A window.fetch JavaScript polyfill by GitHub.

/**
 * Fetch Polyfill v3.0.0
 * https://github.com/github/fetch
 * (c) GitHub, Inc., MIT License
 */
var support = {
  searchParams: "URLSearchParams" in self,
  iterable: "Symbol" in self && "iterator" in Symbol,
  blob:
    "FileReader" in self &&
    "Blob" in self &&
    (function() {
      try {
        new Blob();
        return true;
      } catch (e) {
        return false;
      }
    })(),
  formData: "FormData" in self,
  arrayBuffer: "ArrayBuffer" in self
};

function isDataView(obj) {
  return obj && DataView.prototype.isPrototypeOf(obj);
}

if (support.arrayBuffer) {
  var viewClasses = [
    "[object Int8Array]",
    "[object Uint8Array]",
    "[object Uint8ClampedArray]",
    "[object Int16Array]",
    "[object Uint16Array]",
    "[object Int32Array]",
    "[object Uint32Array]",
    "[object Float32Array]",
    "[object Float64Array]"
  ];

  var isArrayBufferView =
    ArrayBuffer.isView ||
    function(obj) {
      return (
        obj && viewClasses.indexOf(Object.prototype.toString.call(obj)) > -1
      );
    };
}

function normalizeName(name) {
  if (typeof name !== "string") {
    name = String(name);
  }
  if (/[^a-z0-9\-#$%&'*+.^_`|~]/i.test(name)) {
    throw new TypeError("Invalid character in header field name");
  }
  return name.toLowerCase();
}

function normalizeValue(value) {
  if (typeof value !== "string") {
    value = String(value);
  }
  return value;
}

// Build a destructive iterator for the value list
function iteratorFor(items) {
  var iterator = {
    next: function() {
      var value = items.shift();
      return { done: value === undefined, value: value };
    }
  };

  if (support.iterable) {
    iterator[Symbol.iterator] = function() {
      return iterator;
    };
  }

  return iterator;
}

export function Headers(headers) {
  this.map = {};

  if (headers instanceof Headers) {
    headers.forEach(function(value, name) {
      this.append(name, value);
    }, this);
  } else if (Array.isArray(headers)) {
    headers.forEach(function(header) {
      this.append(header[0], header[1]);
    }, this);
  } else if (headers) {
    Object.getOwnPropertyNames(headers).forEach(function(name) {
      this.append(name, headers[name]);
    }, this);
  }
}

Headers.prototype.append = function(name, value) {
  name = normalizeName(name);
  value = normalizeValue(value);
  var oldValue = this.map[name];
  this.map[name] = oldValue ? oldValue + ", " + value : value;
};

Headers.prototype["delete"] = function(name) {
  delete this.map[normalizeName(name)];
};

Headers.prototype.get = function(name) {
  name = normalizeName(name);
  return this.has(name) ? this.map[name] : null;
};

Headers.prototype.has = function(name) {
  return this.map.hasOwnProperty(normalizeName(name));
};

Headers.prototype.set = function(name, value) {
  this.map[normalizeName(name)] = normalizeValue(value);
};

Headers.prototype.forEach = function(callback, thisArg) {
  for (var name in this.map) {
    if (this.map.hasOwnProperty(name)) {
      callback.call(thisArg, this.map[name], name, this);
    }
  }
};

Headers.prototype.keys = function() {
  var items = [];
  this.forEach(function(value, name) {
    items.push(name);
  });
  return iteratorFor(items);
};

Headers.prototype.values = function() {
  var items = [];
  this.forEach(function(value) {
    items.push(value);
  });
  return iteratorFor(items);
};

Headers.prototype.entries = function() {
  var items = [];
  this.forEach(function(value, name) {
    items.push([name, value]);
  });
  return iteratorFor(items);
};

if (support.iterable) {
  Headers.prototype[Symbol.iterator] = Headers.prototype.entries;
}

function consumed(body) {
  if (body.bodyUsed) {
    return Promise.reject(new TypeError("Already read"));
  }
  body.bodyUsed = true;
}

function fileReaderReady(reader) {
  return new Promise(function(resolve, reject) {
    reader.onload = function() {
      resolve(reader.result);
    };
    reader.onerror = function() {
      reject(reader.error);
    };
  });
}

function readBlobAsArrayBuffer(blob) {
  var reader = new FileReader();
  var promise = fileReaderReady(reader);
  reader.readAsArrayBuffer(blob);
  return promise;
}

function readBlobAsText(blob) {
  var reader = new FileReader();
  var promise = fileReaderReady(reader);
  reader.readAsText(blob);
  return promise;
}

function readArrayBufferAsText(buf) {
  var view = new Uint8Array(buf);
  var chars = new Array(view.length);

  for (var i = 0; i < view.length; i++) {
    chars[i] = String.fromCharCode(view[i]);
  }
  return chars.join("");
}

function bufferClone(buf) {
  if (buf.slice) {
    return buf.slice(0);
  } else {
    var view = new Uint8Array(buf.byteLength);
    view.set(new Uint8Array(buf));
    return view.buffer;
  }
}

function Body() {
  this.bodyUsed = false;

  this._initBody = function(body) {
    this._bodyInit = body;
    if (!body) {
      this._bodyText = "";
    } else if (typeof body === "string") {
      this._bodyText = body;
    } else if (support.blob && Blob.prototype.isPrototypeOf(body)) {
      this._bodyBlob = body;
    } else if (support.formData && FormData.prototype.isPrototypeOf(body)) {
      this._bodyFormData = body;
    } else if (
      support.searchParams &&
      URLSearchParams.prototype.isPrototypeOf(body)
    ) {
      this._bodyText = body.toString();
    } else if (support.arrayBuffer && support.blob && isDataView(body)) {
      this._bodyArrayBuffer = bufferClone(body.buffer);
      // IE 10-11 can't handle a DataView body.
      this._bodyInit = new Blob([this._bodyArrayBuffer]);
    } else if (
      support.arrayBuffer &&
      (ArrayBuffer.prototype.isPrototypeOf(body) || isArrayBufferView(body))
    ) {
      this._bodyArrayBuffer = bufferClone(body);
    } else {
      this._bodyText = body = Object.prototype.toString.call(body);
    }

    if (!this.headers.get("content-type")) {
      if (typeof body === "string") {
        this.headers.set("content-type", "text/plain;charset=UTF-8");
      } else if (this._bodyBlob && this._bodyBlob.type) {
        this.headers.set("content-type", this._bodyBlob.type);
      } else if (
        support.searchParams &&
        URLSearchParams.prototype.isPrototypeOf(body)
      ) {
        this.headers.set(
          "content-type",
          "application/x-www-form-urlencoded;charset=UTF-8"
        );
      }
    }
  };

  if (support.blob) {
    this.blob = function() {
      var rejected = consumed(this);
      if (rejected) {
        return rejected;
      }

      if (this._bodyBlob) {
        return Promise.resolve(this._bodyBlob);
      } else if (this._bodyArrayBuffer) {
        return Promise.resolve(new Blob([this._bodyArrayBuffer]));
      } else if (this._bodyFormData) {
        throw new Error("could not read FormData body as blob");
      } else {
        return Promise.resolve(new Blob([this._bodyText]));
      }
    };

    this.arrayBuffer = function() {
      if (this._bodyArrayBuffer) {
        return consumed(this) || Promise.resolve(this._bodyArrayBuffer);
      } else {
        return this.blob().then(readBlobAsArrayBuffer);
      }
    };
  }

  this.text = function() {
    var rejected = consumed(this);
    if (rejected) {
      return rejected;
    }

    if (this._bodyBlob) {
      return readBlobAsText(this._bodyBlob);
    } else if (this._bodyArrayBuffer) {
      return Promise.resolve(readArrayBufferAsText(this._bodyArrayBuffer));
    } else if (this._bodyFormData) {
      throw new Error("could not read FormData body as text");
    } else {
      return Promise.resolve(this._bodyText);
    }
  };

  if (support.formData) {
    this.formData = function() {
      return this.text().then(decode);
    };
  }

  this.json = function() {
    return this.text().then(JSON.parse);
  };

  return this;
}

// HTTP methods whose capitalization should be normalized
var methods = ["DELETE", "GET", "HEAD", "OPTIONS", "POST", "PUT"];

function normalizeMethod(method) {
  var upcased = method.toUpperCase();
  return methods.indexOf(upcased) > -1 ? upcased : method;
}

export function Request(input, options) {
  options = options || {};
  var body = options.body;

  if (input instanceof Request) {
    if (input.bodyUsed) {
      throw new TypeError("Already read");
    }
    this.url = input.url;
    this.credentials = input.credentials;
    if (!options.headers) {
      this.headers = new Headers(input.headers);
    }
    this.method = input.method;
    this.mode = input.mode;
    this.signal = input.signal;
    if (!body && input._bodyInit != null) {
      body = input._bodyInit;
      input.bodyUsed = true;
    }
  } else {
    this.url = String(input);
  }

  this.credentials = options.credentials || this.credentials || "same-origin";
  if (options.headers || !this.headers) {
    this.headers = new Headers(options.headers);
  }
  this.method = normalizeMethod(options.method || this.method || "GET");
  this.mode = options.mode || this.mode || null;
  this.signal = options.signal || this.signal;
  this.referrer = null;

  if ((this.method === "GET" || this.method === "HEAD") && body) {
    throw new TypeError("Body not allowed for GET or HEAD requests");
  }
  this._initBody(body);
}

Request.prototype.clone = function() {
  return new Request(this, { body: this._bodyInit });
};

function decode(body) {
  var form = new FormData();
  body
    .trim()
    .split("&")
    .forEach(function(bytes) {
      if (bytes) {
        var split = bytes.split("=");
        var name = split.shift().replace(/\+/g, " ");
        var value = split.join("=").replace(/\+/g, " ");
        form.append(decodeURIComponent(name), decodeURIComponent(value));
      }
    });
  return form;
}

function parseHeaders(rawHeaders) {
  var headers = new Headers();
  // Replace instances of \r\n and \n followed by at least one space or horizontal tab with a space
  // https://tools.ietf.org/html/rfc7230#section-3.2
  var preProcessedHeaders = rawHeaders.replace(/\r?\n[\t ]+/g, " ");
  preProcessedHeaders.split(/\r?\n/).forEach(function(line) {
    var parts = line.split(":");
    var key = parts.shift().trim();
    if (key) {
      var value = parts.join(":").trim();
      headers.append(key, value);
    }
  });
  return headers;
}

Body.call(Request.prototype);

export function Response(bodyInit, options) {
  if (!options) {
    options = {};
  }

  this.type = "default";
  this.status = options.status === undefined ? 200 : options.status;
  this.ok = this.status >= 200 && this.status < 300;
  this.statusText = "statusText" in options ? options.statusText : "OK";
  this.headers = new Headers(options.headers);
  this.url = options.url || "";
  this._initBody(bodyInit);
}

Body.call(Response.prototype);

Response.prototype.clone = function() {
  return new Response(this._bodyInit, {
    status: this.status,
    statusText: this.statusText,
    headers: new Headers(this.headers),
    url: this.url
  });
};

Response.error = function() {
  var response = new Response(null, { status: 0, statusText: "" });
  response.type = "error";
  return response;
};

var redirectStatuses = [301, 302, 303, 307, 308];

Response.redirect = function(url, status) {
  if (redirectStatuses.indexOf(status) === -1) {
    throw new RangeError("Invalid status code");
  }

  return new Response(null, { status: status, headers: { location: url } });
};

export var DOMException = self.DOMException;
try {
  new DOMException();
} catch (err) {
  DOMException = function(message, name) {
    this.message = message;
    this.name = name;
    var error = Error(message);
    this.stack = error.stack;
  };
  DOMException.prototype = Object.create(Error.prototype);
  DOMException.prototype.constructor = DOMException;
}

export function fetch(input, init) {
  return new Promise(function(resolve, reject) {
    var request = new Request(input, init);

    if (request.signal && request.signal.aborted) {
      return reject(new DOMException("Aborted", "AbortError"));
    }

    var xhr = new XMLHttpRequest();

    function abortXhr() {
      xhr.abort();
    }

    xhr.onload = function() {
      var options = {
        status: xhr.status,
        statusText: xhr.statusText,
        headers: parseHeaders(xhr.getAllResponseHeaders() || "")
      };
      options.url =
        "responseURL" in xhr
          ? xhr.responseURL
          : options.headers.get("X-Request-URL");
      var body = "response" in xhr ? xhr.response : xhr.responseText;
      resolve(new Response(body, options));
    };

    xhr.onerror = function() {
      reject(new TypeError("Network request failed"));
    };

    xhr.ontimeout = function() {
      reject(new TypeError("Network request failed"));
    };

    xhr.onabort = function() {
      reject(new DOMException("Aborted", "AbortError"));
    };

    xhr.open(request.method, request.url, true);

    if (request.credentials === "include") {
      xhr.withCredentials = true;
    } else if (request.credentials === "omit") {
      xhr.withCredentials = false;
    }

    if ("responseType" in xhr && support.blob) {
      xhr.responseType = "blob";
    }

    request.headers.forEach(function(value, name) {
      xhr.setRequestHeader(name, value);
    });

    if (request.signal) {
      request.signal.addEventListener("abort", abortXhr);

      xhr.onreadystatechange = function() {
        // DONE (success or failure)
        if (xhr.readyState === 4) {
          request.signal.removeEventListener("abort", abortXhr);
        }
      };
    }

    xhr.send(
      typeof request._bodyInit === "undefined" ? null : request._bodyInit
    );
  });
}

fetch.polyfill = true;

if (!self.fetch) {
  self.fetch = fetch;
  self.Headers = Headers;
  self.Request = Request;
  self.Response = Response;
}