A little convenience for many `fetch()` use cases

quick-fetchy.js

"use strict";

const qfApi = {
  form: (src) => {
    let form = new FormData()
    for(let name in src) {
      form.append(name, src[name])
    }
    return form
  },
  params: (src) => {
    let params = new URLSearchParams()
    for(let name in src) {
      params.append(name, src[name])
    }
    return params
  },
  headers: (res) => {
    let headObj = {}
    for(let [name, val] of res.headers) {
      headObj[name] = val
    }
    return headObj;
  },
  readersFull: {
    'application/json': 'json',
  },
  readersStart: {
    'text': 'text',
    'image': 'blob',
  },
  mimeReader: (res, mime) => {
    const starting = mime.split('/')[0]
    return (res[qfApi.readersFull[mime]] && res[qfApi.readersFull[mime]] ||
            res[qfApi.readersStart[starting]] &&
            res[qfApi.readersStart[starting]] || res.text)
  },
  process: (res) => {
    const mime = qfApi.headers(res)['content-type'];
    if(res.ok && mime) {
      return qfApi.mimeReader(res, mime).call(res);
    } else if(res.ok) {
      return res.text();
    } else {
      throw new Error(`Fetch of ${url} failed with ${res.status}`);
    }
  },
  post: (url, json, form) => {
    return fetch(url, {method: 'POST', credentials: 'same-origin',
                  body: form && qfApi.form(form) || JSON.stringify(json)
    }).then(qfApi.process);
  },
  get: (url, params) => (
    fetch(`${url}?${qfApi.params(params)}`,
          {credentials:'same-origin'}).then(qfApi.process)
  )
}

// Ad-hoc test cases

qfApi.post('http://httpbin.org/post', {foo: 'bar'})
    .then ((data) => {
      console.log('JSON echo', data.json);
    })
    .catch((failure) => {
      console.log('FAIL', failure);
    });

qfApi.post('http://httpbin.org/post', false, {foo: 'bar'})
    .then ((data => {
      console.log('FORM echo', data.form);
    }))

qfApi.get('http://httpbin.org/get', {foo: 'bar'})
    .then ((data) => {
      console.log('GET JSON Echo', data.args);
    });

qfApi.get('http://httpbin.org/html')
    .then ((html) => {
      console.log('HTML', html);
    });