};
Which I pass children to in another component's render() method:
<Two>
<Img src="/photos/tomato.jpg"/>
<Img src="/photos/tomato.jpg"/>
</Two>
Why is this.props.children not an array? More importantly, how can I get it to be one?

Found a better solution to this after some digging in the React.Children source. It looks like a .toArray() method has been added in React 0.14, soon to be released.
Once it is out we will be able to simply do something like this:
let children = React.Children.toArray(this.props.children);
It's documented in https://reactjs.org/docs/react-api.html#reactchildren

I found this solution. It will render all children, one or more.
const BigMama = ({ children, styles, className }) => {
return (
<div
styles={{styles}}
className={(className ? className : '')}
>
{
React.Children.map(children, (child) =>
<React.Fragment>{child}</React.Fragment>)
}
</div>)
}
<BigMama
styles={{border: 'solid groove'}}
className='bass-player'
>
<h1>Foo</h1>
<h2>Bar</h2>
<h3>Baz</h3>
<h4>Impossibru!</h4>
<BigMama>

If you want to do something with the results (eg. render the array in a list), this is how you can use children. The React docs talk about how the map function works with children, as it's an opaque structure.
export const List = ({ children }) => {
return (
<ul>
{React.Children.map(children, child => <li>{child}</li>)}
</ul>
)
}
So then you can call
<List>
{item.name}
{item.price}
</List>

There are many ways and some are mentioned above already but if you wanna keep it simple and want to achieve this without any utility then below should work
const content = [
<Img src="/photos/tomato.jpg"/>,
<Img src="/photos/tomato.jpg"/>
];
<Two>
{content}
</Two>

You might find the spread syntax useful in this case. Have you tried it out?
<div>
{ [...this.props.children] }
</div>
Combine with map to manipulate the output.
<div>
{ [...this.props.children].map(obj => <div style="someStyling"> {obj} </div> ) }
</div>

Is it because is it a DOM node? Try console.log(this.props.children) you will notice that it logged a array of objects (note that each object contains information of the child element of the component). I read that node and array are not the same tho they have a same format. Visit Javascript DOMNode List for more information.