xhr-in-fetch.js

class Dispatch extends EventTarget {
  dispatch(eventName) {
    const ev = new Event(eventName);
    if (("on" + eventName) in this) {
      this["on" + eventName](ev);
    }
    this.dispatchEvent(ev);
  }
}
class XMLHttpRequest extends Dispatch {
  open(method, url) {
    this.url = url;
    this.method = method;
    this.headers = new Headers();
    this.readyState = 0;
    this.responseType = "";
    this.upload = new Dispatch();
    this._controller = new AbortController();
  }
  setRequestHeader(key, value) {
    x.set(key, value);
  }
  abort() {
    this.upload.dispatch("abort");
    this._controller.abort();
  }
  send(payload) {
    this.readyState = 1;
    this.dispatch("readystatechanged");
    this.upload.dispatch("loadstart");
    fetch(this.url, {
      method: this.method,
      credentials: this.withCredentials,
      headers: this.headers,
      signal: this._controller.signal,
      body: payload
    }).then(response => {
      this.readyState = 2;
      this.responseURL = response.url;
      this.responseType = response.type;
      this.status = response.status;
      this.statusText = response.statusText;
      switch (this.responseType) {
        case "": return response.text();
        case "arraybuffer": return response.arrayBuffer();
        case "blob": return response.blob();
        case "document": return response.text(); // todo pass through XMLParser
        case "json": return response.json();
        //todo stream and status 3 and progress
      }
    }).then(value => {
      this.readyState = 4;
      this.dispatch("readystatechanged");
      this.response = value;
      this.upload.dispatch("load");
      this.dispatch("load");
      this.upload.dispatch("loadend");
      this.dispatch("loadend");
    }, err => {
      this.dispatch("error");
      this.upload.dispatch("error")
      this.readyState = 4;
      this.dispatch("readystatechanged");
    });
  }
  get responseText() {
    if(this.response) {
      if (this.responseType === "arraybuffer") {
        return String.fromCharCode.apply(null, new Uint16Array(this.response)); // slower than StringEncoder/StringDecoder API, but meh
      }
      if (this.responseType === "json") {
        return JSON.stringify(this.response);
      }
      if(this.response === "blob") {
        return ""; //
      }
      return this.response;
    }
  }
  
}

var x = new XMLHttpRequest();
x.open("GET", "./");
x.onload = () => console.log("Onload called");
x.addEventListener("load", console.log);
x.send();

small fix in line 21
this.headers.set(key, value);