let base64 = require('base-64');

let url = 'http://eu.httpbin.org/basic-auth/user/passwd';
let username = 'user';
let password = 'passwd';

let headers = new Headers();

//headers.append('Content-Type', 'text/json');
headers.append('Authorization', 'Basic ' + base64.encode(username + ":" + password));

fetch(url, {method:'GET',
        headers: headers,
        //credentials: 'user:passwd'
       })
.then(response => response.json())
.then(json => console.log(json));
//.done();


headers.set('Authorization', 'Basic ' + Buffer.from(username + ":" + password).toString('base64'));


npm install base-64 --save
import { encode } from "base-64";

 const response = await fetch(URL, {
  method: 'post',
  headers: new Headers({
    'Authorization': 'Basic ' + encode(username + ":" + password),
    'Content-Type': 'application/json'
  }),
  body: JSON.stringify({
    "PassengerMobile": "xxxxxxxxxxxx",
    "Password": "xxxxxxx"
  })
});
const posts = await response.json();


// space in "C ontent-Type"
const headers = {
  "C ontent-Type": "text/xml",
  "Breaking-Bad": "<3",
};
fetch("https://example.com/", { headers });