fetchJson.js

const API_URL = 'https://..';

// returns true for plain object literals, like {foo: 'bar'}, false for other cases, like instances of classes, like lodash.isPlainObject
const isPlainObject = obj => obj && Object.getPrototypeOf(obj) === Object.prototype || Object.getPrototypeOf(obj) === null;

export function fetchJson(url, { body, headers, ...o } = {}) {
  const isJson = isPlainObject(body); // most of the time we send plain 'json' objects
  return fetch(url[0] === '/' ? API_URL + url : url, {
      headers: {
        ...isJson && {'Content-Type': 'application/json'},
        ...headers
      },
      body: isJson ? JSON.stringify(body) : body,
      ...o
  }).then(async r => {
      const data = await r.json().catch(() => ({}));
    
      if (r.ok) return data;

      // your logic to extract error message:
      const err = new Error(data && data.message || data);
      err.status = r.status;
      throw err;
  });
}

// for axios fans
export default {
  get: (url, opts) => fetchJson(url, {method: 'GET', ...opts}),
  post: (url, body, opts) => fetchJson(url, {method: 'POST', body, ...opts}),
  put: (url, body, opts) => fetchJson(url, {method: 'PUT', body, ...opts}),
  delete: (url, opts) => fetchJson(url, {method: 'DELETE', ...opts}),
};