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Component is loading twice [duplicate]

I don't know why my React component is rendering twice. So I am pulling a phone number from params and saving it to state so I can search through Firestore. Everything seems to be working fine except it renders twice... The first one renders the phone number and zero points. The second time it renders all the data is displayed correctly. Can someone guide me to the solution.
class Update extends Component {
constructor(props) {
super(props);
const { match } = this.props;
this.state = {
phoneNumber: match.params.phoneNumber,
points: 0,
error: ''
}
}
getPoints = () => {
firebase.auth().onAuthStateChanged((user) => {
if(user) {
const docRef = database.collection('users').doc(user.uid).collection('customers').doc(this.state.phoneNumber);
docRef.get().then((doc) => {
if (doc.exists) {
const points = doc.data().points;
this.setState(() => ({ points }));
console.log(points);
} else {
// doc.data() will be undefined in this case
console.log("No such document!");
const error = 'This phone number is not registered yet...'
this.setState(() => ({ error }));
}
}).catch(function(error) {
console.log("Error getting document:", error);
});
} else {
history.push('/')
}
});
}
componentDidMount() {
if(this.state.phoneNumber) {
this.getPoints();
} else {
return null;
}
}
render() {
return (
<div>
<div>
<p>{this.state.phoneNumber} has {this.state.points} points...</p>
<p>Would you like to redeem or add points?</p>
</div>
<div>
<button>Redeem Points</button>
<button>Add Points</button>
</div>
</div>
);
}
}
export default Update;

You are running your app in strict mode. Go to index.js and comment strict mode tag. You will find a single render.
This happens is an intentional feature of the React.StrictMode. It only happens in development mode and should help to find accidental side effects in the render phase.
From the docs:
Strict mode can’t automatically detect side effects for you, but it can help you spot them by making them a little more deterministic. This is done by intentionally double-invoking the following functions:...
^ In this case the render function.
Official documentation of what might cause re-rendering when using React.StrictMode:
https://reactjs.org/docs/strict-mode.html#detecting-unexpected-side-effects

This is because of React Strict Mode code.
Remove -> React.StrictMode, from ReactDOM.render code.
Will render 2 times on every re-render:
ReactDOM.render(
<React.StrictMode>
<App />
</React.StrictMode>,
document.getElementById('root')
);
Will render 1 time:
ReactDOM.render(
<>
<App />
</>,
document.getElementById('root')
);

React is rendering the component before getPoints finishing the asynchronous operation.
So the first render shows the initial state for points which is 0, then componentDidMount is called and triggers the async operation.
When the async operation is done and the state been updated, another render is triggered with the new data.
If you want, you can show a loader or an indicator that the data is being fetched and is not ready yet to display with conditional rendering.
Just add another Boolean key like isFetching, set it to true when you call the server and set it to false when the data is received.
Your render can look something like this:
render() {
const { isFetching } = this.state;
return (
<div>
{isFetching ? (
<div>Loading...</div>
) : (
<div>
<p>
{this.state.phoneNumber} has {this.state.points} points...
</p>
<p>Would you like to redeem or add points?</p>
<div>
<button>Redeem Points</button>
<button>Add Points</button>
</div>
</div>
)}
</div>
);
}

React.StrictMode, makes it render twice, so that we do not put side effects in following locations
constructor
componentWillMount (or UNSAFE_componentWillMount)
componentWillReceiveProps (or UNSAFE_componentWillReceiveProps)
componentWillUpdate (or UNSAFE_componentWillUpdate)
getDerivedStateFromProps
shouldComponentUpdate
render
setState updater functions (the first argument)
All these methods are called more than once, so it is important to avoid having side-effects in them. If we ignore this principle it is likely to end up with inconsistent state issues and memory leaks.
React.StrictMode cannot spot side-effects at once, but it can help us find them by intentionally invoking twice some key functions.
These functions are:
Class component constructor, render, and shouldComponentUpdate methods
Class component static getDerivedStateFromProps method
Function component bodies
State updater functions (the first argument to setState)
Functions passed to useState, useMemo, or useReducer
This behaviour definitely has some performance impact, but we should not worry since it takes place only in development and not in production.
credit: https://mariosfakiolas.com/blog/my-react-components-render-twice-and-drive-me-crazy/

it is done intentionally by react to avoid this
remove
<React.StrictMode> </React.StrictMode>
from index.js

I worked around this by providing a custom hook. Put the hook below into your code, then:
// instead of this:
useEffect( ()=> {
console.log('my effect is running');
return () => console.log('my effect is destroying');
}, []);
// do this:
useEffectOnce( ()=> {
console.log('my effect is running');
return () => console.log('my effect is destroying');
});
Here is the code for the hook:
export const useEffectOnce = ( effect => {
const destroyFunc = useRef();
const calledOnce = useRef(false);
const renderAfterCalled = useRef(false);
if (calledOnce.current) {
renderAfterCalled.current = true;
}
useEffect( () => {
if (calledOnce.current) {
return;
}
calledOnce.current = true;
destroyFunc.current = effect();
return ()=> {
if (!renderAfterCalled.current) {
return;
}
if (destroyFunc.current) {
destroyFunc.current();
}
};
}, []);
};
See this blog for the explanation.

Well, I have created a workaround hook for this. Check this, if it helps:
import { useEffect } from "react";
const useDevEffect = (cb, deps) => {
let ran = false;
useEffect(() => {
if (ran) return;
cb();
return () => (ran = true);
}, deps);
};
const isDev = !process.env.NODE_ENV || process.env.NODE_ENV === "development";
export const useOnceEffect = isDev ? useDevEffect : useEffect;
CodeSandbox Demo: https://github.com/akulsr0/react-18-useeffect-twice-fix

React internally monitors & manages its render cycles using its virtual dom and its diffing algorithms, so you need not worry about the number of re-renders. Let the re-renders to be managed by react. Even though the render function is getting invoked, there are sub components which doesn't gets refreshed on ui, if there is no props or state change inside it. Every setstate function call will inform react to check the diffing algorithm, and invoke the render function.
So in your case, since you have a setstate defined inside the getPoints function, it tells react to rerun the diffing process through the render function.

Provide alternative render method for React Component

In my app I have several form field components that all behave the same way but look slightly different. I want to be able to use the state and class methods of a single formfield component while providing some sort of alternative render method so that I can customize the appearance of this element on the fly. I know I can wrap children and then use the props.children in the component. But I'm looking to re-use the components methods somehow:
class Parent extends React.Component {
render() {
<div className="parent">
<ChildFormField
renderAlternate={self => {
return (
<div className="child--alternate">
<input onChange={self.doThing} />
</div>
);
}}
/>
</div>
}
}
// And the child component look something like...
class ChildFormField extends React.Component {
state = {
value: null
}
doThing = value => {
return this.setState({ value });
}
render() {
if (this.props.renderAlternate !== undefined) {
return this.props.renderAlternate();
}
// standard return
return <div />;
}
}
I'm relatively new to React outside of its basic usage. Is there a recommended way to achieve this functionality?

Your renderAlternate function expects a parameter self. So you need to pass this when calling it.
return this.props.renderAlternate(this);
See https://codesandbox.io/s/w759j6pl6k as an example of your code.

This recipe is known as render prop. It's widely used where it's suitable, but here it looks like bad design decision, primarily because it exists to access component self instance. This is the case for inheritance:
class AlternateChildFormField extends ChildFormField {
render() {
return (
<div className="child--alternate">
<input onChange={this.doThing} />
</div>
);
}
}
In React, function composition is usually preferred, but inheritance is acceptable solution if it serves a good purpose. ChildFormField requires doThing to be a method and not helper function because it needs to access this.setState.
An alternative is to use React 16.7 hooks and functional components. This way the same component can be expressed with composition:
const useThingState = () => {
const [state, setState] = useState({ value: null });
return value => {
return setState({ value });
};
}
const ChildFormField = props => {
// not used here
// const doThing = useThingState();
return <div />;
}
const AlternateChildFormField = props => {
const doThing = useThingState();
return (
<div className="child--alternate">
<input onChange={doThing} />
</div>
);
}

The common way this is managed in React ecosystem are High Order Components:
ref: https://reactjs.org/docs/higher-order-components.html
ref: https://medium.com/backticks-tildes/reusing-react-component-logic-with-higher-order-component-3fbe284beec9
Further, what you are looking for maybe a reuse of state logic.
As React 16.7-alpha you can use hooks to reuse state logic with functional components but APIs are subject of possible breaking changes.
ref: https://medium.com/#nicolaslopezj/reusing-logic-with-react-hooks-8e691f7352fa

Why is my React component is rendering twice?

I don't know why my React component is rendering twice. So I am pulling a phone number from params and saving it to state so I can search through Firestore. Everything seems to be working fine except it renders twice... The first one renders the phone number and zero points. The second time it renders all the data is displayed correctly. Can someone guide me to the solution.
class Update extends Component {
constructor(props) {
super(props);
const { match } = this.props;
this.state = {
phoneNumber: match.params.phoneNumber,
points: 0,
error: ''
}
}
getPoints = () => {
firebase.auth().onAuthStateChanged((user) => {
if(user) {
const docRef = database.collection('users').doc(user.uid).collection('customers').doc(this.state.phoneNumber);
docRef.get().then((doc) => {
if (doc.exists) {
const points = doc.data().points;
this.setState(() => ({ points }));
console.log(points);
} else {
// doc.data() will be undefined in this case
console.log("No such document!");
const error = 'This phone number is not registered yet...'
this.setState(() => ({ error }));
}
}).catch(function(error) {
console.log("Error getting document:", error);
});
} else {
history.push('/')
}
});
}
componentDidMount() {
if(this.state.phoneNumber) {
this.getPoints();
} else {
return null;
}
}
render() {
return (
<div>
<div>
<p>{this.state.phoneNumber} has {this.state.points} points...</p>
<p>Would you like to redeem or add points?</p>
</div>
<div>
<button>Redeem Points</button>
<button>Add Points</button>
</div>
</div>
);
}
}
export default Update;

You are running your app in strict mode. Go to index.js and comment strict mode tag. You will find a single render.
This happens is an intentional feature of the React.StrictMode. It only happens in development mode and should help to find accidental side effects in the render phase.
From the docs:
Strict mode can’t automatically detect side effects for you, but it can help you spot them by making them a little more deterministic. This is done by intentionally double-invoking the following functions:...
^ In this case the render function.
Official documentation of what might cause re-rendering when using React.StrictMode:
https://reactjs.org/docs/strict-mode.html#detecting-unexpected-side-effects

This is because of React Strict Mode code.
Remove -> React.StrictMode, from ReactDOM.render code.
Will render 2 times on every re-render:
ReactDOM.render(
<React.StrictMode>
<App />
</React.StrictMode>,
document.getElementById('root')
);
Will render 1 time:
ReactDOM.render(
<>
<App />
</>,
document.getElementById('root')
);

React is rendering the component before getPoints finishing the asynchronous operation.
So the first render shows the initial state for points which is 0, then componentDidMount is called and triggers the async operation.
When the async operation is done and the state been updated, another render is triggered with the new data.
If you want, you can show a loader or an indicator that the data is being fetched and is not ready yet to display with conditional rendering.
Just add another Boolean key like isFetching, set it to true when you call the server and set it to false when the data is received.
Your render can look something like this:
render() {
const { isFetching } = this.state;
return (
<div>
{isFetching ? (
<div>Loading...</div>
) : (
<div>
<p>
{this.state.phoneNumber} has {this.state.points} points...
</p>
<p>Would you like to redeem or add points?</p>
<div>
<button>Redeem Points</button>
<button>Add Points</button>
</div>
</div>
)}
</div>
);
}

React.StrictMode, makes it render twice, so that we do not put side effects in following locations
constructor
componentWillMount (or UNSAFE_componentWillMount)
componentWillReceiveProps (or UNSAFE_componentWillReceiveProps)
componentWillUpdate (or UNSAFE_componentWillUpdate)
getDerivedStateFromProps
shouldComponentUpdate
render
setState updater functions (the first argument)
All these methods are called more than once, so it is important to avoid having side-effects in them. If we ignore this principle it is likely to end up with inconsistent state issues and memory leaks.
React.StrictMode cannot spot side-effects at once, but it can help us find them by intentionally invoking twice some key functions.
These functions are:
Class component constructor, render, and shouldComponentUpdate methods
Class component static getDerivedStateFromProps method
Function component bodies
State updater functions (the first argument to setState)
Functions passed to useState, useMemo, or useReducer
This behaviour definitely has some performance impact, but we should not worry since it takes place only in development and not in production.
credit: https://mariosfakiolas.com/blog/my-react-components-render-twice-and-drive-me-crazy/

it is done intentionally by react to avoid this
remove
<React.StrictMode> </React.StrictMode>
from index.js

I worked around this by providing a custom hook. Put the hook below into your code, then:
// instead of this:
useEffect( ()=> {
console.log('my effect is running');
return () => console.log('my effect is destroying');
}, []);
// do this:
useEffectOnce( ()=> {
console.log('my effect is running');
return () => console.log('my effect is destroying');
});
Here is the code for the hook:
export const useEffectOnce = ( effect => {
const destroyFunc = useRef();
const calledOnce = useRef(false);
const renderAfterCalled = useRef(false);
if (calledOnce.current) {
renderAfterCalled.current = true;
}
useEffect( () => {
if (calledOnce.current) {
return;
}
calledOnce.current = true;
destroyFunc.current = effect();
return ()=> {
if (!renderAfterCalled.current) {
return;
}
if (destroyFunc.current) {
destroyFunc.current();
}
};
}, []);
};
See this blog for the explanation.

Well, I have created a workaround hook for this. Check this, if it helps:
import { useEffect } from "react";
const useDevEffect = (cb, deps) => {
let ran = false;
useEffect(() => {
if (ran) return;
cb();
return () => (ran = true);
}, deps);
};
const isDev = !process.env.NODE_ENV || process.env.NODE_ENV === "development";
export const useOnceEffect = isDev ? useDevEffect : useEffect;
CodeSandbox Demo: https://github.com/akulsr0/react-18-useeffect-twice-fix

React internally monitors & manages its render cycles using its virtual dom and its diffing algorithms, so you need not worry about the number of re-renders. Let the re-renders to be managed by react. Even though the render function is getting invoked, there are sub components which doesn't gets refreshed on ui, if there is no props or state change inside it. Every setstate function call will inform react to check the diffing algorithm, and invoke the render function.
So in your case, since you have a setstate defined inside the getPoints function, it tells react to rerun the diffing process through the render function.

How to handle inputs added by button in React [duplicate]

We should avoid method binding inside render because during re-rendering it will create the new methods instead of using the old one, that will affect the performance.
So for the scenarios like this:
<input onChange = { this._handleChange.bind(this) } ...../>
We can bind _handleChange method either in constructor:
this._handleChange = this._handleChange.bind(this);
Or we can use property initializer syntax:
_handleChange = () => {....}
Now lets consider the case where we want to pass some extra parameter, lets say in a simple todo app, onclick of item i need to delete the item from array, for that i need to pass either the item index or the todo name in each onClick method:
todos.map(el => <div key={el} onClick={this._deleteTodo.bind(this, el)}> {el} </div>)
For now just assume that todo names are unique.
As per DOC:
The problem with this syntax is that a different callback is created
each time the component renders.
Question:
How to avoid this way of binding inside render method or what are the alternatives of this?
Kindly provide any reference or example, thanks.

First: A simple solution will be to create a component for the content inside a map function and pass the values as props and when you call the function from the child component you can pass the value to the function passed down as props.
Parent
deleteTodo = (val) => {
console.log(val)
}
todos.map(el =>
<MyComponent val={el} onClick={this.deleteTodo}/>
)
MyComponent
class MyComponent extends React.Component {
deleteTodo = () => {
this.props.onClick(this.props.val);
}
render() {
return <div onClick={this.deleteTodo}> {this.props.val} </div>
}
}
Sample snippet
class Parent extends React.Component {
_deleteTodo = (val) => {
console.log(val)
}
render() {
var todos = ['a', 'b', 'c'];
return (
<div>{todos.map(el =>
<MyComponent key={el} val={el} onClick={this._deleteTodo}/>
)}</div>
)
}
}
class MyComponent extends React.Component {
_deleteTodo = () => {
console.log('here'); this.props.onClick(this.props.val);
}
render() {
return <div onClick={this._deleteTodo}> {this.props.val} </div>
}
}
ReactDOM.render(<Parent/>, 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>
EDIT:
Second: The other approach to it would be to use memoize and return a function
constructor() {
super();
this._deleteTodoListener = _.memoize(
this._deleteTodo, (element) => {
return element.hashCode();
}
)
}
_deleteTodo = (element) => {
//delete handling here
}
and using it like
todos.map(el => <div key={el} onClick={this._deleteTodoListener(el)}> {el} </div>)
P.S. However this is not a best solution and will still result in
multiple functions being created but is still an improvement over the
initial case.
Third: However a more appropriate solution to this will be to add an attribute to the topmost div and get the value from event like
_deleteTodo = (e) => {
console.log(e.currentTarget.getAttribute('data-value'));
}
todos.map(el => <div key={el} data-value={el} onClick={this._deleteTodo}> {el} </div>)
However, in this case the attributes are converted to string using toString method and hence and object will be converted to [Object Object] and and array like ["1" , "2", "3"] as "1, 2, 3"

How to avoid this way of binding inside render method or what are the
alternatives of this?
If you care about re-rendering then shouldComponentUpdate and PureComponent are your friends and they will help you optimize rendering.
You have to extract "Child" component from the "Parent" and pass always the same props and implement shouldComponentUpdate or use PureComponent. What we want is a case when we remove a child, other children shouldn't be re-rendered.
Example
import React, { Component, PureComponent } from 'react';
import { render } from 'react-dom';
class Product extends PureComponent {
render() {
const { id, name, onDelete } = this.props;
console.log(`<Product id=${id} /> render()`);
return (
<li>
{id} - {name}
<button onClick={() => onDelete(id)}>Delete</button>
</li>
);
}
}
class App extends Component {
constructor(props) {
super(props);
this.state = {
products: [
{ id: 1, name: 'Foo' },
{ id: 2, name: 'Bar' },
],
};
this.handleDelete = this.handleDelete.bind(this);
}
handleDelete(productId) {
this.setState(prevState => ({
products: prevState.products.filter(product => product.id !== productId),
}));
}
render() {
console.log(`<App /> render()`);
return (
<div>
<h1>Products</h1>
<ul>
{
this.state.products.map(product => (
<Product
key={product.id}
onDelete={this.handleDelete}
{...product}
/>
))
}
</ul>
</div>
);
}
}
render(<App />, document.getElementById('root'));
Demo: https://codesandbox.io/s/99nZGlyZ
Expected behaviour
<App /> render()
<Product id=1... render()
<Product id=2... render()
When we remove <Product id=2 ... only <App /> is re-rendered.
render()
To see those messages in demo, open the dev tools console.
The same technique is used and described in article: React is Slow, React is Fast: Optimizing React Apps in Practice by François Zaninotto.

Documentation encourages to use data-attributes and access them from within evt.target.dataset:
_deleteTodo = (evt) => {
const elementToDelete = evt.target.dataset.el;
this.setState(prevState => ({
todos: prevState.todos.filter(el => el !== elementToDelete)
}))
}
// and from render:
todos.map(
el => <div key={el} data-el={el} onClick={this._deleteTodo}> {el} </div>
)
Also note that this makes sense only when you have performance issues:
Is it OK to use arrow functions in render methods?
Generally speaking, yes, it is OK, and it is often the easiest way to
pass parameters to callback functions.
If you do have performance issues, by all means, optimize!

This answer https://stackoverflow.com/a/45053753/2808062 is definitely exhaustive, but I'd say fighting excessive re-renders instead of just re-creating the tiny callback would bring you more performance improvements. That's normally achieved by implementing a proper shouldComponentUpdate in the child component.
Even if the props are exactly the same, the following code will still re-render children unless they prevent it in their own shouldComponentUpdate (they might inherit it from PureComponent):
handleChildClick = itemId => {}
render() {
return this.props.array.map(itemData => <Child onClick={this.handleChildClick} data={itemData})
}
Proof: https://jsfiddle.net/69z2wepo/92281/.
So, in order to avoid re-renders, the child component has to implement shouldComponentUpdate anyway. Now, the only reasonable implementation is completely ignoring onClick regardless of whether it has changed:
shouldComponentUpdate(nextProps) {
return this.props.array !== nextProps.array;
}

React - set state using button and pass it as props

I want to use the 'compare' button to toggle the compare state to true or false.
Next I want to pass this compare state to pivot as props.
I am literally using the same code as in the react documentation when looking at the Toggle class. https://facebook.github.io/react/docs/handling-events.html
The only thing I changed is the name isToggleOn to compare.
When looking at the console client side I get following error every time the component renders:
modules.js?hash=5bd264489058b9a37cb27e36f529f99e13f95b78:3941 Warning: setState(...): Cannot update during an existing state transition (such as within render or another component's constructor). Render methods should be a pure function of props and state; constructor side-effects are an anti-pattern, but can be moved to componentWillMount.`
My code is following:
class Dashboard extends Component {
constructor(props) {
super(props);
this.state = { compare: true };
this.handleClick = this.handleClick.bind(this);
}
handleClick(button) {
if (button === 'compare') {
this.setState(prevState => ({
compare: !prevState.compare,
}));
}
}
render() {
return (
<Grid>
<div className="starter-template">
<h1>This is the dashboard page.</h1>
<p className="lead">
Use this document as a way to quickly start any new project.<br />{' '}
All you get is this text and a mostly barebones HTML document.
</p>
</div>
<ButtonToolbar>
<button onClick={this.handleClick('compare')}>
{this.state.compare ? 'AGGREGATE' : 'COMPARE'}
</button>
</ButtonToolbar>
<PivotTable
ready={this.props.isReady}
data={this.props.gapData}
compare={this.state.compare}
/>
</Grid>
);
}
}
export default (DashboardContainer = createContainer(() => {
// Do all your reactive data access in this method.
// Note that this subscription will get cleaned up when your component is unmounted
const handle = Meteor.subscribe('weekly-dashboard');
return {
isReady: handle.ready(),
gapData: WeeklyDashboard.find({}).fetch(),
};
}, Dashboard));
Any advice on how to fix this?

The reason is this line
<button onClick={this.handleClick('compare')}>
This will call the handleClick function while executing render function. You can fix by:
<button onClick={() => this.handleClick('compare')}>
Or
const handleBtnClick = () => this.handleClick('compare');
...
<button onClick={this.handleBtnClick}>
...
I prefer the latter