Rest.js

export default class RestJson {
    constructor(endpoint, options = {}) {
        if (!window.fetch) {
            console.error("Sorry you cannot use RestJson Class, your browser doesn't support Fetch API.");
            console.error("Please try with another browser or update it to a new version.");
        }

        const defaultOptions = {
            headers: {'Content-Type': 'application/json'}
        };

        this.endpoint = endpoint;
        this.options = Object.assign(defaultOptions, options);
        this.queryString = '';
        this.id = '';
    }

    withParams(params) {
        if (Object.keys(params).length) {
            const query = new URLSearchParams();

            Object.entries(params).forEach(param => query.set(param[0], param[1]));

            this.queryString = `?${query.toString()}`;
        }

        return this;
    }

    withId(id) {
        if (typeof id === 'number') {
            this.id = `/${id}`;
        }

        return this;
    }

    async baseFetch(method, payload = null) {
        if (payload) {
            this.options.body = JSON.stringify(payload);
        }

        this.options.method = method;

        let response = await fetch(
            this.endpoint + this.id + this.queryString,
            this.options
        );

        if (!response.ok) {
            throw new Error(response.status);
        }

        this.options.body = null;
        this.queryString = '';
        this.id = '';

        if (response.status === 204) {
            return response;
        }

        return response.json();
    }

    get() {
        return this.baseFetch('GET');
    }

    post(data) {
        return this.baseFetch('POST', data);
    }

    put(data) {
        return this.baseFetch('PUT', data);
    }

    patch(data) {
        return this.baseFetch('PATCH', data);
    }

    delete() {
        return this.baseFetch('DELETE');
    }
}

How to use it

let restJson = new RestJson('https://rest.api.com/endpoint');

// GET
restJson
  .get()
  .then(response => {
    // do something...
  });

// POST
restJson
  .post('id') // <== fill in with item id
  .then(transaction => {
    // do something...
    }
  });