Develop Reference

Why was "this" lost? [duplicate]

class PlayerControls extends React.Component {
constructor(props) {
super(props)
this.state = {
loopActive: false,
shuffleActive: false,
}
}
render() {
var shuffleClassName = this.state.toggleActive ? "player-control-icon active" : "player-control-icon"
return (
<div className="player-controls">
<FontAwesome
className="player-control-icon"
name='refresh'
onClick={this.onToggleLoop}
spin={this.state.loopActive}
/>
<FontAwesome
className={shuffleClassName}
name='random'
onClick={this.onToggleShuffle}
/>
</div>
);
}
onToggleLoop(event) {
// "this is undefined??" <--- here
this.setState({loopActive: !this.state.loopActive})
this.props.onToggleLoop()
}
I want to update loopActive state on toggle, but this object is undefined in the handler. According to the tutorial doc, I this should refer to the component. Am I missing something?

ES6 React.Component doesn't auto bind methods to itself. You need to bind them yourself in constructor. Like this:
constructor (props){
super(props);
this.state = {
loopActive: false,
shuffleActive: false,
};
this.onToggleLoop = this.onToggleLoop.bind(this);
}

There are a couple of ways.
One is to add
this.onToggleLoop = this.onToggleLoop.bind(this); in the constructor.
Another is arrow functions
onToggleLoop = (event) => {...}.
And then there is onClick={this.onToggleLoop.bind(this)}.

Write your function this way:
onToggleLoop = (event) => {
this.setState({loopActive: !this.state.loopActive})
this.props.onToggleLoop()
}
Fat Arrow Functions
the binding for the keyword this is the same outside and inside the fat arrow function. This is different than functions declared with function, which can bind this to another object upon invocation. Maintaining the this binding is very convenient for operations like mapping: this.items.map(x => this.doSomethingWith(x)).

I ran into a similar bind in a render function and ended up passing the context of this in the following way:
{someList.map(function(listItem) {
// your code
}, this)}
I've also used:
{someList.map((listItem, index) =>
<div onClick={this.someFunction.bind(this, listItem)} />
)}

in my case this was the solution = () => {}
methodName = (params) => {
//your code here with this.something
}

You should notice that this depends on how function is invoked
ie: when a function is called as a method of an object, its this is set to the object the method is called on.
this is accessible in JSX context as your component object, so you can call your desired method inline as this method.
If you just pass reference to function/method, it seems that react will invoke it as independent function.
onClick={this.onToggleLoop} // Here you just passing reference, React will invoke it as independent function and this will be undefined
onClick={()=>this.onToggleLoop()} // Here you invoking your desired function as method of this, and this in that function will be set to object from that function is called ie: your component object

If you are using babel, you bind 'this' using ES7 bind operator
https://babeljs.io/docs/en/babel-plugin-transform-function-bind#auto-self-binding
export default class SignupPage extends React.Component {
constructor(props) {
super(props);
}
handleSubmit(e) {
e.preventDefault();
const data = {
email: this.refs.email.value,
}
}
render() {
const {errors} = this.props;
return (
<div className="view-container registrations new">
<main>
<form id="sign_up_form" onSubmit={::this.handleSubmit}>
<div className="field">
<input ref="email" id="user_email" type="email" placeholder="Email" />
</div>
<div className="field">
<input ref="password" id="user_password" type="new-password" placeholder="Password" />
</div>
<button type="submit">Sign up</button>
</form>
</main>
</div>
)
}
}

I want to give an explanation of why this is undefined:
If we use this in a function that is not an arrow function, this is bound to a global object when not in strict mode. But with strict mode, this will be undefined (https://www.w3schools.com/js/js_this.asp).
And ES6 modules are always in strict mode (javascript: use strict is unnecessary inside of modules).
You can bind this in onToggleLoop function with the instance of PlayerControls component by using bind method inside the constructor:
constructor(props) {
super(props)
this.state = {
loopActive: false,
shuffleActive: false,
}
this.onToggleLoop = this.onToggleLoop.bind(this)
}
Or use the arrow function instead:
onToggleLoop = (event) => {
this.setState({loopActive: !this.state.loopActive})
this.props.onToggleLoop()
}
The arrow function does not have context, so this in the arrow function will represent the object that defined the arrow function.

If you call your created method in the lifecycle methods like componentDidMount... then you can only use the this.onToggleLoop = this.onToogleLoop.bind(this) and the fat arrow function onToggleLoop = (event) => {...}.
The normal approach of the declaration of a function in the constructor wont work because the lifecycle methods are called earlier.

In my case, for a stateless component that received the ref with forwardRef, I had to do what it is said here https://itnext.io/reusing-the-ref-from-forwardref-with-react-hooks-4ce9df693dd
From this (onClick doesn't have access to the equivalent of 'this')
const Com = forwardRef((props, ref) => {
return <input ref={ref} onClick={() => {console.log(ref.current} } />
})
To this (it works)
const useCombinedRefs = (...refs) => {
const targetRef = React.useRef()
useEffect(() => {
refs.forEach(ref => {
if (!ref) return
if (typeof ref === 'function') ref(targetRef.current)
else ref.current = targetRef.current
})
}, [refs])
return targetRef
}
const Com = forwardRef((props, ref) => {
const innerRef = useRef()
const combinedRef = useCombinedRefs(ref, innerRef)
return <input ref={combinedRef } onClick={() => {console.log(combinedRef .current} } />
})

You can rewrite how your onToggleLoop method is called from your render() method.
render() {
var shuffleClassName = this.state.toggleActive ? "player-control-icon active" : "player-control-icon"
return (
<div className="player-controls">
<FontAwesome
className="player-control-icon"
name='refresh'
onClick={(event) => this.onToggleLoop(event)}
spin={this.state.loopActive}
/>
</div>
);
}
The React documentation shows this pattern in making calls to functions from expressions in attributes.

How to remove ReactJS warning related to using a `value` prop to a form field without an `onChange` handler when the handler is actually defined?

So have an input field:
<input
id={itemId}
onChange={handleChange}
type="number"
value={packed}
/>
And here is my onChange function:
handleChange(e) {
const { items, onUpdateQuantity } = this.props;
const updateItem = items.filter((item) =>
item.itemId === e.target.id,
);
const itemQuantity = toNumber(e.target.value);
updateItem.total += itemQuantity;
onUpdateQuantity(e.target.id, itemQuantity);
}
So why is React still complaining about an onChange handler not being defined when it already is? I don't want to add a defaultValue prop, as that causes bugs in my app. Any ideas?

That is coming because your value is not changing anywhere. As you can see from docs for controlled components, the value of the input is this.state.value and the onChange method changes the value inside the input by changing this.state.value.
As far as I can see, when you input a value inside the input (<input/>) element, the value of that element is not changing. It is always whatever the value of packed is. That is why you're getting the error.

Make sure you bind you function in the constructor
class bla extends Component {
constructor(props){
super(props);
this.handleChange = this.handleChange.bind(this);
}
}
or if using stage 0 to have you have your function in this way.
handleChange = () => {}
and if not
handleChange () {
return (e) => {};
}
Also, if your using a class component you should call your handleChange with this.handleChange if your passing it to functional component then what you have should be fine

https://reactjs.org/docs/handling-events.html
You have to be careful about the meaning of this in JSX callbacks. In JavaScript, class methods are not bound by default. If you forget to bind this.handleClick and pass it to onClick, this will be undefined when the function is actually called.
This is not React-specific behavior; it is a part of how functions work in JavaScript. Generally, if you refer to a method without () after it, such as onClick={this.handleClick}, you should bind that method.
Don't forget to use this.handleChange in you JSX
Example:
class Toggle extends React.Component {
constructor(props) {
super(props);
this.state = {isToggleOn: true};
// This binding is necessary to make `this` work in the callback
this.handleClick = this.handleClick.bind(this);
}
handleClick() {
this.setState(prevState => ({
isToggleOn: !prevState.isToggleOn
}));
}
render() {
return (
<button onClick={this.handleClick}>
{this.state.isToggleOn ? 'ON' : 'OFF'}
</button>
);
}
}
ReactDOM.render(
<Toggle />,
document.getElementById('root')
);

Deprecation warning using this.refs

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
})
}
}

Why this.state is undefined in react native?

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}

ReactJS - MouseClick gets triggered without a click

I'm new to React.JS and trying to create a click event on an element inside a rendered component.
Here is my code:
class InputPanel extends React.Component{
handleClick(i,j) {
this.props.dispatch(actions.someMethod());
// e.preventDefault();
}
render() {
const { dispatch, board } = this.props;
return(
<div>
{
board.map((row, i) => (
<div>{row.map((cell, j) => <div className="digit"
onClick={this.handleClick(i,j)}>{cell}</div>)}</div>
))
}
</div>
);
}
};
My problem is that "handleClick" gets triggered after page load without any mouse clicked!
I've read about React.JS lifecycle and thought about registering to click event in componentDidMount method, but i'm really not sure about it:
Is there any easier way ? (or: Am I doing something wrong that triggers click ?)
If adding componentDidMount method is the right way - how can I get the element I create in render method ?

You should not use .bind when passing the callback as a prop. There’s a ESLint rule for that. You can read more about how to pass callback without breaking React performance here.
Summary:
make sure you aren’t calling functions but pass functions as handlers in your props.
make sure you do not create functions on every render, for that, you need to bind your handlers in parent component, pass correct the required data (such as indices of iteration) down the child component and have it call the parent’s handler with the data it has
Ideally you’d create another component for the rows and pass the callback there. Moreover, ideally you’d bind the onClick in the parent component’s constructor (or componentWillMount). Otherwise every time render runs a new function is created (in both anonymous function handler () => { this.onClick() } and this.onClick.bind and defeat React’s vdom diff causing every row to rerender every time.
So:
class InputPanel extends React.Component{
constructor() {
super();
this.handleClick = this.handleClick.bind(this);
}
handleClick(i,j) {
this.props.dispatch(actions.someMethod());
// e.preventDefault();
}
render() {
const { dispatch, board } = this.props;
return(
<div>
{board.map((row, i) => <div>
{row.map((cell, j) => <Digit
onClick={this.handleClick})
i={i}
j={j}
cell={cell}
/>)}
</div>)}
</div>
);
}
};
class Digit extends React.Component {
constructor() {
super();
this.handleClick = this.handleClick.bind(this);
}
handleClick() {
this.props.onClick(this.props.i, this.props.j);
}
render() {
return <div
className="digit"
onClick={this.handleClick}
>{this.props.cell}</div>
}
}

It is because you are calling this.handleClick() function instead of providing a function definition as onClick prop.
Try changing the div line like this:
<div className="digit" onClick={ () => this.handleClick(i,j) }>{cell}</div>
Also you have to bind this.handleClick() function. You can add a constructor and bind all the member functions of a class there. that's the best practice in ES6.
constructor(props, context) {
super(props, context);
this.handleClick = this.handleClick.bind(this);
}

You call this function in render. You should only transfer function and bind params.
onClick={this.handleClick.bind(null,i,j)}

You should use .bind().
class InputPanel extends React.Component{
handleClick(i,j) {
this.props.dispatch(actions.someMethod());
// e.preventDefault();
}
render() {
const { dispatch, board } = this.props;
return(
<div>
{
board.map((row, i) => (
<div>{row.map((cell, j) => <div className="digit"
onClick={this.handleClick.bind(null,i,j)}>{cell}</div>)}</div>
))
}
</div>
);
}
};