[reactjs] What's the difference between "super()" and "super(props)" in React when using es6 classes?

When is it important to pass props to super(), and why?

class MyComponent extends React.Component {
  constructor(props) {
    super(); // or super(props) ?
  }
}

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.

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.

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 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.

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.
}

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

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.

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 
  }
  // ...
}