super(props) in React

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.

In JavaScript, super refers to the parent class constructor. (In our example, it points to the React.Component implementation.)

Importantly, you can’t use this in a constructor until after you’ve called the parent constructor. JavaScript won’t let you:

class Checkbox extends React.Component {
  constructor(props) {
    // 🔴 Can’t use `this` yet
    super(props);
    // ✅ Now it’s okay though
    this.state = { isOn: true };
  }
  // ...
}