get all response headers and a parsed JSON result with "Fetch API" on ES2015

fetchApiHeadersGetter.js

(() => {
  const promise = fetch("//api.github.com/users/octocat/orgs");
  
  promise.then(res => {
    const headers = res.headers;
    
    for (let key of headers.keys()) {
      console.log(`${key} : ${headers.get(key)}`);
    }
    return res.json(); // return JSON object, not serialized string
  })
  .then(json => console.log(json));
}).call(this);