Develop Reference

Fetch and array of objects from a database and displaying it with react

I was trying to fetch an array of objects when I encounter this problem. Sorry for my messy code, I'm a beginner.
export class App extends Component {
state ={
character:[]
}
componentDidMount(){
fetch('https://swapi.dev/api/people/').then(data => data.json()).then(res =>{
this.setState(() =>{
const ar = res.results
return {
character: ar
}
})
})
}
render() {
return (
<div>
test
{console.log(this.state.character[0])}
</div>
)
}
}
That code works fine for me at first until I changed the console log to {console.log(this.state.character[0].name)} it says "cannot read property of an undefined" even though the first line displays the object perfectly

Your App component fetches the data in componentDidMount life cycle method which will execute after the first mount. {console.log(this.state.character[0].name)} shows error "cannot read property of undefined because when the component mounts for first time, the data is not available yet, which means this.state.character is still an empty array. So when try to access this.state.character[0].name, its like saying give me the value of undefined.name which will give you an error because name property does not exist on undefined. To fix this issue you can check if character.length in a conditional and then try to access .name.
Example
render() {
return (
<div>
test
{this.state.character.length && <p>{this.state.character[0].name}</p>}
</div>
)
}

Fetch is asynchronous so you will take some time to get the result but before that your jsx is called already. So it is good to use conditional jsx. Please check the complete example as below where I shown all the name of character array under li element.
export class App extends React.Component {
state = {
character: []
};
componentDidMount() {
fetch('https://swapi.dev/api/people/').then(data => data.json()).then(res => {
this.setState(() => {
const ar = res.results;
return {
character: ar
}
})
})
}
render() {
return (
<div>
test
{
this.state.character && this.state.character.length && this.state.character.map((item, index) => {
return <li key={index}>{item.name}</li>
})
}
</div>
)
}
}
export default App;

So that will not work. Data is not there yet. There are bunch of ways to workaround it. Most simple one is to have object structure predefined in the state object, which is not very dynamic. Otherwise simple .length + if + else will do the job.

Why is my React Component Not Rendering HTML, but logs correctly?

I'm using Redux to store some API-called data, and in my attempt to render it to screen, I've encountered some issues. Basically, I'm using Redux and Fetch to store all the pulled Products from the Stripe API into a state(prop?) called "products", it's structure looks like this:
{ products:
product1ID: {
items: [
0: {json data from Stripe Product API}
]
},
product2ID: {
items: [
0: {json data from Stripe Product API}
]
},
etc...
}
This is not the most effective structure, and will likely need to be refactored. However, that's not the issue at the moment, because I can, at least, return the data using console.log(). The real issue is, when an element is inside the .forEach / .map loop, I cannot render my component, or any HTML to the page. Here's my "ProductList" component, that breaks down the structure above:
import React from 'react';
import { connect } from 'react-redux';
import ProductListItem from './Items/ProductListItem'
import selectProducts from '../../../_selectors/products.selector';
const ProductList = (props) => (
<div>
<h1>Stripe Product List</h1> //This Prints to Screen
{
Object.entries(props.products).forEach(element => {
return element[1].items.map((product) => {
//console.log(product) <-- this logs the json API data correctly! However, nothing prints to screen using HTML elements below!
return <ProductListItem key={product.id} {...product} /> //This does not display!!! Why?
})
})
}
</div>
);
const mapStateToProps = (state) => {
return {
products: selectProducts(state.productsById)
};
};
export default connect(mapStateToProps)(ProductList);
And heres the ProductListItem, just in case you need it:
import React from 'react';
import { Link } from 'react-router-dom';
const ProductListItem = ({ id, name, metadata }) => (
<Link to={`/memberships/${id}`} className="membership red">
<div className="membership__price">
<span className="membership__price_price-label">${metadata.price_start}</span>
<span className="membership__price_mo-label">/mo</span>
</div>
<div className="membership__info">
<h2>{name}</h2>
<p>{metadata.service_description}</p>
</div>
</Link>
);
export default ProductListItem;
So what gives? It logs the correct data, but won't output any HTML. I can't even get a <p> tag to return when I replace the return <ProductListItem key={product.id} {...product} /> line with something simple, like: return <p>test</p>.
I'm fairly new to React and Redux. Any help is appreciated.

Within JSX you are attempting to return an array of React elements using the Array#forEach method.
The difference between Array#forEach and something like Array#map is the returning value.
The Array.forEach() method does not return a value, disregarding the values returned within its callback.
The Array.map() method returns an array with the values returned from its callback.
Solution
To fix your issue, you should replace Array#forEach with Array#map:
Object.entries(props.products).forEach
with the following:
Object.entries(props.products).map

.map() working outside of render function, but fails when used inside render

I'm not a very experienced developer, so I may be missing something simple. I have an array of objects that I am getting from my redux state (this.props.assignments.assignments). I am getting a .map() is not a function when trying to call displayAssignments() in my render return. However, I have another function that is attached to an onClick that is doing the same map and logging "name" into the console, and that one works as expected (when i comment out displayAssignments()). I have no idea why the .map() would work on my onClick but not on displayAssignments.
I included the showAssignments function simply to test if my data could even be mapped...which confused me even more because that one works...
import React, { Component } from 'react';
import PropTypes from 'prop-types';
import { connect } from 'react-redux';
import { getAssignments } from '../../Actions/assignmentActions.js';
class Assignments extends Component {
componentDidMount() {
this.props.getAssignments();
};
showAssignments = () => {
console.log(this.props.assignments.assignments);
this.props.assignments.assignments.map(assignment => (
console.log(assignment.name)
));
};
displayAssignments = () => {
this.props.assignments.assignments.map(assignment => {
return (
<div>
<p>{assignment.name}</p>
<p>{assignment.description}</p>
</div>
)
})
};
render() {
return (
<div>
<h1>Assignments</h1>
<button onClick={this.showAssignments}>Click Me</button>
{this.displayAssignments()}
</div>
);
};
};
Assignments.propTypes = {
getAssignments: PropTypes.func.isRequired,
assignments: PropTypes.object.isRequired
};
const mapStateToProps = state => ({
assignments: state.assignments
});
export default connect(mapStateToProps, { getAssignments })(Assignments);
Expected: to render name and description of assignments by way of mapping
Actual: getting this.props.assignments.assignments.map is not a function

You should return your map() as a value of displayAssigments:
displayAssignments = () => {
return this.props.assignments.assignments.map(assignment => {
return (
<div>
<p>{assignment.name}</p>
<p>{assignment.description}</p>
</div>
)
})
};
Or just remove the curly bracket which wrap .map():
displayAssignments = () => this.props.assignments.assignments.map(assignment =>
<div>
<p>{assignment.name}</p>
<p>{assignment.description}</p>
</div>
)
For your error is not function, make sure that this.props.assignments.assignments is an Array.

Your problem is that, given the fact that you call getAssignments() in the componentDidMount method, it means there is a section of time between when the component is mounted and the assignment field is populated in the store in which there are no assignments, hence why you get the .map() is not a function error.
The click handler is only called after the asynchronous operation finishes so assignments is defined by then.
To fix this, you can change your default state for the assignments field to be:
{
"assignments":[]
}
Another problem with your code, as #radonirina-maminiaina has mentioned, is that your displayAssignments function does not return the result of the maps so the HTML will be blank
The Array.map function creates a new array from the given array with the function provided applied to each element.
Another thing to consider is why you need assignments.assignments to get to the list of assignments. You could probably change your actions or reducer to remove the nesting.

Rendering an array of objects React

Good afternoon,
I am trying to display data that is provided to my application by an instance of MongoDB before the initial render. I have yet been successful in doing so either running into errors or warnings.
This is the part of the render method I am working with.
<div className="right-column">
<div className="pipeline">
{this.props.tournamentList.map((x,i) => {
return <li key={i}>{x.name}</li>
})}
</div>
</div>
this.props.tournamentList has a value of an array of objects like so:
tournamentList:
Array[15]
0:
{…}
1:
{…}
2:
{…} ...
This list comes to my application through the componentWillMount lifecycle method, so before the initial render. To me I should be able to iterate through the array and make a dynamically generated list of tournaments provided by my database.
Yet with the code I provided I am getting this warning:
Uncaught (in promise) Error: Objects are not valid as a React child (found: object with keys {prelims, outRounds, notes}). If you meant to render a collection of children, use an array instead or wrap the object using createFragment(object) from the React add-ons. Check the render method ofBuildTournament.
I tried this approach, creating called displayTournaments and calling it inside the div with the class of "pipeline" but nothing happens, no errors no render:
displayTournaments(){
const { tournamentList } = this.props;
tournamentList.map((x,i) => {
return <li key={i}>{x.name}</li>
})
}
Clearly I am doing something wrong but I don't know what. Is this an instance where I should be using keyed fragments as suggested by the error message? Would anyone smarter than myself be willing to lend some insight?
Cheers.

Update:
Sorry, I misunderstood your question. Kyle is correct with the loading state.
In addition, using a library like lodash will allow you to map over objects in a more natural manner. The native javascript map method doesn't handle objects all that well.
https://www.npmjs.com/package/lodash
you use it much the same way. just
import _ from lodash
then
_.map(objectToMap, (x) => <Component key={x}>{x.thing}</Component>)

Here would be a simple solution that would have a loading state, error state, and success state.
The first thing to note is you will need to use Object.keys() to your object in order to map over the array of keys since you cannot map plain objects. You should also note that the map will return the key of each object so in order to target key values pairs you will need to use a syntax like this tournaments[key].name rather than just doing tournament.name as you are targeting an object with in an object and then grabbing the value.
Let me know if you need any more help with this
import React from 'react'
import Loader from '../Loader'
const resultList = ({ tournaments, isFetching = true }) => (
<div>
{
isFetching
? <div>
<Loader /><br />
<span>Loading…</span>
</div>
: <div>
{
Object.keys(tournaments).length
? <div>
{
tournaments.map((key) => (
<section id={tournaments[key].id} key={key}>
<p>{tournaments[key].name}</p>
</section>
))
}
</div>
: <div>
<p>There are no tournaments....</p>
</div>
}
</div>
}
</div>
);
export default resultList

You are going to need to have a loading state if you get your data in the componentWillMount or componentDidMount lifecycle hooks. The below example will illustrate how this is done.
class ComponentThatGetsAsyncData extends PureComponent {
constructor( props ) {
super( props );
this.state = {
tournamentList: [ ]
}
}
componentDidMount() {
// use any http library you choose
axios.get( "/some_url" )
.then( ( res ) => {
// this will trigger a re-render
this.setState({
tournamentList: res.data
});
})
.catch( ( err ) => {
// handle errors
});
}
render() {
const { tournamentList } = this.state;
// i'd use something other than index for key
// on your initial render your async data will not be available
// so you give a loading indicator and when your http call
// completes it will update state, triggering a re-render
return (
{
tournamentList ?
tournamentList.map((x,i) => {
return <li key={i}>{x.name}</li>
}) :
<div className="loading">Loading...</div>
}
);
}
}

Invariant Violation: Objects are not valid as a React child

In my component's render function I have:
render() {
const items = ['EN', 'IT', 'FR', 'GR', 'RU'].map((item) => {
return (<li onClick={this.onItemClick.bind(this, item)} key={item}>{item}</li>);
});
return (
<div>
...
<ul>
{items}
</ul>
...
</div>
);
}
everything renders fine, however when clicking the <li> element I receive the following error:
Uncaught Error: Invariant Violation: Objects are not valid as a React
child (found: object with keys {dispatchConfig, dispatchMarker,
nativeEvent, target, currentTarget, type, eventPhase, bubbles,
cancelable, timeStamp, defaultPrevented, isTrusted, view, detail,
screenX, screenY, clientX, clientY, ctrlKey, shiftKey, altKey,
metaKey, getModifierState, button, buttons, relatedTarget, pageX,
pageY, isDefaultPrevented, isPropagationStopped, _dispatchListeners,
_dispatchIDs}). If you meant to render a collection of children, use an array instead or wrap the object using createFragment(object) from
the React add-ons. Check the render method of Welcome.
If I change to this.onItemClick.bind(this, item) to (e) => onItemClick(e, item) inside the map function everything works as expected.
If someone could explain what I am doing wrong and explain why do I get this error, would be great
UPDATE 1:
onItemClick function is as follows and removing this.setState results in error disappearing.
onItemClick(e, item) {
this.setState({
lang: item,
});
}
But I cannot remove this line as I need to update state of this component

I was having this error and it turned out to be that I was unintentionally including an Object in my JSX code that I had expected to be a string value:
return (
<BreadcrumbItem href={routeString}>
{breadcrumbElement}
</BreadcrumbItem>
)
breadcrumbElement used to be a string but due to a refactor had become an Object. Unfortunately, React's error message didn't do a good job in pointing me to the line where the problem existed. I had to follow my stack trace all the way back up until I recognized the "props" being passed into a component and then I found the offending code.
You'll need to either reference a property of the object that is a string value or convert the Object to a string representation that is desirable. One option might be JSON.stringify if you actually want to see the contents of the Object.

So I got this error when trying to display the createdAt property which is a Date object. If you concatenate .toString() on the end like this, it will do the conversion and eliminate the error. Just posting this as a possible answer in case anyone else ran into the same problem:
{this.props.task.createdAt.toString()}

I just got the same error but due to a different mistake: I used double braces like:
{{count}}
to insert the value of count instead of the correct:
{count}
which the compiler presumably turned into {{count: count}}, i.e. trying to insert an Object as a React child.

Just thought I would add to this as I had the same problem today, turns out that it was because I was returning just the function, when I wrapped it in a <div> tag it started working, as below
renderGallery() {
const gallerySection = galleries.map((gallery, i) => {
return (
<div>
...
</div>
);
});
return (
{gallerySection}
);
}
The above caused the error. I fixed the problem by changing the return() section to:
return (
<div>
{gallerySection}
</div>
);
...or simply:
return gallerySection

React child(singular) should be type of primitive data type not object or it could be JSX tag(which is not in our case). Use Proptypes package in development to make sure validation happens.
Just a quick code snippet(JSX) comparision to represent you with idea :
Error : With object being passed into child
<div>
{/* item is object with user's name and its other details on it */}
{items.map((item, index) => {
return <div key={index}>
--item object invalid as react child--->>>{item}</div>;
})}
</div>
Without error : With object's property(which should be primitive, i.e. a string value or integer value) being passed into child.
<div>
{/* item is object with user's name and its other details on it */}
{items.map((item, index) => {
return <div key={index}>
--note the name property is primitive--->{item.name}</div>;
})}
</div>
TLDR; (From the source below) : Make sure all of the items you're rendering in JSX are primitives and not objects when using React. This error usually happens because a function involved in dispatching an event has been given an unexpected object type (i.e passing an object when you should be passing a string) or part of the JSX in your component is not referencing a primitive (i.e. this.props vs this.props.name).
Source - codingbismuth.com

Mine had to do with forgetting the curly braces around props being sent to a presentational component:
Before:
const TypeAheadInput = (name, options, onChange, value, error) => {
After
const TypeAheadInput = ({name, options, onChange, value, error}) => {

I too was getting this "Objects are not valid as a React child" error and for me the cause was due to calling an asynchronous function in my JSX. See below.
class App extends React.Component {
showHello = async () => {
const response = await someAPI.get("/api/endpoint");
// Even with response ignored in JSX below, this JSX is not immediately returned,
// causing "Objects are not valid as a React child" error.
return (<div>Hello!</div>);
}
render() {
return (
<div>
{this.showHello()}
</div>
);
}
}
What I learned is that asynchronous rendering is not supported in React. The React team is working on a solution as documented here.

Mine had to do with unnecessarily putting curly braces around a variable holding a HTML element inside the return statement of the render() function. This made React treat it as an object rather than an element.
render() {
let element = (
<div className="some-class">
<span>Some text</span>
</div>
);
return (
{element}
)
}
Once I removed the curly braces from the element, the error was gone, and the element was rendered correctly.

For anybody using Firebase with Android, this only breaks Android. My iOS emulation ignores it.
And as posted by Apoorv Bankey above.
Anything above Firebase V5.0.3, for Android, atm is a bust. Fix:
npm i --save firebase#5.0.3
Confirmed numerous times here
https://github.com/firebase/firebase-js-sdk/issues/871

I also have the same problem but my mistake is so stupid. I was trying to access object directly.
class App extends Component {
state = {
name:'xyz',
age:10
}
render() {
return (
<div className="App">
// this is what I am using which gives the error
<p>I am inside the {state}.</p>
//Correct Way is
<p>I am inside the {this.state.name}.</p>
</div>
);
}
}

Typically this pops up because you don't destructure properly. Take this code for example:
const Button = text => <button>{text}</button>
const SomeForm = () => (
<Button text="Save" />
)
We're declaring it with the = text => param. But really, React is expecting this to be an all-encompassing props object.
So we should really be doing something like this:
const Button = props => <button>{props.text}</button>
const SomeForm = () => (
<Button text="Save" />
)
Notice the difference? The props param here could be named anything (props is just the convention that matches the nomenclature), React is just expecting an object with keys and vals.
With object destructuring you can do, and will frequently see, something like this:
const Button = ({ text }) => <button>{text}</button>
const SomeForm = () => (
<Button text="Save" />
)
...which works.
Chances are, anyone stumbling upon this just accidentally declared their component's props param without destructuring.

Just remove the curly braces in the return statement.
Before:
render() {
var rows = this.props.products.map(product => <tr key={product.id}><td>{product.name}</td><td>{product.price}</td></tr>);
return {rows}; // unnecessary
}
After:
render() {
var rows = this.props.products.map(product => <tr key={product.id}><td>{product.name}</td><td>{product.price}</td></tr>);
return rows; // add this
}

I had the same problem because I didn't put the props in the curly braces.
export default function Hero(children, hero ) {
return (
<header className={hero}>
{children}
</header>
);
}
So if your code is similar to the above one then you will get this error.
To resolve this just put curly braces around the props.
export default function Hero({ children, hero }) {
return (
<header className={hero}>
{children}
</header>
);
}

I got the same error, I changed this
export default withAlert(Alerts)
to this
export default withAlert()(Alerts).
In older versions the former code was ok , but in later versions it throws an error. So use the later code to avoid the errror.

This was my code:
class App extends Component {
constructor(props){
super(props)
this.state = {
value: null,
getDatacall : null
}
this.getData = this.getData.bind(this)
}
getData() {
// if (this.state.getDatacall === false) {
sleep(4000)
returnData("what is the time").then(value => this.setState({value, getDatacall:true}))
// }
}
componentDidMount() {
sleep(4000)
this.getData()
}
render() {
this.getData()
sleep(4000)
console.log(this.state.value)
return (
<p> { this.state.value } </p>
)
}
}
and I was running into this error. I had to change it to
render() {
this.getData()
sleep(4000)
console.log(this.state.value)
return (
<p> { JSON.stringify(this.state.value) } </p>
)
}
Hope this helps someone!

If for some reason you imported firebase. Then try running npm i --save firebase#5.0.3. This is because firebase break react-native, so running this will fix it.

In my case it was i forgot to return a html element frm the render function and i was returning an object . What i did was i just wrapped the {items} with a html element - a simple div like below
<ul>{items}</ul>

Just remove the async keyword in the component.
const Register = () => {
No issues after this.

In my case, I added a async to my child function component and encountered this error. Don't use async with child component.

I got this error any time I was calling async on a renderItem function in my FlatList.
I had to create a new function to set my Firestore collection to my state before calling said state data inside my FlatList.

My case is quite common when using reduce but it was not shared here so I posted it.
Normally, if your array looks like this:
[{ value: 1}, {value: 2}]
And you want to render the sum of value in this array. JSX code looks like this
<div>{array.reduce((acc, curr) => acc.value + curr.value)}</div>
The problem happens when your array has only one item, eg: [{value: 1}].
(Typically, this happens when your array is the response from server so you can not guarantee numbers of items in that array)
The reduce function returns the element itself when array has only one element, in this case it is {value: 1} (an object), it causes the Invariant Violation: Objects are not valid as a React child error.

You were just using the keys of object, instead of the whole object!
More details can be found here: https://github.com/gildata/RAIO/issues/48
import React, { Component } from 'react';
import PropTypes from 'prop-types';
class SCT extends Component {
constructor(props, context) {
super(props, context);
this.state = {
data: this.props.data,
new_data: {}
};
}
componentDidMount() {
let new_data = this.state.data;
console.log(`new_data`, new_data);
this.setState(
{
new_data: Object.assign({}, new_data)
}
)
}
render() {
return (
<div>
this.state.data = {JSON.stringify(this.state.data)}
<hr/>
<div style={{color: 'red'}}>
{this.state.new_data.name}<br />
{this.state.new_data.description}<br />
{this.state.new_data.dependtables}<br />
</div>
</div>
);
}
}
SCT.propTypes = {
test: PropTypes.string,
data: PropTypes.object.isRequired
};
export {SCT};
export default SCT;
<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>

If you are using Firebase and seeing this error, it's worth to check if you're importing it right. As of version 5.0.4 you have to import it like this:
import firebase from '#firebase/app'
import '#firebase/auth';
import '#firebase/database';
import '#firebase/storage';
Yes, I know. I lost 45 minutes on this, too.

I just put myself through a really silly version of this error, which I may as well share here for posterity.
I had some JSX like this:
...
{
...
<Foo />
...
}
...
I needed to comment this out to debug something. I used the keyboard shortcut in my IDE, which resulted in this:
...
{
...
{ /* <Foo /> */ }
...
}
...
Which is, of course, invalid -- objects are not valid as react children!

I'd like to add another solution to this list.
Specs:
"react": "^16.2.0",
"react-dom": "^16.2.0",
"react-redux": "^5.0.6",
"react-scripts": "^1.0.17",
"redux": "^3.7.2"
I encountered the same error:
Uncaught Error: Objects are not valid as a React child (found: object
with keys {XXXXX}). If you meant to render a collection of children,
use an array instead.
This was my code:
let payload = {
guess: this.userInput.value
};
this.props.dispatch(checkAnswer(payload));
Solution:
// let payload = {
// guess: this.userInput.value
// };
this.props.dispatch(checkAnswer(this.userInput.value));
The problem was occurring because the payload was sending the item as an object. When I removed the payload variable and put the userInput value into the dispatch everything started working as expected.

If in case your using Firebase any of the files within your project.
Then just place that import firebase statement at the end!!
I know this sounds crazy but try it!!

I have the same issue, in my case,
I update the redux state, and new data parameters did not match old parameters, So when I want to access some parameters it through this Error,
Maybe this experience help someone

My issue was simple when i faced the following error:
objects are not valid as a react child (found object with keys {...}
was just that I was passing an object with keys specified in the error while trying to render the object directly in a component using {object} expecting it to be a string
object: {
key1: "key1",
key2: "key2"
}
while rendering on a React Component, I used something like below
render() {
return this.props.object;
}
but it should have been
render() {
return this.props.object.key1;
}

If using stateless components, follow this kind of format:
const Header = ({pageTitle}) => (
<h1>{pageTitle}</h1>
);
export {Header};
This seemed to work for me

Something like this has just happened to me...
I wrote:
{response.isDisplayOptions &&
{element}
}
Placing it inside a div fixed it:
{response.isDisplayOptions &&
<div>
{element}
</div>
}