Simple fetch API wrapper

fetch-api-wrapper.js

(function(global) {
    'use strict';

    var JSON_TYPE_NAME = 'type';
    
    function getQueryString(queryParams) {
                var params = Object.keys(queryParams).map(function(paramName) {
            return [paramName, encodeURIComponent(queryParams[paramName])].join('=');
        });
        return '?' + params.join('&');
    }

    function checkStatus(response) {
        if (response.status >= 200 && response.status < 300 || response.status === 304) {
            return Promise.resolve(response);
        } else {
            var error = new Error(response.statusText || response.status);
            error.response = response;
            return Promise.reject(error);
        }
    }

    function setHeaders(headers) {
        headers = headers || {};
        headers['X-Requested-With'] = 'XMLHttpRequest';
        return headers;
    }

    function setCredentials(options) {
        options = options || {};

        if (!options.credentials) {
            options.credentials = 'same-origin';
        }

        return options;
    }

    function toJson(response) {
        return response.json();
    }

    function toText(response) {
        return response.text();
    }

    function fetchMain(method, url, data, options, type) {
        var responseAs = toText;

        options = options || {};
        type = type || JSON_TYPE_NAME;

        options.method = method;
        options.headers = setHeaders(options.headers);

        if (type === JSON_TYPE_NAME) {
            responseAs = toJson;
            options.headers.Accept = 'application/json';
        }

        if (method === 'get') {
            url += getQueryString(data);
        } else {
            options.body = data;
        }

        return global.fetch(url, setCredentials(options)).then(checkStatus).then(responseAs);
    }

    function getFetch(url, data, options, type) {
        return fetchMain('get',  url, data, options, type);
    }

    function postFetch(url, data, options, type) {
        return fetchMain('post', url, data, options, type);
    }

    global.fetchWrapper = {
        get: getFetch,
        post: postFetch
    };
}(window));