index.js

;(function (window) {

  function defaults (target, obj) {
    for (var prop in obj) target[prop] = target[prop] || obj[prop]
  }

  function getQuery (queryParams) {
    return '?' + serialize(queryParams);
  }

  function serialize(obj, prefix) {
    var str = [], p;
    for(p in obj) {
      if (obj.hasOwnProperty(p)) {
        if (Array.isArray(obj[p])) {
          var values = obj[p];
          values.forEach(v => {
            str.push(encodeURIComponent(p) + "[]=" + encodeURIComponent(v));
          });
        } else {
          var k = prefix ? prefix + "[" + p + "]" : p, v = obj[p];
          str.push((v !== null && typeof v === "object") ?
            serialize(v, k) :
            encodeURIComponent(k) + "=" + encodeURIComponent(v));
        }
      }
    }
    return str.join("&");
  }

  function _fetch (method, url, opts, data, queryParams) {
    opts.method = method
    opts.headers = opts.headers || {}
    opts.responseAs = (opts.responseAs && ['json', 'text', 'response'].indexOf(opts.responseAs) >= 0) ? opts.responseAs : 'json'

    defaults(opts.headers, {
      'Accept': 'application/json',
      'Content-Type': 'application/json'
    })

    if (queryParams) {
      url += getQuery(queryParams)
    }

    if (data) {
        opts.body = JSON.stringify(data);
    } else {
        delete opts.body;
    }

    return fetchival.fetch(url, opts)
      .then(function (response) {
        if (response.status >= 200 && response.status < 300) {
          if(opts.responseAs=="response")
            return response
          if (response.status == 204)
            return null;
          return response[opts.responseAs]();
        }
        var err = new Error(response.statusText)
        err.response = response
        throw err
      })
  }

  function fetchival (url, opts) {
    opts = opts || {}

    var _ = function (u, o) {
      // Extend parameters with previous ones
      u = url + '/' + u
      o = o || {}
      defaults(o, opts)
      return fetchival(u, o)
    }

    _.get = function (queryParams) {
      return _fetch('GET', url, opts, null, queryParams)
    }

    _.post = function (data) {
      return _fetch('POST', url, opts, data)
    }

    _.put = function (data) {
      return _fetch('PUT', url, opts, data)
    }

    _.patch = function (data) {
      return _fetch('PATCH', url, opts, data)
    }

    _.delete = function () {
      return _fetch('DELETE', url, opts)
    }

    return _
  }

  // Expose fetch so that other polyfills can be used
  // Bind fetch to window to avoid TypeError: Illegal invocation
  fetchival.fetch = typeof fetch !== 'undefined' ? fetch.bind(window) : null

  // Support CommonJS, AMD & browser
  if (typeof exports === 'object')
    module.exports = fetchival
  else if (typeof define === 'function' && define.amd)
    define(function() { return fetchival })
  else
    window.fetchival = fetchival

})(typeof window != 'undefined' ? window : undefined);