I just started learning React and JavaScript.

While going through the tutorial, I got to this example code of a component, which creates a toggle button.

This is part of the code:

class Toggle extends React.Component {

    constructor(props) {

        super(props);

        this.state = {isToggleOn: true};

        this.handleClick = this.handleClick.bind(this);

    }



    handleClick() {

        this.setState(prevState => ({    // prevState?

            isToggleOn: !prevState.isToggleOn

        }));

    }

2 things that are bugging me here:

1. Where did the prevState argument come from?
   
   I don't see anything like var prevState = this.state; before calling it, and still, it works.


2. The syntax of the arrow function: why the parentheses after the arrow?
   
   Why doesn't the usual arg => { statement; } syntax work here?

Sorry for the newbie questions...