为什么我们能够在react组件的构造函数中绑定函数?

import React from 'react';
class A extends React.Component {
  constructor(props){
    super(props);
    this.a = 1;
    this.fn = this.fn.bind(this);
  }
  fn(){
    console.log(this.a);
  }
}

<script src="https://cdnjs.cloudflare.com/ajax/libs/react/16.6.3/umd/react.production.min.js"></script>
<script src="https://cdnjs.cloudflare.com/ajax/libs/react-dom/16.6.3/umd/react-dom.production.min.js"></script>