Develop Reference

React JS - Props is not shown in the rendered HTML [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

Combine two object keys in and array without loosing the value

I have an object that I'm mapping through its keys in my react component to display header titles.
obj1 = {
TERM_NOTE: 'TERM_NOTE',
EQUITY: 'EQUITY',
REVENUE_SHARING_NOTE: 'REVENUE_SHARING_NOTE',
CONVERTIBLE_NOTES: 'CONVERTIBLE_NOTES',
SAFE: 'SAFE',
FUNDS: 'FUNDS',
}
const keytermsSecurityTypes = Object.keys(obj1);
keytermsSecurityTypes.forEach((type) => {
listAsPerSecurityType[type] = props.investments.filter(i => get(i, 'offering.keyTerms.securities') === type);
listHeaderAsPerSecurityType[type] = listMetaByType.filter(i => i.securityType.includes('ALL') || i.securityType.includes(type));
});
and the variables mentioned before are used inside the React component:
keytermsSecurityTypes.map(type => (
<>
{listAsPerSecurityType[type] && listAsPerSecurityType[type].length
&& (
<>
<Header as="h5" className="investment-list" content={getSecurityHeaderTitle(type)} />
<ListTable listData={listAsPerSecurityType[type]} header={listHeaderAsPerSecurityType[type]}/>
</>
)}
</>
))
for CONVERTIBLE_NOTES and SAFE keys, title should display as "Convertible Securities" for both as one title. However I still need to keep the values of both keys as they are being used in the same component as well.
While I got the title to display the same string for both keys, I still can't figure out how to combine them into one title.
Also, considering I'm still new to React and javascript, is there a better way/best practice to do this?
Desired output:
Design spec
Problem output:
Problem
Thank you.

Change react props within component as optimization

Problem
*That is not exactly my case but here is what matters.
Imagine an app that renders a list of elements:
const ElementsList = ({elements, <onElementTextChange>}) => (
<div className='ElementsList'>
{elements.map(el => (
<Element
key={el.id}
element={el}
/>
))}
</div>
)
Elements are kept in state of the parent component that renders ElementsList. Each element is a simple object:
this.state = {elements: [
{id: 1, text: 'apple'},
{id: 23, text: 'banana'},
]}
...
render() { ... <ElementsList elements={this.state.elements}/> ... }
Each element renders with the input field nearby to change its text:
export const Element = ({element, <onTextChange>}) => (
<div className='Element'>
<div className='element-text'>{element.text}</div>
<input type='text' value={element.text} onChange={???}></input>
</div>
)
Consider that there can be a lot of elements (the more the app can handle the merrier).
Now the question is how should we go about updating texts through those inputs.
Solutions
I see 2 options (some combinations of them are possible):
1. Proper react way (is it?) - notify parent component about the change
at Element: onChange={evt => onTextChange(evt.target.value)}
at ElementsList: onTextChange={newText => onElementTextChange(el, newText)}
at parent component: onElementTextChange={ (element, newText) => {make new elements array with the updated element somehow; setState(elements)} }
That solution seems too awkward to implement. Also it is slow compared to the alternatives below. Events happening on input change (correct me if I'm wrong):
notify parent component about the change through the chain of components
parent component constructs new elements array and calls setState
the whole virtual DOM gets updated (including each element)
updated virtual DOM is compared to the previous virtual DOM version
one Element is updated in the real DOM
2. Update props of the Element directly and forceUpdate()
rewrite Element to be a class instead of functional component
at Element: onChange={evt => {element.text = evt.target.value; this.forceUpdate()}}
Events happening on input change:
props.element.text is changed and forceUpdate is called
only one Element in the virtual DOM is updated
updated subtree of the virtual DOM (one Element) is compared to the old virtual DOM
one Element is updated in the real DOM
Questions
Currently I'm using an intermediate approach but more on the "proper react way" side. And it isn't fast enough.
I like the second option more, it requires less code and seems like it should work much faster (in theory).
Will I get significantly better perfomance by using the second approach? (Remember that there are thouthands of elements)
What specific problems this approach can lead to? Yeah, yeah, I know the code is less transparent and this not how it is meant to be.
Do you see any other way to get the perfomance up? I'd be happy to try even less react-like suggestions, might consider giving up on react partially or entirely.

Immutable data structures address your performance concerns through techniques like memoization and structural sharing.
You should look into using Immutable.js. Then you can use React.PureComponent to do efficient value equality checks on large data structures.
One downside is that Immutable.js data structures do not mix well with regular mutable javascript data structures, which can be frustrating. Some languages like Elm or ClojureScript offer first class support for these kinds of data structures, making it a much more consistent experience.

you can do like this
this.state = {elements: [
{id: 1, text: 'apple'},
{id: 23, text: 'banana'},
]}
// creating a update function which you will pass down to the child
updateState = (id, val) => {
const newElements = this.state.elements;
newElements.map(item => {
if(item.id === id) item.text = val;
});
this.setState({
elements: newElements,
})
}
render() { ... <ElementsList elements={this.state.elements} updateState ={this.updateState}/> ... }
now in the ElementsList
const ElementsList = ({elements, updateState, <onElementTextChange>}) => (
<div className='ElementsList'>
{elements.map(el => (
<Element
key={el.id}
element={el}
updateState = {updateState}
/>
))}
</div>
)
now in the element
export class Element extends React.Component{
state = {
value: '',
}
onChangeValue = (e) => {
this.setState({ value: e.target.value});
// calling the parent function directly
this.props.updateState(this.props.element.id, e.target.value);
}
componentDidMount(){
this.setState({
value: this.props.element.text,
})
}
render(){
return(
<div className='Element'>
<div className='element-text'>{element.text}</div>
<input type='text' value={this.state.value} onChange={this.onChangeValue}></input>
</div>
)
}
}

Assign unique id to component as it renders using Redux

I recently started learning how to use Redux as a way to manage state in my applications and I like it so far, but I'm having trouble assigning props dynamically to to children of a parent component.
Here is what I am working with, basic parent with children scenario:
<Layout>
<Child />
<Child />
<Child />
</Layout>
I want each child to have a unique id so childProps.id for the first child would be 1, the second child would be 2 and so on.
First here is the assignId function I want to fire when the component is rendered:
export function assignId(number){
return{
type: "ASSIGN",
payload: number
}
}
Here is the reducer listening for this function:
export default function reducer(state={
id: 0
}, action){
switch (action.type){
case "ASSIGN": {
return {...state, id: action.payload}
}
}
return state
}
Finally the <Child /> component itself:
import React from "react";
import { activeBox, beginSession, unselect, assignId } from '../actions/boxActions'
export default class Child extends React.Component {
componentWillMount(){
var i = 1
this.props.dispatch(assignId(i));
i++
}
render(){
return (
<div className="box"></div>
)
}
}
componentWillMount works. I have var i = 1 it sets childProps.id to 1 for all of my <Child /> components. Simply, how would I make a function that makes each childProps.id unique for each <Child /> component based on it's order within the <Layout /> parent?

I think you're adding complexity with zero value. The id attribute of a DOM element has for sure to be unique, so you're on a right track, however
in React, you don't manage elements by their IDs; take a look at references to corresponding DOM elements instead,
generating a globally unique identifier comes at cost of maintaining that generating function, and
if you plan to have an arbitrary number of Child components coming from somewhere and being reflected from an iterable, why not using Array#map?
So
render() {
const childElements = [1, 2, 3];
return (
<Layout>
<Child id="1" />
<Child id="2" />
<Child id="3" />
</Layout>
);
}
turns into
render() {
const childElements = [1, 2, 3];
return (
<Layout>
{childElements.map((id, index) => (
<Child
key={index}
id={id} />
))}
</Layout>
);
}
From your code, I can see that you want to dispatch an action to assign an element with an ID, while that ID isn't even stored anywhere. If it really isn't stored and used for any purpose, there's no need in it.
A general rule in React is to make things simple, not complicated. You have data, you reflect this data into DOM using components (and the hierarchy they form), and that's it. If you need to listen to DOM events, you use onClick, onChange and other props that are bound to DOM event listeners, and that's it. Once you want to use a lib that works with DOM, try to stick to refs instead of ids, because those are guaranteed to refer to existing DOM element, and reference by id may lead to nowhere when a component is being updated.

Contrary to what others are saying, it makes good sense to assign IDs to array items in the store. When you render them you can pass this ID as the key prop to each child component. This starts to make great sense when you insert, delete or reorder array items. React will use this key to help it find the right component that matches the array item. This can speed up rendering massively when store item indices change around.
This is the reason the key prop exists and it's also why you should not simply use the index of the item. The index isn't the same thing as the identity.
Now, to generate and store such a unique ID, do something like this:
let nextId = 0;
function addItem(item) {
return {
type: 'ADD_ITEM',
payload: {
...item,
id: nextId++
}
};
}
function reducer(state = [], action) {
switch action.type {
case 'ADD_ITEM':
return [...state, action.payload]
break;
default:
return state;
}
}
You can then assign this ID to the key prop of each child. It'll be unique for the session and that's all you probably need. If not you can generate any other ID you like, such as a GUID. In any case, the important thing is that you want the ID to be a part of the item from the beginning and that the ID is not and should not be assigned to the component, but to the item the component is going to display. The component assumes the identity of the item found in the store and not the other way around.

At the parent level, get your children IDs in an array, loop through them, and assign the id as a prop of the child.
Something like this:
<Layout>
{childrenIds.map((i) => { return <Child id={i} />} )}
</Layout>

Simple Add / Remove Component Pattern

I'm quite confused about this pattern in React. I have seen it in other places it but it does not seem correct.
Given the below code:
/** #jsx React.DOM */
var React = window.React = require('react'),
Item = require("./ui/Item");
var ItemApp = React.createClass({
getInitialState: function() {
return {
items: [ "Example Item" ] // we want one thing to render initially
}; // so prepopulate with one thing
},
getDefaultProps: function () {
return {
index: 1
}
},
onAddItem: function (e) {
var debts = this.state.debts.push( "New Item " + this.props.index);
this.setState({ items: this.state.items });
this.props.index++;
},
onRemoveItem: function (i) {
// splice here
this.state.items.splice(i, 1);
this.setState({ items: this.state.items });
this.props.index--;
},
render: function () {
var items = this.state.items;
var closeFn = this.onRemoveItem;
return (
<div className="col-lg-12">
<div className="cold-md-3"></div>
<div className="col-md-3 col-md-offset-9">
<button type="button" className="btn btn-default" onClick={this.onAddItem}><span className="glyphicon glyphicon-plus"></span> Add New Debt</button>
</div>
{items.map(function (item, i) {
return <Item name={item} closeFn={closeFn.bind(null, i)} key={i} />;
})}
</div>
);
}
});
The example above works like intended (it's a list and you can add and remove things at will) however whenever I add something renders the whole thing over again.
So after only adding one component the whole thing has rendered 3 times. After clicking twice it will have rendered 5 times. You can see why this might be a problem.
This seems really inefficient is there a better way to do this?

If you want to manually manage the dom, react probably isn't for you. You can always do it better than the abstraction if you really want to.
In react, it's usually best to aim for code that can clearly express its intent to react, and let it do its thing.
You code is close, the only problem is your key attributes don't uniquely reflect the items in the array. Index based keys are 100% fine for when you have a stack. I gave a more detailed answer to a similar question.
To actually solve this here, because any item can be removed at any time, you need to encode a unique identifier in the item itself. Instead of storing an array of strings ["a", "b", "c"] you should store an array of objects [{id: '1', text: "a"}, {id: '2', text: "b"}, {id: '3', text: "c"}]. You can then use the id as your key and it'll be more efficient.
This is unrelated to the question, but this.props.index-- is modifying props, which is bad and unreliable. If you need to keep a counter for some reason, set it directly on this in componentWillMount. If you need to communicate something up the tree, you accept a callback as a prop, and call that.
Also <Item name={item} closeFn={closeFn.bind(null, i)} key={i} /> doesn't describe any way to get data back out of the Item, other than when deletion is requested.