Develop Reference

When to use plain state over constructor(props)?

I know that their are some similar questions like this, however im still confused on the following.
When to use state like this
Class Example extends Component {
state = {
name: ''
}
}
Over constructor props
Class Example extends Component{
constructor(props) {
super(props);
this.state = {
name: ''
}
}
}
Does it have something to do with method binding ? because i can use onChange fine without binding it like this:
this.onChange = this.onChange.bind(this)
I can still call onChange like
onChange = (e) => {
}
<Component onChange = {this.onChange} />

The difference in your examples is purely syntactical, not functional. You can use both of them with the same effect.
Generally you assign a state in the constructor function when you want to compute the state based on some runtime logic (eg. based on the passed props). In that case you'll write it like that:
class Example extends Component {
constructor(props) {
this.state = {
fullName: props.firstName + ' ' + props.lastName
}
}
}

in react new version you can initialise state directly without using this
constructor(Props){
super(props)
this.state ={
}
}
both of them are correct. you can use any of them.
i will choose first one because it is less code
and for any method you can directly use them like
onChange = (e) => {
}
<Component onChange = {this.onChange} />
there is no need to bind method in new versions of react

Constructor Method in React

I have read the React Docs regarding the constructor method and what it can be used for as far as setting state and binding functions but is it really necessary in most cases?
What's the difference between doing
export default class MyClass extends Component {
constructor(props) {
super(props);
this.state = {
foo: 'bar',
};
this.member = 'member';
this.someFunction = this.anotherFunction(num);
}
anotherFunction = (num) => num * 2;
render() {
// render jsx here
}
}
and simply putting all that outside the constructor like
export default class MyClass extends Component {
state = {
foo: 'bar',
};
member = 'member';
someFunction = this.anotherFunction(num);
anotherFunction = (num) => num * 2;
render() {
// render jsx here
}
}
Is one option preferred over the other and are there any performance issues I should know about? This has been bugging me for a bit and I can't seem to find a concrete answer out there.

Your two examples are functionally identical, but the key thing is that assigning things outside of class methods, but inside of body of a class, like you have with everything other than render and constructor, is not standard ES6, and will only work via Babel. That syntax is the proposed class property syntax.

Seamless way to bind reference to a class method with class instance [duplicate]

I'm new to using ES6 classes with React, previously I've been binding my methods to the current object (show in first example), but does ES6 allow me to permanently bind a class function to a class instance with arrows? (Useful when passing as a callback function.) I get errors when I try to use them as you can with CoffeeScript:
class SomeClass extends React.Component {
// Instead of this
constructor(){
this.handleInputChange = this.handleInputChange.bind(this)
}
// Can I somehow do this? Am i just getting the syntax wrong?
handleInputChange (val) => {
console.log('selectionMade: ', val);
}
So that if I were to pass SomeClass.handleInputChange to, for instance setTimeout, it would be scoped to the class instance, and not the window object.

Your syntax is slightly off, just missing an equals sign after the property name.
class SomeClass extends React.Component {
handleInputChange = (val) => {
console.log('selectionMade: ', val);
}
}
This is an experimental feature. You will need to enable experimental features in Babel to get this to compile. Here is a demo with experimental enabled.
To use experimental features in babel you can install the relevant plugin from here. For this specific feature, you need the transform-class-properties plugin:
{
"plugins": [
"transform-class-properties"
]
}
You can read more about the proposal for Class Fields and Static Properties here

No, if you want to create bound, instance-specific methods you will have to do that in the constructor. However, you can use arrow functions for that, instead of using .bind on a prototype method:
class SomeClass extends React.Component {
constructor() {
super();
this.handleInputChange = (val) => {
console.log('selectionMade: ', val, this);
};
…
}
}
There is an proposal which might allow you to omit the constructor() and directly put the assignment in the class scope with the same functionality, but I wouldn't recommend to use that as it's highly experimental.
Alternatively, you can always use .bind, which allows you to declare the method on the prototype and then bind it to the instance in the constructor. This approach has greater flexibility as it allows modifying the method from the outside of your class.
class SomeClass extends React.Component {
constructor() {
super();
this.handleInputChange = this.handleInputChange.bind(this);
…
}
handleInputChange(val) {
console.log('selectionMade: ', val, this);
}
}

You are using arrow function and also binding it in constructor. So you no need to do binding when you use arrow functions
class SomeClass extends React.Component {
handleInputChange = (val) => {
console.log('selectionMade: ', val);
}
}
OR you need to bind a function only in constructor when you use normal function like below
class SomeClass extends React.Component {
constructor(props){
super(props);
this.handleInputChange = this.handleInputChange.bind(this);
}
handleInputChange(val){
console.log('selectionMade: ', val);
}
}
Also binding a function directly in render is not recommended. It should always be in constructor

I know this question has been sufficiently answered, but I just have a small contribution to make (for those who don't want to use the experimental feature). Because of the problem of having to bind multiple function binds in the constructor and making it look messy, I came up with a utility method that once bound and called in the constructor, does all the necessary method bindings for you automatically.
Assume I have this class with the constructor:
//src/components/PetEditor.jsx
import React from 'react';
class PetEditor extends React.Component {
constructor(props){
super(props);
this.state = props.currentPet || {tags:[], photoUrls: []};
this.tagInput = null;
this.htmlNode = null;
this.removeTag = this.removeTag.bind(this);
this.handleChange = this.handleChange.bind(this);
this.modifyState = this.modifyState.bind(this);
this.handleKeyUp = this.handleKeyUp.bind(this);
this.addTag = this.addTag.bind(this);
this.removeTag = this.removeTag.bind(this);
this.savePet = this.savePet.bind(this);
this.addPhotoInput = this.addPhotoInput.bind(this);
this.handleSelect = this.handleSelect.bind(this);
}
// ... actual method declarations omitted
}
It looks messy, doesn't it?
Now I created this utility method
//src/utils/index.js
/**
* NB: to use this method, you need to bind it to the object instance calling it
*/
export function bindMethodsToSelf(objClass, otherMethodsToIgnore=[]){
const self = this;
Object.getOwnPropertyNames(objClass.prototype)
.forEach(method => {
//skip constructor, render and any overrides of lifecycle methods
if(method.startsWith('component')
|| method==='constructor'
|| method==='render') return;
//any other methods you don't want bound to self
if(otherMethodsToIgnore.indexOf(method)>-1) return;
//bind all other methods to class instance
self[method] = self[method].bind(self);
});
}
All I now need to do is import that utility, and add a call to my constructor, and I don't need to bind each new method in the constructor anymore.
New constructor now looks clean, like this:
//src/components/PetEditor.jsx
import React from 'react';
import { bindMethodsToSelf } from '../utils';
class PetEditor extends React.Component {
constructor(props){
super(props);
this.state = props.currentPet || {tags:[], photoUrls: []};
this.tagInput = null;
this.htmlNode = null;
bindMethodsToSelf.bind(this)(PetEditor);
}
// ...
}

React getting props value in from another component [duplicate]

When is it important to pass props to super(), and why?
class MyComponent extends React.Component {
constructor(props) {
super(); // or super(props) ?
}
}

There is only one reason when one needs to pass props to super():
When you want to access this.props in constructor.
Passing:
class MyComponent extends React.Component {
constructor(props) {
super(props)
console.log(this.props)
// -> { icon: 'home', … }
}
}
Not passing:
class MyComponent extends React.Component {
constructor(props) {
super()
console.log(this.props)
// -> undefined
// Props parameter is still available
console.log(props)
// -> { icon: 'home', … }
}
render() {
// No difference outside constructor
console.log(this.props)
// -> { icon: 'home', … }
}
}
Note that passing or not passing props to super has no effect on later uses of this.props outside constructor. That is render, shouldComponentUpdate, or event handlers always have access to it.
This is explicitly said in one Sophie Alpert's answer to a similar question.
The documentation—State and Lifecycle, Adding Local State to a Class, point 2—recommends:
Class components should always call the base constructor with props.
However, no reason is provided. We can speculate it is either because of subclassing or for future compatibility.
(Thanks #MattBrowne for the link)

In this example, you are extending the React.Component class, and per the ES2015 spec, a child class constructor cannot make use of this until super() has been called; also, ES2015 class constructors have to call super() if they are subclasses.
class MyComponent extends React.Component {
constructor() {
console.log(this); // Reference Error
}
render() {
return <div>Hello {this.props.name}</div>;
}
}
By contrast:
class MyComponent extends React.Component {
constructor() {
super();
console.log(this); // this logged to console
}
render() {
return <div>Hello {this.props.name}</div>;
}
}
More detail as per this excellent stack overflow answer
You may see examples of components created by extending the React.Component class that do not call super() but you'll notice these don't have a constructor, hence why it is not necessary.
class MyOtherComponent extends React.Component {
render() {
return <div>Hi {this.props.name}</div>;
}
}
One point of confusion I've seen from some developers I've spoken to is that the components that have no constructor and therefore do not call super() anywhere, still have this.props available in the render() method. Remember that this rule and this need to create a this binding for the constructor only applies to the constructor.

When you pass props to super, the props get assigned to this. Take a look at the following scenario:
constructor(props) {
super();
console.log(this.props) //undefined
}
How ever when you do :
constructor(props) {
super(props);
console.log(this.props) //props will get logged.
}

When implementing the constructor() function inside a React component, super() is a requirement. Keep in mind that your MyComponent component is extending or borrowing functionality from the React.Component base class.
This base class has a constructor() function of its own that has some code inside of it, to setup our React component for us.
When we define a constructor() function inside our MyComponent class, we are essentially, overriding or replacing the constructor() function that is inside the React.Component class, but we still need to ensure that all the setup code inside of this constructor() function still gets called.
So to ensure that the React.Component’s constructor() function gets called, we call super(props). super(props) is a reference to the parents constructor() function, that’s all it is.
We have to add super(props) every single time we define a constructor() function inside a class-based component.
If we don’t we will see an error saying that we have to call super(props).
The entire reason for defining this constructor() funciton is to initialize our state object.
So in order to initialize our state object, underneath the super call I am going to write:
class App extends React.Component {
constructor(props) {
super(props);
this.state = {};
}
// React says we have to define render()
render() {
return <div>Hello world</div>;
}
};
So we have defined our constructor() method, initialized our state object by creating a JavaScript object, assigning a property or key/value pair to it, assigning the result of that to this.state. Now of course this is just an example here so I have not really assigned a key/value pair to the state object, its just an empty object.

Dan Abramov wrote an article on this topic:
Why Do We Write super(props)?
And the gist of it is that it's helpful to have a habit of passing it to avoid this scenario, that honestly, I don't see it unlikely to happen:
// Inside React
class Component {
constructor(props) {
this.props = props;
// ...
}
}
// Inside your code
class Button extends React.Component {
constructor(props) {
super(); // 😬 We forgot to pass props
console.log(props); // ✅ {}
console.log(this.props); // 😬 undefined
}
// ...
}

As per source code
function ReactComponent(props, context) {
this.props = props;
this.context = context;
}
you must pass props every time you have props and you don't put them into this.props manually.

super() is used to call the parent constructor.
super(props) would pass props to the parent constructor.
From your example, super(props) would call the React.Component constructor passing in props as the argument.
More information on super:
https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Operators/super

For react version 16.6.3, we use super(props) to initialize state element name : this.props.name
constructor(props){
super(props);
}
state = {
name:this.props.name
//otherwise not defined
};

Here we won't get this in the constructor so it will return undefined, but we will be able to fetch this outside the constructor function
class MyComponent extends React.Component {
constructor() {
console.log(this); // Reference Error i.e return undefined
}
render() {
return <div>Hello {this.props.name}</div>;
}
}
If we are using super(), then we can fetch the "this" variable inside the constructor as well
class MyComponent extends React.Component {
constructor() {
super();
console.log(this); // this logged to console
}
render() {
return <div>Hello {this.props.name}</div>;
}
}
So when we are using super(); we will be able to fetch this but this.props will be undefined in the constructor. But other than constructor, this.props will not return undefined.
If we use super(props), then we can use this.props value inside the constructor as well
Sophie Alpert's Answer
If you want to use this.props in the constructor, you need to pass
props to super. Otherwise, it doesn’t matter because React sets .props
on the instance from the outside immediately after calling the
constructor.

Here is the fiddle I've made:jsfiddle.net. It shows that props are assigned not in the constructor by default. As I understand they are assinged in the method React.createElement. Hence super(props) should be called only when the superclass's constructor manually assings props to this.props. If you just extend the React.Component calling super(props) will do nothing with props. Maybe It will be changed in the next versions of React.

How to use arrow functions (public class fields) as class methods?

I'm new to using ES6 classes with React, previously I've been binding my methods to the current object (show in first example), but does ES6 allow me to permanently bind a class function to a class instance with arrows? (Useful when passing as a callback function.) I get errors when I try to use them as you can with CoffeeScript:
class SomeClass extends React.Component {
// Instead of this
constructor(){
this.handleInputChange = this.handleInputChange.bind(this)
}
// Can I somehow do this? Am i just getting the syntax wrong?
handleInputChange (val) => {
console.log('selectionMade: ', val);
}
So that if I were to pass SomeClass.handleInputChange to, for instance setTimeout, it would be scoped to the class instance, and not the window object.

Your syntax is slightly off, just missing an equals sign after the property name.
class SomeClass extends React.Component {
handleInputChange = (val) => {
console.log('selectionMade: ', val);
}
}
This is an experimental feature. You will need to enable experimental features in Babel to get this to compile. Here is a demo with experimental enabled.
To use experimental features in babel you can install the relevant plugin from here. For this specific feature, you need the transform-class-properties plugin:
{
"plugins": [
"transform-class-properties"
]
}
You can read more about the proposal for Class Fields and Static Properties here

No, if you want to create bound, instance-specific methods you will have to do that in the constructor. However, you can use arrow functions for that, instead of using .bind on a prototype method:
class SomeClass extends React.Component {
constructor() {
super();
this.handleInputChange = (val) => {
console.log('selectionMade: ', val, this);
};
…
}
}
There is an proposal which might allow you to omit the constructor() and directly put the assignment in the class scope with the same functionality, but I wouldn't recommend to use that as it's highly experimental.
Alternatively, you can always use .bind, which allows you to declare the method on the prototype and then bind it to the instance in the constructor. This approach has greater flexibility as it allows modifying the method from the outside of your class.
class SomeClass extends React.Component {
constructor() {
super();
this.handleInputChange = this.handleInputChange.bind(this);
…
}
handleInputChange(val) {
console.log('selectionMade: ', val, this);
}
}

You are using arrow function and also binding it in constructor. So you no need to do binding when you use arrow functions
class SomeClass extends React.Component {
handleInputChange = (val) => {
console.log('selectionMade: ', val);
}
}
OR you need to bind a function only in constructor when you use normal function like below
class SomeClass extends React.Component {
constructor(props){
super(props);
this.handleInputChange = this.handleInputChange.bind(this);
}
handleInputChange(val){
console.log('selectionMade: ', val);
}
}
Also binding a function directly in render is not recommended. It should always be in constructor

I know this question has been sufficiently answered, but I just have a small contribution to make (for those who don't want to use the experimental feature). Because of the problem of having to bind multiple function binds in the constructor and making it look messy, I came up with a utility method that once bound and called in the constructor, does all the necessary method bindings for you automatically.
Assume I have this class with the constructor:
//src/components/PetEditor.jsx
import React from 'react';
class PetEditor extends React.Component {
constructor(props){
super(props);
this.state = props.currentPet || {tags:[], photoUrls: []};
this.tagInput = null;
this.htmlNode = null;
this.removeTag = this.removeTag.bind(this);
this.handleChange = this.handleChange.bind(this);
this.modifyState = this.modifyState.bind(this);
this.handleKeyUp = this.handleKeyUp.bind(this);
this.addTag = this.addTag.bind(this);
this.removeTag = this.removeTag.bind(this);
this.savePet = this.savePet.bind(this);
this.addPhotoInput = this.addPhotoInput.bind(this);
this.handleSelect = this.handleSelect.bind(this);
}
// ... actual method declarations omitted
}
It looks messy, doesn't it?
Now I created this utility method
//src/utils/index.js
/**
* NB: to use this method, you need to bind it to the object instance calling it
*/
export function bindMethodsToSelf(objClass, otherMethodsToIgnore=[]){
const self = this;
Object.getOwnPropertyNames(objClass.prototype)
.forEach(method => {
//skip constructor, render and any overrides of lifecycle methods
if(method.startsWith('component')
|| method==='constructor'
|| method==='render') return;
//any other methods you don't want bound to self
if(otherMethodsToIgnore.indexOf(method)>-1) return;
//bind all other methods to class instance
self[method] = self[method].bind(self);
});
}
All I now need to do is import that utility, and add a call to my constructor, and I don't need to bind each new method in the constructor anymore.
New constructor now looks clean, like this:
//src/components/PetEditor.jsx
import React from 'react';
import { bindMethodsToSelf } from '../utils';
class PetEditor extends React.Component {
constructor(props){
super(props);
this.state = props.currentPet || {tags:[], photoUrls: []};
this.tagInput = null;
this.htmlNode = null;
bindMethodsToSelf.bind(this)(PetEditor);
}
// ...
}