请在构造函数中绑定getAll函数，或者您可以使用箭头函数

export class HttpClient {
  constructor(url) {
    this.url = url;
    this.xhr = new XMLHttpRequest();
    this.xhr.onloadend = event => {
      console.log("ONLOADEND",this.xhr.response);
      return this.xhr.result;
    };

    this.setHeader = this.setHeader.bind(this);
    this.get = this.get.bind(this);
    this.post = this.post.bind(this);
  }

  setHeader() {
    this.xhr.setRequestHeader("Content-Type","application/json");
  }

  get(async,header) {
    return new Promise((resolve,reject) => {
      this.xhr.open("GET",this.url,async);
      this.xhr.setRequestHeader(header.name,header.value);
      this.xhr.send();
      console.log("xhr",this.xhr.response);
      resolve(this.xhr.response);
      // return nothing
    });
  }

  post(async,data,reject) => {
      this.xhr.open("POST",this.url + "create",async);
      this.setHeader(header);
      this.xhr.send(data);
      console.log("RESPONSE",this.xhr.response);
      resolve(this.xhr.response);
    });
  }
}

export class TodoList extends HttpClient {
  constructor() {
    super("http://todoapp.test/api/");
    this.items = [];
    this.header = new Headers();
    this.header.set("Accept-Encoding","application/json");

    this.add = this.add.bind(this);
    this.getAll = this.getAll.bind(this);
  }

  add(todo) {
    this.post(true,JSON.stringify(todo),this.header);
    this.items.push(todo);
  }

  getAll() {
    this.get(true,this.header).then(result => {
      const data = !!result ? result : "[]";
      const items = JSON.parse(data);
      console.log("result:",result);
      items.forEach(item => {
        const todo = new Todo(item.id,item.name,item.status);
        this.items.push(todo);
      });
      console.log("items block:",this.items);
    });
  }
}