Develop Reference

State from react useState is updated when key property is used but requires useEffect or similar method to update otherwise

I have the following component:
import React, { useState } from "react";
import { FormControl, TextField } from "#material-ui/core";
interface IProps {
text?: string;
id: number;
onValueChange: (text: string, id: number) => void;
placeholder: string;
}
export const QuestionTextRow: React.FC<IProps> = (props) => {
const [item, onItemChange] = useState(props.text);
const onChange = (e: React.FormEvent<HTMLInputElement>) => {
const newValue = e.currentTarget.value;
onItemChange(newValue);
props.onValueChange(newValue, props.id);
};
return (
<>
<FormControl fullWidth>
<div>{props.text}</div>
<TextField
aria-label="question-text-row"
onDragStart={(e) => {
e.preventDefault();
e.stopPropagation();
}}
value={item}
onChange={(ev: React.ChangeEvent<HTMLInputElement>): void => {
onChange(ev);
}}
/>
</FormControl>
</>
);
};
It is rendered via the following component:
const renderQuestionOptions = (id: number): JSX.Element => {
const item = props.bases.find((x) => x.sortableId === id);
if (!item) return <> </>;
return (
<div className={classes.questionPremiseRow}>
<div className={classes.rowOutline}>
<QuestionOptionsSortableRow item={item} isDisabled={false} onClickRow={onClickBasisRow}>
<QuestionTextRow
text={item.text ? item.text.text : ""}
id={item.sortableId}
onValueChange={basisValueChanged}
placeholder={intl.formatMessage({ id: "question.create.basis.row.placeholder" })}
></QuestionTextRow>
</QuestionOptionsSortableRow>
</div>
</div>
);
};
It renders the following list:
As you can see props.text and useState item from props.text are rendered equally. If props.text is updated it does not reflect on useState though.
https://stackoverflow.com/a/53846698/3850405
I can solve it by useEffect to make it work:
useEffect(() => {
onItemChange(props.text);
}, [props.text]);
https://reactjs.org/docs/hooks-effect.html
https://stackoverflow.com/a/54866051/3850405
However If I add key={`${item.text?.text}-${item.sortableId}`} to QuestionTextRow it will work without using useEffect. How come?
I know a static unique key should be used but would it not be the same result if key={item.uniqueId} was used?
https://www.npmjs.com/package/react-key-index
The argument passed to useState is the initial state much like setting
state in constructor for a class component and isn't used to update
the state on re-render
https://stackoverflow.com/a/43892905/3850405

However If I add key={${item.text?.text}-${item.sortableId}} to
QuestionTextRow it will work without using useEffect. How come?
That is because of reconciliation. In react, when on one render you have say:
<SomeComponent key={1}/>
If on next render you render same component (at the same place) with different key, say:
<SomeComponent key={2}/>
React will destroy instance related to previous component and create a new instance for this one, hence the useState inside that component will be initialized with the provided text property once again (like when you created the component first time).
If the key is same for some component on previous and next renders and you just change some other props, in this case the component is re-rendered (no instance destroyed), that's why you didn't see the text property reflected in state.
Sometimes it can be tricky to copy props to state like you have in your useEffect solution, I recommend you read this post, it is about classes but same ideas apply.

ReactJs - Input Component behaves strangely when its returned from a function

Scenario
I declared a react component that renders a simple html input tag.
const MyComponent = (props) => (
<input
defaultValue="test"
onChange={(e) => {
props.setTitle(e.target.value);
}}
/>
);
Then I declared a function that takes a setState as a parameter and returns that component with the setState inside the input's onChange.
const getComponent = (setTitle) => (props) => (
<input
defaultValue="test"
onChange={(e) => {
setTitle(e.target.value);
}}
/>
);
Then I called my function to get the component and to render it:
const Root = () => {
const [title, setTitle] = React.useState('');
const Component = getComponent(setTitle);
return (
<div>
<div>{title}</div>
<Component />{' '}
</div>
);
};
Expected:
The input element behaves normally, and changes its value
Reality:
The input loses focus after each character typed, and won't retain its value.
Here is a simple example of the error:
CodeSandbox

The reason this is happening is that when your code comes to this line:
const Component = getComponent(setTitle);
This generates new function (i.e. new instance) that is not rendered again, but mounted again. That is the reason you get unfocused from field.
There is no way you will make this work in this way, it's just not meant to be working like this. When you do it once when you are exporting it, than its ok, but every time === new instance.
If this is just an experiment that you are trying, than ok. But there is no reason not to pass setState as prop to that component.

I found a solution that let me use the function and keep the component from mounting each time.
If you put the function call inside the jsx, the component won't remount each render.
const Root = () => {
const [title, setTitle] = React.useState('');
const Component = ;
const someprops = {};
return (
<div>
<div>{title}</div>
{getComponent(setTitle)(someprops)}
</div>
);
};

Passing a custom argument to the eventListener 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;
}

Wrapping a Component means it loses focus on rerender

I have a long form in react. Before, it had a bunch of components that were defined as such:
<input
type='text'
value={this.state.form.nameOfFormField}
onChange={this.updateForm('nameOfFormField')} />
Where updateForm is a function in the form of (field) => (e) => {}, to make code reuse easier.
I wanted to make this easier to maintain, so I created a component, SpecialInput, which was defined as such:
const SpecialInputBuilder = (form, onChange) => ({ field, ..props }) => (
<input
type='text'
value={form[field]}
onChange={onChange(field)}
{...props} />
)
Now, I could define the Input during render like so:
const SpecialInput = SpecialInputBuilder(this.state.form, this.updateForm)
And use it in the component like this:
<SpecialInput field='nameOfFormField' />
Obviously, this is much more succinct. But this also means that the input field will drop focus every time input is entered into the field (i.e., when updateForm is called), because SpecialInput is defined every time the render function is called. Defining a key to each element does not seem to at all alleviate the problem. How can I fix this while still using this simpler component? Is there a middle ground?

Why not just change your input builder to just be a react component?
const SpecialInput = (props) => {
return (
<input
value={props.form[props.field]}
{...props}
type={props.type || 'text'}
onChange={() => props.onChange(props.field)}
/>
)
}
and just use it the same way.
<SpecialInput field='nameOfFormField' onChange={this.updateForm} form={this.state.form} />

I had a similar approach but ended up changing it to this;
(1) Input typer child component:
import React, { Component } from 'react';
class FreeTextField extends Component {
inputValueFn(e) {
this.props.userInput(this.props.responseObject, e.target.value);
}
render() {
return (
<div className="input-group">
<label>{this.props.buttonText ? this.props.buttonText : "Firstname"}</label>
<input type={this.props.type} placeholder="" className="form-control" defaultValue={this.props.defaultValue} onChange={this.inputValueFn.bind(this)} />
</div>
);
}
}
export default FreeTextField;
(2) From the parent component you can specify all relavent child attr via props
// import
import FreeTextField from './pathTo/FreeTextField';
// Initial state
this.state = {
responseObject: {}
}
// onChange it updates the responseObject
userInput(fieldKey,value) {
let responseObject = this.state.responseObject;
responseObject[fieldKey] = value;
this.setState({responseObject:responseObject});
}
// component render()
<FreeTextField
buttonText="First Name"
type="text"
formObjectKey="first_name"
userInput{this.userInput.bind(this)} />

The main issue is that your onChange call is executing as soon as it's rendered, instead of a reference to a function to be called when the input changes.
// this executes immediately
onChange={onChange(field)}
// this is a reference to the function with a prop prepended
onChange={onChange.bind(this,field)}

How to access a child's state in React

I have the following structure:
FormEditor - holds multiple instances of FieldEditor
FieldEditor - edits a field of the form and saving various values about it in its state
When a button is clicked within FormEditor, I want to be able to collect information about the fields from all FieldEditor components, information that's in their state, and have it all within FormEditor.
I considered storing the information about the fields outside of FieldEditor's state and put it in FormEditor's state instead. However, that would require FormEditor to listen to each of its FieldEditor components as they change and store their information in its state.
Can't I just access the children's state instead? Is it ideal?

Just before I go into detail about how you can access the state of a child component, please make sure to read Markus-ipse's answer regarding a better solution to handle this particular scenario.
If you do indeed wish to access the state of a component's children, you can assign a property called ref to each child. There are now two ways to implement references: Using React.createRef() and callback refs.
Using React.createRef()
This is currently the recommended way to use references as of React 16.3 (See the documentation for more information). If you're using an earlier version then see below regarding callback references.
You'll need to create a new reference in the constructor of your parent component and then assign it to a child via the ref attribute.
class FormEditor extends React.Component {
constructor(props) {
super(props);
this.FieldEditor1 = React.createRef();
}
render() {
return <FieldEditor ref={this.FieldEditor1} />;
}
}
In order to access this kind of ref, you'll need to use:
const currentFieldEditor1 = this.FieldEditor1.current;
This will return an instance of the mounted component so you can then use currentFieldEditor1.state to access the state.
Just a quick note to say that if you use these references on a DOM node instead of a component (e.g. <div ref={this.divRef} />) then this.divRef.current will return the underlying DOM element instead of a component instance.
Callback Refs
This property takes a callback function that is passed a reference to the attached component. This callback is executed immediately after the component is mounted or unmounted.
For example:
<FieldEditor
ref={(fieldEditor1) => {this.fieldEditor1 = fieldEditor1;}
{...props}
/>
In these examples the reference is stored on the parent component. To call this component in your code, you can use:
this.fieldEditor1
and then use this.fieldEditor1.state to get the state.
One thing to note, make sure your child component has rendered before you try to access it ^_^
As above, if you use these references on a DOM node instead of a component (e.g. <div ref={(divRef) => {this.myDiv = divRef;}} />) then this.divRef will return the underlying DOM element instead of a component instance.
Further Information
If you want to read more about React's ref property, check out this page from Facebook.
Make sure you read the "Don't Overuse Refs" section that says that you shouldn't use the child's state to "make things happen".

If you already have an onChange handler for the individual FieldEditors I don't see why you couldn't just move the state up to the FormEditor component and just pass down a callback from there to the FieldEditors that will update the parent state. That seems like a more React-y way to do it, to me.
Something along the line of this perhaps:
const FieldEditor = ({ value, onChange, id }) => {
const handleChange = event => {
const text = event.target.value;
onChange(id, text);
};
return (
<div className="field-editor">
<input onChange={handleChange} value={value} />
</div>
);
};
const FormEditor = props => {
const [values, setValues] = useState({});
const handleFieldChange = (fieldId, value) => {
setValues({ ...values, [fieldId]: value });
};
const fields = props.fields.map(field => (
<FieldEditor
key={field}
id={field}
onChange={handleFieldChange}
value={values[field]}
/>
));
return (
<div>
{fields}
<pre>{JSON.stringify(values, null, 2)}</pre>
</div>
);
};
// To add the ability to dynamically add/remove fields, keep the list in state
const App = () => {
const fields = ["field1", "field2", "anotherField"];
return <FormEditor fields={fields} />;
};
Original - pre-hooks version:
class FieldEditor extends React.Component {
constructor(props) {
super(props);
this.handleChange = this.handleChange.bind(this);
}
handleChange(event) {
const text = event.target.value;
this.props.onChange(this.props.id, text);
}
render() {
return (
<div className="field-editor">
<input onChange={this.handleChange} value={this.props.value} />
</div>
);
}
}
class FormEditor extends React.Component {
constructor(props) {
super(props);
this.state = {};
this.handleFieldChange = this.handleFieldChange.bind(this);
}
handleFieldChange(fieldId, value) {
this.setState({ [fieldId]: value });
}
render() {
const fields = this.props.fields.map(field => (
<FieldEditor
key={field}
id={field}
onChange={this.handleFieldChange}
value={this.state[field]}
/>
));
return (
<div>
{fields}
<div>{JSON.stringify(this.state)}</div>
</div>
);
}
}
// Convert to a class component and add the ability to dynamically add/remove fields by having it in state
const App = () => {
const fields = ["field1", "field2", "anotherField"];
return <FormEditor fields={fields} />;
};
ReactDOM.render(<App />, document.body);

As the previous answers said, try to move the state to a top component and modify the state through callbacks passed to its children.
In case that you really need to access to a child state that is declared as a functional component (hooks) you can declare a ref in the parent component, and then pass it as a ref attribute to the child, but you need to use React.forwardRef and then the hook useImperativeHandle to declare a function you can call in the parent component.
Take a look at the following example:
const Parent = () => {
const myRef = useRef();
return <Child ref={myRef} />;
}
const Child = React.forwardRef((props, ref) => {
const [myState, setMyState] = useState('This is my state!');
useImperativeHandle(ref, () => ({getMyState: () => {return myState}}), [myState]);
})
Then you should be able to get myState in the Parent component by calling:
myRef.current.getMyState();

It's 2020 and lots of you will come here looking for a similar solution but with Hooks (they are great!) and with the latest approaches in terms of code cleanliness and syntax.
So as previous answers had stated, the best approach to this kind of problem is to hold the state outside of child component fieldEditor. You could do that in multiple ways.
The most "complex" is with a global context (state) that both parent and children could access and modify. It's a great solution when components are very deep in the tree hierarchy and so it's costly to send props in each level.
In this case I think it's not worth it, and a more simple approach will bring us the results we want, just using the powerful React.useState().
An approach with a React.useState() hook - way simpler than with Class components
As said, we will deal with changes and store the data of our child component fieldEditor in our parent fieldForm. To do that we will send a reference to the function that will deal and apply the changes to the fieldForm state, you could do that with:
function FieldForm({ fields }) {
const [fieldsValues, setFieldsValues] = React.useState({});
const handleChange = (event, fieldId) => {
let newFields = { ...fieldsValues };
newFields[fieldId] = event.target.value;
setFieldsValues(newFields);
};
return (
<div>
{fields.map(field => (
<FieldEditor
key={field}
id={field}
handleChange={handleChange}
value={fieldsValues[field]}
/>
))}
<div>{JSON.stringify(fieldsValues)}</div>
</div>
);
}
Note that React.useState({}) will return an array with position 0 being the value specified on call (Empty object in this case), and position 1 being the reference to the function
that modifies the value.
Now with the child component, FieldEditor, you don't even need to create a function with a return statement. A lean constant with an arrow function will do!
const FieldEditor = ({ id, value, handleChange }) => (
<div className="field-editor">
<input onChange={event => handleChange(event, id)} value={value} />
</div>
);
Aaaaand we are done, nothing more. With just these two slim functional components we have our end goal "access" our child FieldEditor value and show it off in our parent.
You could check the accepted answer from 5 years ago and see how Hooks made React code leaner (by a lot!).
Hope my answer helps you learn and understand more about Hooks, and if you want to check a working example here it is.

Now you can access the InputField's state which is the child of FormEditor.
Basically, whenever there is a change in the state of the input field (child), we are getting the value from the event object and then passing this value to the Parent where in the state in the Parent is set.
On a button click, we are just printing the state of the input fields.
The key point here is that we are using the props to get the input field's id/value and also to call the functions which are set as attributes on the input field while we generate the reusable child input fields.
class InputField extends React.Component{
handleChange = (event)=> {
const val = event.target.value;
this.props.onChange(this.props.id , val);
}
render() {
return(
<div>
<input type="text" onChange={this.handleChange} value={this.props.value}/>
<br/><br/>
</div>
);
}
}
class FormEditorParent extends React.Component {
state = {};
handleFieldChange = (inputFieldId , inputFieldValue) => {
this.setState({[inputFieldId]:inputFieldValue});
}
// On a button click, simply get the state of the input field
handleClick = ()=>{
console.log(JSON.stringify(this.state));
}
render() {
const fields = this.props.fields.map(field => (
<InputField
key={field}
id={field}
onChange={this.handleFieldChange}
value={this.state[field]}
/>
));
return (
<div>
<div>
<button onClick={this.handleClick}>Click Me</button>
</div>
<div>
{fields}
</div>
</div>
);
}
}
const App = () => {
const fields = ["field1", "field2", "anotherField"];
return <FormEditorParent fields={fields} />;
};
ReactDOM.render(<App/>, mountNode);

You may access the child state by passing a callback to the child component.
const Parent = () => {
return (
<Child onSubmit={(arg) => {
console.log('accessing child state from parent callback: ', arg)
}}
/>
)
}
const Child = ({onSubmit}) => {
const [text, setText] = useState('');
return (
<>
<input value={text} onChange={setText}>
<button onClick={() => onSubmit(text)} />
</>
)
}
Now if you click the button in the child component, you will execute the function passed from the parent and have access to the child component's state variables.