What is the difference in method declaration?

Question

JavaScript

class TripHistory extends Component {

    scrollHandler = throttle((e) => {
        console.log(e);
    }, 300);
}

class TripHistory extends Component {

    constructor(){
        super(props);
        this.scrollHandler = throttle(this.scrollHandler,300)
    }

    scrollHandler = (e) => {
        console.log(e);
    }
}

The question is, what is the difference between these two methods? Why e.persist()? And how to properly troll React SyntheticEvent.