I have a react component, which has properties and state. Some fields of state contain input data (uplifted from input control), but there is also fields in the state that must be Calculated based on current State and Props:
The question: what is the best way to update calculated fields of the state (based on other fields of state and props)?
Ugly way to do it:
componentDidUpdate(){
this.setState({calculatedField:calculate(this.props,this.state)}))
}
In this case I get infinite loop of updates or in the best case (if I use PureComponent) double rendering invocation.
The best solution I found so far (but still ugly):
Is to create a calculated object in state, which contains calculated fields and updated in componentWillUpdate avoiding setState:
componentWillUpdate(nextProps,nextState){
nextState.calculated.field1=f(nextProps,nextState)
}
class ParentComponent extends React.Component {
constructor(props, ctx) {
super(props,ctx)
this.state={A:"2"}
}
render() {
console.log("rendering ParentComponent")
return <div>
<label>A=<input value={this.state.A} onChange={e=>{this.setState({A:e.target.value})}} /></label> (stored in state of Parent component)
<ChildComponent A={this.state.A} />
</div>
}
}
class ChildComponent extends React.PureComponent {
constructor(props,ctx) {
super(props,ctx);
this.state={
B:"3",
Calculated:{}
}
}
render() {
console.log("rendering ChildComponent")
return <div>
<label>B=<input value={this.state.B} onChange={e=>{this.setState({B:e.target.value})}} /></label> (stored in state of Child component state)
<div>
f(A,B)=<b>{this.state.Calculated.result||""}</b>(stored in state of Child component)
<button onClick={e=>{ this.setState({Calculated:{result:new Date().toTimeString()}}) }}>Set manual value</button>
</div>
</div>
}
componentWillUpdate(nextProps, nextState) {
this.state.Calculated.result = getCalculatedResult(nextProps.A, nextState.B)
}
componentWillReceiveProps(nextProps) {
this.state.Calculated.result = getCalculatedResult(nextProps.A, this.state.B)
}
componentWillMount() {
this.state.Calculated.result = getCalculatedResult(this.props.A, this.state.B)
}
}
function getCalculatedResult(a,b) {
const aNum = Number(a)||0
const bNum = Number(b)||0;
const result = (aNum*bNum).toString();
return result;
}
ReactDOM.render(<ParentComponent/>, document.getElementById('root'));
<script src="https://cdnjs.cloudflare.com/ajax/libs/react/16.2.0/umd/react.development.js"></script>
<script src="https://cdnjs.cloudflare.com/ajax/libs/react-dom/16.2.0/umd/react-dom.development.js"></script>
<div id="root"></div>
This is also ugly solution and React does not recommended to mutate state avoiding setState. So what is right solution for that?
NOTE:
In my real application I cannot recalculate f(a,b) every single time during rendering, because it's actually complex object, so I need to cache it somehow and the best way is in the state.
If you are using React 16.8.0 and above, you can use React hooks API. I think it's useMemo() hook you might need. For example:
import React, { useMemo } from 'react'
const MyComponent = ({ ...props }) => {
const calculatedValue = useMemo(
() => {
// Do expensive calculation and return.
},
[a, b]
)
return (
<div>
{ calculatedValue }
</div>
)
}
For more details, refer to the React documentation
I wouldn't advice you to store your calculated value inside your state. My approach would be more like this:
import PropTypes from 'prop-types';
import React from 'react';
class Component extends React.Component {
static defaultProps = { value: 0 };
static propTypes = { value: PropTypes.number };
state = { a: 0, b: 0 };
result = () => this.state.a + this.state.b + this.props.value;
updateA = e => this.setState({ a: +e.target.value });
updateB = e => this.setState({ b: +e.target.value });
render() {
return (
<div>
A: <input onChange={this.updateA} value={this.state.a} />
B: <input onChange={this.updateB} value={this.state.b} />
Result: {this.result()}
</div>
);
}
}
The problem with storing the calculation inside your state is, that the calculation can be mutated by multiple sources. If you use my solution, there is no way, that anything can overwrite the calculation WITHOUT using the correct function to calculate them.
You can save calculated result in this.calculated instead of this.state. It is dependent data. All data which causes update and render is already in state and props.
class Component extends React.Component {
constructor(props) {
super(props)
state = {
b: 0
}
}
updateThis = (event) => {
this.setState({ b: event.target.value });
}
componentWillUpdate(nextProps,nextState){
this.calculated.field1=f(nextProps.a, nextState.b)
}
render() {
return (
<form>
A = <input onChange={this.props.updateParent} value={this.props.a} /> <br>
B = <input onChange={this.updateThis} value={this.state.b} /> <br>
f(A,B) = {this.calculated.field1} <br>
</form>
);
}
}
class ParentComponent extends React.Component {
constructor(props) {
super(props)
state = {
a: 0
}
}
render() {
return (
<Component
updateParent={event=>this.setState({a: event.target.value})}
a={this.state.a}
/>
}
}
}
You're first attempt is the right way to solve this problem. However, you need to add a check to see if state has actually changed:
componentDidUpdate(prevProps, prevState){
if(prevState.field !== this.state.field){
this.setState({calculatedField:calculate(this.props,this.state)}))
}
}
shouldComponentUpdate(nextProps, nextState) {
return this.state.calculatedField !== nextState.calculatedField
}
You need to check the pieces of state and props that you use in your calculate method and make sure they have changed before updating state again. This will prevent the infinite loop.
It looks like the "state" is the place to store everything (even computed values) you'll need to use on the render function, but usually we have the problem you describe.
Since React 16.3 a new approach for this situation has been given in the way of the static getDerivedStateFromProps (nextProps, prevState) "lifecycle hook".
You should update at least to this version if you haven't, and follow the advice given by the React Team on their blog.
Here is the official documentation for this functionality.
The issue here is that this function is invoked before every render, and being "static" you cannot access the current instance previous props, which is usually needed to decide if the computed value must be generated again or not (I suppose this is your case, as you have stated your computation process is heavy). In this case, the React team suggests to store in the state the values of the related props, so they can be compared with the new ones:
if (nextProps.propToCompute !== prevState.propToComputePrevValue) {
return {
computedValue: Compute(nextProp.propToCompute),
propToComputePrevValue: nextProps.propToCompute
};
}
return null;
Do not include redundant information in your state.
A simplified example is having firstName and lastName in your state. If we want to display the full name in your render method, you would simply do:
render() {
return <span>{`${this.state.firstName} ${this.state.lastName}`}</span>
}
I like this example because it's easy to see that adding a fullName in our state, that just holds ${this.state.firstName} ${this.state.lastName} is unnecessary. We do string concatenation every time our component renders, and we're okay with that because it's a cheap operation.
In your example, your calculation is cheap so you should do it in the render method as well.
I have a React component and I want to toggle a css class when clicked.
So I have this:
export class myComponent extends React.Component {
constructor() {
super();
this.state = { clicked: false };
this.handleClick = this.handleClick.bind(this);
}
render() {
return (
<div>
<div onClick={this.clicked}><span ref="btn" className="glyphicon"> </span></div>
</div>
);
}
handleClick() {
this.refs.btn.classList.toggle('active');
}
componentDidMount() {
this.refs.btn.addEventListener('click', this.handleClick);
this.setState({
clicked: this.state.clicked = true,
});
}
componentWillUnmount() {
this.refs.btn.removeEventListener('click', this.handleClick);
this.setState({
clicked: this.state.clicked = false,
});
}
}
This problem is that ESLint keeps telling me "this.refs" is depreciated.
What do I do instead? How can I fix it so it's not using depreciated code?
The Lint rule you are referring to is called no-string-refs and warns you with:
"Using string literals in ref attributes is deprecated (react/no-string-refs)"
You are getting this warning because have implemented the deprecated way of using refs (by using strings). Depending on your React version, you can do:
React 16.3 and later
constructor() {
super();
this.btnRef= React.createRef();
this.state = { clicked: false };
this.handleClick = this.handleClick.bind(this);
}
render() {
return (
<div>
<div onClick={this.addVote}><span ref={this.btnRef} className="glyphicon"> </span></div>
</div>
);
}
React 16.2 and older
constructor() {
super();
this.btnRef; //not necessary to declare the variable here, but I like to make it more visible.
this.state = { clicked: false };
this.handleClick = this.handleClick.bind(this);
}
render() {
return (
<div>
<div onClick={this.addVote}><span ref={(el) => this.btnRef = el} className="glyphicon"> </span></div>
</div>
);
}
For even better readability, you could also do:
render() {
let myRef = (el) => this.btnRef = el;
return (
<div>
<div onClick={this.addVote}><span ref={myRef} className="glyphicon"> </span></div>
</div>
);
}
Have a look at what the official documentation says on Refs and the DOM, and this section in particular:
Legacy API: String Refs
If you worked with React before, you might be
familiar with an older API where the ref attribute is a string, like
"textInput", and the DOM node is accessed as this.refs.textInput. We
advise against it because string refs have some issues, are considered
legacy, and are likely to be removed in one of the future releases. If
you're currently using this.refs.textInput to access refs, we
recommend the callback pattern instead.
The reason this ESLint rule exists is that string Refs are on their way out. However, for the code above I would recommend to not use a Ref in the first place.
Don't Overuse Refs
React's advantage is that it is declarative. Meaning, we have state and an expression (returned JSX) of how the UI (more precisely the DOM) should look given a certain state.
Whatever can be done using just state and UI expression, should be done this way. The problem with the use of a Ref in the code above is that it makes the code imperative. We can't understand how the DOM will look just from the JSX. Here is how you could achieve the same result in a declarative way:
export class myComponent extends React.Component {
constructor(props) {
super(props);
this.state = {
active: false
};
}
handleClick = () => { // with arrow function there is no need for binding.
this.setState(
prevState => {
return {
active: !prevState.active
}
}
)
}
render() {
return (
<div>
<span
onClick={this.handleClick}
className={`glyphicon ${this.state.active && "active"}`}
>
Hello World
</span>
</div>
);
}
}
Refs should be used when state and UI expression aren't enough, and you need access to the actual DOM. For example, focusing on an input field, scrolling to an element, or getting the exact width and height of an element.
If you do use Refs, avoid string refs
String refs harm performance, aren't composable, and are on there way out.
string refs have some issues, are considered legacy, and are likely to
be removed in one of the future releases. [Official React documentation]
[resource1][1], [resource2][1]
Option #1: Use React.createRef
class MyComponent extends Component {
constructor(props) {
super(props)
this.myRef = React.createRef() // create a ref object
}
render() {
return <div ref={this.myRef}></div> // Attach the ref property to a dom element
}
}
Option #2: Use a ref callback
class MyComponent extends Component {
constructor(props){ // Optional, declare a class field
super(props)
this.myRef=null
}
render() {
return <div ref={ (ref) => this.myRef=ref }></div>
} // Attach the dom element to a class field
}
you can try a more declarative way. I changed your code to reflect this. You just need to remind that a component will refresh and call render in every state/props change. So, we can create the class of your element inside render method.
import React from 'react'
export default class myComponent extends React.Component {
constructor() {
super();
this.state = { clicked: false };
this.handleClick = this.handleClick.bind(this);
}
render() {
let btnClass = 'glyphicon'
if(this.state.clicked){
btnClass+=' active'
}
return (
<div>
<div onClick={this.handleClick}><span ref="btn" className={btnClass}> </span></div>
</div>
);
}
handleClick() {
this.setState({
clicked: !this.state.clicked
})
}
}
I am a complete newbie in react native, react.js, and javascript. I am Android developer so would like to give RN a try.
Basically, the difference is in onPress;
This code shows 'undefined' when toggle() runs:
class LoaderBtn extends Component {
constructor(props) {
super(props);
this.state = { loading: false };
}
toggle() {
console.log(this.state);
// let state = this.state.loading;
console.log("Clicked!")
// this.setState({ loading: !state })
}
render() {
return (
<Button style={{ backgroundColor: '#468938' }} onPress={this.toggle}>
<Text>{this.props.text}</Text>
</Button>
);
}
}
but this code works:
class LoaderBtn extends Component {
constructor(props) {
super(props);
this.state = { loading: false };
}
toggle() {
console.log(this.state);
// let state = this.state.loading;
console.log("Clicked!")
// this.setState({ loading: !state })
}
render() {
return (
<Button style={{ backgroundColor: '#468938' }} onPress={() => {this.toggle()}}>
<Text>{this.props.text}</Text>
</Button>
);
}
}
Can you explain me the difference, please?
In Java / Kotlin we have method references, basically it passes the function if signatures are the same, like onPress = () => {} and toggle = () => {}
But in JS it doesn't work :(
The issue is that in the first example toggle() is not bound to the correct this.
You can either bind it in the constructor:
constructor(props) {
super(props);
this.toggle = this.toggle.bind(this);
...
Or use an instance function (OK under some circumstances):
toggle = () => {
...
}
This approach requires build changes via stage-2 or transform-class-properties.
The caveat with instance property functions is that there's a function created per-component. This is okay if there aren't many of them on the page, but it's something to keep in mind. Some mocking libraries also don't deal with arrow functions particularly well (i.e., arrow functions aren't on the prototype, but on the instance).
This is basic JS; this article regarding React Binding Patterns may help.
I think what is happening is a matter of scope. When you use onPress={this.toggle} this is not what you are expecting in your toggle function. However, arrow functions exhibit different behavior and automatically bind to this. You can also use onPress={this.toggle.bind(this)}.
Further reading -
ES6 Arrow Functions
.bind()
What is happening in this first example is that you have lost scope of "this". Generally what I do is to define all my functions in the constructor like so:
constructor(props) {
super(props);
this.state = { loading: false };
this.toggle = this.toggle.bind(this);
}
In the second example, you are using ES6 syntax which will automatically bind this (which is why this works).
Then inside of you onPress function, you need to call the function that you built. So it would look something like this,
onPress={this.toggle}
I'm stuck changing the array of object value of checkbox. I'm even not sure my current approach is correct or not, because I have no clue what to do next. Assume fruits data is an ajax call, it's should come from componentWillMount right? should I set the state like this.setState({input:ajaxresponse})? I'm lost, it's hard for me because I was using jquery. Just now I also read about redux, I heard people said in redux you don't use setState, omg I'm lost.
http://jsbin.com/dodanahani/1/edit
class HelloWorldComponent extends React.Component {
constructor(){
super()
this.handleChange = this.handleChange.bind(this);
this.state = {
input: {}
}
}
handleChange(e,key){
console.log(e.target.checked)
// how to update state here?
}
render() {
const data = {
"fruits":[
{"name":"banana","value":true},
{"name":"watermelon","value":false},
{"name":"lemon","value":true},
]
}
return (
<div>
{data.fruits.map(obj =>
<div key={obj.name}>
<label>{obj.name}</label>
<input onChange={(e) => this.handleChange(e, obj.name)} type="checkbox" defaultChecked={obj.value} />
</div>
)}
<p>{this.state.fruits}</p>
</div>
);
}
}
What I want to achieve is simple : user get api data, and he know his previously saved data, bind to view (checkboxes), and he can update them again. I'm stuck at sending back the changes.
I have the following React component:
export default class MyComponent extends React.Component {
onSubmit(e) {
e.preventDefault();
var title = this.title;
console.log(title);
}
render(){
return (
...
<form className="form-horizontal">
...
<input type="text" className="form-control" ref={(c) => this.title = c} name="title" />
...
</form>
...
<button type="button" onClick={this.onSubmit} className="btn">Save</button>
...
);
}
};
The console is giving me undefined - any ideas what's wrong with this code?
There are three answers here, depending on the version of React you're (forced to) work(ing) with, and whether you want to use hooks.
First things first:
It's important to understand how React works, so you can do things properly (protip: it's super worth running through the React tutorial on the React website. It's well written, and covers all the basics in a way that actually explains how to do things). "Properly" here means that you're not writing a web page, you're writing the user interface for an application that happens to be rendered in a browser; all the actual user interface work happens in React, not in "what you're used to from writing a web page" (this is why React apps really are "apps", not "web pages").
React applications are rendered based off of two things:
the component's properties as declared by whichever parent creates an instance of that component, which the parent can modify throughout its lifecycle, and
the component's own internal state, which it can modify itself throughout its own lifecycle.
What you're expressly not doing when you use React is generating HTML elements and then using those: when you tell React to use an <input>, for instance, you are not creating an HTML input element, you are instead telling React to create a React input object that happens to render as an HTML input element when you compile your React app for the web, with event handling that is controlled by React.
When using React, what you're doing is generating application UI elements that present the user with (often manipulable) data, with user interaction changing the state of your application in a way that you define - actions performed by the user may update a component's props or state, which React uses as a signal to generate a new UI representation for changed components, which may cause an update of part of your application interface to reflect the new state.
In this programming model, the app's internal state is the final authority, rather than "the UI your users look at and interact with": if a user tries to type something in an input field, and you did not write anything to handle that, nothing will happen: the UI is a reflection of the application state, not the other way around. Effectively, the browser DOM is almost an afterthought in this programming model: it just happens to be a super convenient UI framework that the entire planet is virtually guaranteed to have access to (but it's not the only one React knows how to work with)
A specific example
So with that covered, let's look how a user interacting with an input element works in React. First, we need to get to having a UI element for the user to interact with:
You wrote a component to manage (i.e. both store and present) some string data for your users, with an onChange function for handling user data.
Your component's rendering code is used by React to generate a virtual DOM that contains an input component (not a DOM <input> element), and binds your onChange handler to that component so that it can be called with React event data (so note that this is not a DOM change event listener, and does not get the same event data that regular DOM event listeners do).
The React library then translates that virtual DOM into a UI users can interact with, and that it will update as the application state changes. Since it's running in the browser, it builds an HTML input element.
Then, your user tries to actually interact with that input element:
Your user clicks on the input element and starts typing.
Nothing happens to the input element yet. Instead, the input events get intercepted by React and killed off immediately.
React turns the browser event into a React event, and calls the onChange function for the virtual DOM component with the React event data.
That function may do something, based on what how you wrote it, and in this case you almost certainly wrote it to update the state of your component with what the user (tried to) type.
If a state update gets scheduled, React will run that state update in the near future, which will trigger a render pass after the update.
During the render pass, it checks to see if the state is actually different, and if so, it generates a temporary second virtual DOM, which it compares to (a part of) your application's virtual DOM, determines which set of add/update/remove operations it needs to perform on you application's virtual DOM so that it looks the same as the new temporary one, then applies those operations and throws away the temporary virtual DOM again.
It then updates the UI so that it reflects what the virtual DOM now looks like.
And after all of that, we finally have an updated DOM on the page the user is actually looking at, and they see what they typed in the input element.
So this is completely different from the regular browser model: instead of the user updating the UI data by typing into a text box first and our code reading "the current value of that text box" to figure out what the state is second, React already knows what the state is, and uses events to update the state first, which leads to a UI update second.
And it is important to remember that all of this happens effectively instantly, so to your user it looks like they typed text into an input element in the same way they would for any random web page, but under the hood things couldn't be more different while still leading to the same result.
So, with that covered, let's look at how to get values from elements in React:
Component classes and ES6 (React 16+ and 15.5 transitional)
As of React 16 (and soft-starting with 15.5) the createClass call is no longer supported, and class syntax needs to be used. This changes two things: the obvious class syntax, but also the thiscontext binding that createClass can do "for free", so to ensure things still work make sure you're using "fat arrow" notation for this context preserving anonymous functions in onWhatever handlers, such as the onChange we use in the code here:
class MyComponent extends React.Component {
constructor(props) {
super(props);
this.reset();
}
reset() {
// Always set the initial state in its own function, so that
// you can trivially reset your components at any point.
this.state = {
inputValue: ''
};
}
render() {
return (
// ...
<input value={this.state.inputValue} onChange={evt => this.updateInputValue(evt)}/>
// ...
);
},
updateInputValue(evt) {
const val = evt.target.value;
// ...
this.setState({
inputValue: val
});
}
});
You may also have seen people use bind in their constructor for all their event handling functions, like this:
constructor(props) {
super(props);
this.handler = this.handler.bind(this);
...
}
render() {
return (
...
<element onclick={this.handler}/>
...
);
}
Don't do that.
Almost any time you're using bind, the proverbial "you're doing it wrong" applies. Your class already defines the object prototype, and so already defines the instance context. Don't put bind of top of that; use normal event forwarding instead of duplicating all your function calls in the constructor, because that duplication increases your bug surface, and makes it much harder to trace errors because the problem might be in your constructor instead of where you call your code.
"But then it's constantly making and throwing away functions on rerenders!" and that may be true but you're not going to notice. Nor are your users. If event handler garbage collection is your performance bottleneck, so much has already gone wrong that you need to stop and rethink your design: the reason React works so incredibly well is because it does not update the entire UI, it only updates the parts that change, and in a well designed UI, the time that most of your UI spends not changing drastically outnumbers the time small parts of your UI spend updating.
Function components with hooks (React 16.8+)
As of React 16.8 the function component (i.e. literally just a function that takes some props as argument can be used as if it's an instance of a component class, without ever writing a class) can also be given state, through the use of hooks.
If you don't need full class code, and a single instance function will do, then you can now use the useState hook to get yourself a single state variable, and its update function, which works roughly the same as the above examples, except without the "universal" setState function call and using one dedicated state setter for each value you're working with:
import { useId, useState } from 'react';
function myFunctionalComponentFunction(props) {
const id = useId();
const [input, setInput] = useState(props?.value ?? '');
return (
<div>
<label htmlFor={id}>Please specify:</label>
<input id={id} value={input} onInput={e => setInput(e.target.value)}/>
</div>
);
}
Previously the unofficial distinction between classes and function components was "function components don't have state", so we can't hide behind that one anymore: the difference between function components and classes components can be found spread over several pages in the very well-written react documentation (no shortcut one liner explanation to conveniently misinterpret for you!) which you should read so that you know what you're doing and can thus know whether you picked the best (whatever that means for you) solution to program yourself out of a problem you're having.
React 15 and below, using legacy ES5 and createClass
To do things properly, your component has a state value, which is shown via an input field, and we can update it by making that UI element send change events back into the component:
var Component = React.createClass({
getInitialState: function() {
return {
inputValue: ''
};
},
render: function() {
return (
//...
<input value={this.state.inputValue} onChange={this.updateInputValue}/>
//...
);
},
updateInputValue: function(evt) {
this.setState({
inputValue: evt.target.value
});
}
});
So we tell React to use the updateInputValue function to handle the user interaction, use setState to schedule the state update, and the fact that render taps into this.state.inputValue means that when it rerenders after updating the state, the user will see the update text based on what they typed.
addendum based on comments
Given that UI inputs represent state values (consider what happens if a user closes their tab midway, and the tab is restored. Should all those values they filled in be restored? If so, that's state). That might make you feel like a large form needs tens or even a hundred input forms, but React is about modeling your UI in a maintainable way: you do not have 100 independent input fields, you have groups of related inputs, so you capture each group in a component and then build up your "master" form as a collection of groups.
MyForm:
render:
<PersonalData/>
<AppPreferences/>
<ThirdParty/>
...
This is also much easier to maintain than a giant single form component. Split up groups into Components with state maintenance, where each component is only responsible for tracking a few input fields at a time.
You may also feel like it's "a hassle" to write out all that code, but that's a false saving: developers-who-are-not-you, including future you, actually benefit greatly from seeing all those inputs hooked up explicitly, because it makes code paths much easier to trace. However, you can always optimize. For instance, you can write a state linker
MyComponent = React.createClass({
getInitialState() {
return {
firstName: this.props.firstName || "",
lastName: this.props.lastName || ""
...: ...
...
}
},
componentWillMount() {
Object.keys(this.state).forEach(n => {
let fn = n + 'Changed';
this[fn] = evt => {
let update = {};
update[n] = evt.target.value;
this.setState(update);
});
});
},
render: function() {
return Object.keys(this.state).map(n => {
<input
key={n}
type="text"
value={this.state[n]}
onChange={this[n + 'Changed']}/>
});
}
});
Managed to get the input field value by doing something like this:
import React, { Component } from 'react';
class App extends Component {
constructor(props){
super(props);
this.state = {
username : ''
}
this.updateInput = this.updateInput.bind(this);
this.handleSubmit = this.handleSubmit.bind(this);
}
updateInput(event){
this.setState({username : event.target.value})
}
handleSubmit(){
console.log('Your input value is: ' + this.state.username)
//Send state to the server code
}
render(){
return (
<div>
<input type="text" onChange={this.updateInput}></input>
<input type="submit" onClick={this.handleSubmit} ></input>
</div>
);
}
}
//output
//Your input value is: x
You should use constructor under the class MyComponent extends React.Component
constructor(props){
super(props);
this.onSubmit = this.onSubmit.bind(this);
}
Then you will get the result of title
In react 16, I use
<Input id="number"
type="time"
onChange={(evt) => { console.log(evt.target.value); }} />
Give the <input> a unique id
<input id='title' ...>
and then use the standard Web API to reference it in the DOM
const title = document.getElementById('title').value
No need to continually update the React state with every keypress. Simply get the value when it's required.
In Function Component
useState
Returns a stateful value, and a function to update it.
During the initial render, the returned state (state) is the same as the value passed as the first argument (initialState).
The setState function is used to update the state. It accepts a new state value and enqueues a re-render of the component.
src ---> https://reactjs.org/docs/hooks-reference.html#usestate
useRef
useRef returns a mutable ref object whose .current property is initialized to the passed argument (initialValue). The returned object will persist for the full lifetime of the component.
src ---> https://reactjs.org/docs/hooks-reference.html#useref
import { useRef, useState } from "react";
export default function App() {
const [val, setVal] = useState('');
const inputRef = useRef();
const submitHandler = (e) => {
e.preventDefault();
setVal(inputRef.current.value);
}
return (
<div className="App">
<form onSubmit={submitHandler}>
<input ref={inputRef} />
<button type="submit">Submit</button>
</form>
<p>Submit Value: <b>{val}</b></p>
</div>
);
}
In Function Component :-
export default function App(){
const [state, setState] = useState({
value:'',
show:''
});
const handleChange = (e) => {
setState({value: e.target.value})
}
const submit = () => {
setState({show: state.value})
}
return(
<>
<form onSubmit={()=>submit()}>
<input type="text" value={state.value} onChange={(e)=>handleChange(e)} />
<input type="submit" />
</form>
<h2>{state.show}</h2>
</>
)}
export default class App extends React.Component{
state={
value:'',
show:''
}
handleChange=(e)=>{
this.setState({value:e.target.value})
}
submit=()=>{
this.setState({show:this.state.value})
}
render(){
return(
<>
<form onSubmit={this.submit}>
<input type="text" value={this.state.value} onChange={this.handleChange} />
<input type="submit" />
</form>
<h2>{this.state.show}</h2>
</>
)
}
}
I succeeded in doing this by binding this to the function
updateInputValue(evt) with
this.updateInputValue = this.updateInputValue.bind(this);
However input value={this.state.inputValue} ...
turned out to be no good idea.
Here's the full code in babel ES6 :
class InputField extends React.Component{
constructor(props){
super(props);
//this.state={inputfield: "no value"};
this.handleClick = this.handleClick.bind(this);
this.updateInputValue = this.updateInputValue.bind(this);
}
handleClick(){
console.log("trying to add picture url");
console.log("value of input field : "+this.state.inputfield);
}
updateInputValue(evt){
//console.log("input field updated with "+evt.target.value);
this.state={inputfield: evt.target.value};
}
render(){
var r;
r=<div><input type="text" id="addpixinputfield"
onChange={this.updateInputValue} />
<input type="button" value="add" id="addpix" onClick={this.handleClick}/>
</div>;
return r;
}
}
your error is because of you use class and when use class we need to bind the functions with This in order to work well. anyway there are a lot of tutorial why we should "this" and what is "this" do in javascript.
if you correct your submit button it should be work:
<button type="button" onClick={this.onSubmit.bind(this)} className="btn">Save</button>
and also if you want to show value of that input in console you should use var title = this.title.value;
This simplest way is to use arrow function
Your code with arrow functions
export default class MyComponent extends React.Component {
onSubmit = (e) => {
e.preventDefault();
var title = this.title;
console.log(title);
}
render(){
return (
...
<form className="form-horizontal">
...
<input type="text" className="form-control" ref={(c) => this.title = c} name="title" />
...
</form>
...
<button type="button" onClick={this.onSubmit} className="btn">Save</button>
...
);
}
};
React Version: 17.0.1
a) Using Functional Components
b) Manage state using hook: useState().
Write and Run code as above:
import React, {useState} from 'react';
const InputElement = () => {
const [inputText, setInputText] = useState('');
return (
<div>
<input
onChange={(e) => {
setInputText(e.target.value);
}
}
placeholder='Enter Text'
/>
{inputText}
</div>
);
}
The solving scheme algorithm is similar to a two-way data binding:
input <=> DATA_MODEL <=> Label_Text
// On the state
constructor() {
this.state = {
email: ''
}
}
// Input view ( always check if property is available in state {this.state.email ? this.state.email : ''}
<Input
value={this.state.email ? this.state.email : ''}
onChange={event => this.setState({ email: event.target.value)}
type="text"
name="emailAddress"
placeholder="johdoe#somewhere.com" />
You can get an input value without adding 'onChange' function.
Just add to the input element a 'ref attr:
And then use this.refs to get the input value when you need it.
Change your ref into: ref='title' and delete name='title'
Then delete var title = this.title and write:
console.log(this.refs.title.value)
Also you should add .bind(this) to this.onSubmit
(It worked in my case which was quite similar, but instead of onClick I had onSubmit={...} and it was put in form ( <form onSubmit={...} ></form>))
if you use class component then only 3 steps- first you need to declare state for your input filed for example this.state = {name:''}. Secondly, you need to write a function for setting the state when it changes in bellow example it is setName() and finally you have to write the input jsx for example < input value={this.name} onChange = {this.setName}/>
import React, { Component } from 'react'
export class InputComponents extends Component {
constructor(props) {
super(props)
this.state = {
name:'',
agree:false
}
this.setName = this.setName.bind(this);
this.setAgree=this.setAgree.bind(this);
}
setName(e){
e.preventDefault();
console.log(e.target.value);
this.setState({
name:e.target.value
})
}
setAgree(){
this.setState({
agree: !this.state.agree
}, function (){
console.log(this.state.agree);
})
}
render() {
return (
<div>
<input type="checkbox" checked={this.state.agree} onChange={this.setAgree}></input>
< input value={this.state.name} onChange = {this.setName}/>
</div>
)
}
}
export default InputComponents
export default class MyComponent extends React.Component {
onSubmit(e) {
e.preventDefault();
var title = this.title.value; //added .value
console.log(title);
}
render(){
return (
...
<form className="form-horizontal">
...
<input type="text" className="form-control" ref={input => this.title = input} name="title" />
...
</form>
...
<button type="button" onClick={this.onSubmit} className="btn">Save</button>
...
);
}
};
using uncontrolled fields:
export default class MyComponent extends React.Component {
onSubmit(e) {
e.preventDefault();
console.log(e.target.neededField.value);
}
render(){
return (
...
<form onSubmit={this.onSubmit} className="form-horizontal">
...
<input type="text" name="neededField" className="form-control" ref={(c) => this.title = c}/>
...
</form>
...
<button type="button" className="btn">Save</button>
...
);
}
};