fetch JSON with a callback

fetchJSON.js

function fetchJSON(url, options, callback) {
  if (typeof options === 'function') {
    callback = options
    options = {}
  }

  options = options || {}

  const headers = (options.headers || (options.headers = {}))
  headers.Accept = 'application/json'

  fetch(url, options)
    .then(response => response.json())
    .then(json => callback(null, json), callback)
}