fetchWithMiddleware.js

/* @flow */
/* eslint-disable no-param-reassign, prefer-const */

import { createRequestError } from './createRequestError';
import RelayResponse from './RelayResponse';
import type {
  Middleware,
  MiddlewareNextFn,
  RelayRequestAny,
  MiddlewareRaw,
  MiddlewareRawNextFn,
  FetchResponse,
} from './definition';

function runFetch(req: RelayRequestAny): Promise<FetchResponse> {
  let { url } = req.fetchOpts;
  if (!url) url = '/graphql';

  if (!req.fetchOpts.headers.Accept) req.fetchOpts.headers.Accept = '*/*';
  if (!req.fetchOpts.headers['Content-Type'] && !req.isFormData()) {
    req.fetchOpts.headers['Content-Type'] = 'application/json';
  }

  return fetch(url, (req.fetchOpts: any));
}

// convert fetch response to RelayResponse object
const convertResponse: (next: MiddlewareRawNextFn) => MiddlewareNextFn = (next) => async (req) => {
  const resFromFetch = await next(req);

  const res = await RelayResponse.createFromFetch(resFromFetch);
  if (res.status && res.status >= 400) {
    throw createRequestError(req, res);
  }
  return res;
};

export default function fetchWithMiddleware(
  req: RelayRequestAny,
  middlewares: Middleware[], // works with RelayResponse
  rawFetchMiddlewares: MiddlewareRaw[], // works with raw fetch response
  noThrow?: boolean
): Promise<RelayResponse> {
  // $FlowFixMe
  const wrappedFetch: MiddlewareNextFn = compose(
    ...middlewares,
    convertResponse,
    ...rawFetchMiddlewares
  )((runFetch: any));

  return wrappedFetch(req).then((res) => {
    if (!noThrow && (!res || res.errors || !res.data)) {
      throw createRequestError(req, res);
    }
    return res;
  });
}

/**
 * Composes single-argument functions from right to left. The rightmost
 * function can take multiple arguments as it provides the signature for
 * the resulting composite function.
 *
 * @param {...Function} funcs The functions to compose.
 * @returns {Function} A function obtained by composing the argument functions
 * from right to left. For example, compose(f, g, h) is identical to doing
 * (...args) => f(g(h(...args))).
 */
function compose(...funcs) {
  if (funcs.length === 0) {
    return (arg) => arg;
  } else {
    const last = funcs[funcs.length - 1];
    const rest = funcs.slice(0, -1);
    // $FlowFixMe - Suppress error about promise not being callable
    return (...args) => rest.reduceRight((composed, f) => f((composed: any)), last(...args));
  }
}