在类构造函数中 this.button = document.getElementById('button') / Uncaught TypeError: Cannot read property 'addEventListener' of undefined？

我得到了带有一些代码的 webpack 包：

class View extends EventEmitter {

    constructor() {
        super();
        this.input = document.getElementById('input');
        this.btn = document.getElementById('button');
        this.list = document.getElementById('list');
    }
    addListeners() {
        this.btn.addEventListener('click',this.emit('some', this.inputValue));
    }
   // some other stuff
}

//--------got error : Uncaught TypeError: Cannot read property 'addEventListener' of undefined

但是当我在做：

addListeners() {

// document.getElementById('button') instead of this.btn everything is good 


document.getElementById('button').addEventListener('click',this.emit('some', this.inputValue));
    }