Example: `fetch` JSON file.

fetch.js

// Fetch JSON data
fetch('https://jsonplaceholder.typicode.com/todos/').then(response => {

  // Success
  if (response.ok)
      return response.json(); // Returns to then()

  // Error
  return Promise.reject(response);

}).then(data => {
  console.log(data); // JSON
}).catch(err => {
  console.error(err); // Error
});

// Fetch JSON data and log value of specific response header (x-ratelimit-remaining)
fetch('https://api.github.com/users/donbrae/gists?per_page=15').then(response => {
  
  const headers = {};
  for (let entry of response.headers.entries()) {
    headers[entry[0]] = entry[1];
  }
  
  console.log(`x-ratelimit-remaining: ${headers['x-ratelimit-remaining']}`);
  
  // Success	
  if (response.ok)
    return response.json(); // Returns to then()

  // Error
  return Promise.reject(response);

}).then(data => {
  console.log(data);
}).catch(err => {
  console.error(err); // Error
});