您需要将createXHR函数放在传递给ajax()调用的实际配置上：

import { XMLHttpRequest } from 'xmlhttprequest'

function createXHR() {
  return new XMLHttpRequest();
}

const ajax$ = ajax({
  createXHR, // <--- here
  url: genURL_chan(179),
  crossDomain: true,
  withCredentials: false,
  method: 'POST',
  body: { 'since': 0, 'mode': 'Messages', 'msgCount': 5000},
});

相关：我也回答了你关于仓库的问题，并提供了更多信息：https://github.com/ReactiveX/rxjs/issues/3978#issuecomment-411472389

由于某种原因，is a bug从未真正修复。首先，您必须安装xmlhttprequest，

npm install xmlhttprequest;

你得稍微修改一下然后加一个这个

## CommonJS
var XMLHttpRequest = require("xmlhttprequest").XMLHttpRequest;

## ES2015
import { XMLHttpRequest } from 'xmlhttprequest';

然后把它放到调用ajax()中

createXHR: function () {
  return new XMLHttpRequest();
}

应该是这个样子

import { ajax } from 'rxjs/ajax';
import { XMLHttpRequest } from 'xmlhttprequest';

const ajax$ = ajax({
  url: genURL_chan(179),
  crossDomain: true,
  withCredentials: false,
  method: 'POST',
  body: { 'since': 0, 'mode': 'Messages', 'msgCount': 5000},
});

我们可以使用**new XMLHttpRequest（）**有部分请求选项
options.js

import merge from 'lodash/fp/merge'

const createXHR = () => new XMLHttpRequest()

export const DEFAULT_REQUEST_OPTIONS = {
    createXHR,
    headers: {
        Accept: 'application/json',
        'Content-type': 'application/json'
    },
    method: 'POST',
    timeout: 200000
}

export default merge(DEFAULT_REQUEST_OPTIONS)

request.js

import getOptions from '../../../../../services/utils/get-options'

export default (app, body) => getOptions({
    body,
    url: app.api.clientSearch
})

apiCall.js

import { ajax } from 'rxjs/ajax'
import { map, pluck, shareReplay, switchMap } from 'rxjs/operators'
import env$ from '../../../../services/env'
import request from './request'

export default args => env$.pipe(
    map(env => request(env, args)),
    switchMap(req => ajax(req)),
    pluck('response'),
    shareReplay(1)
)