convert Image url to Base64 with use fetch

fetch-b64.js

const fetch = require('node-fetch');

function btob64(buff) {
  let binary = '';
  const bytes = (new Array()).slice.call(new Uint8Array(buff));

  for(let i = 0; i < bytes.length; ++i){
    binary = binary + String.fromCharCode(bytes[i]);
  }

  return Buffer.from(binary.toString(), 'binary').toString('base64');
}

function get_type(url) {
  const parts = url.split('.');
  const last_one = parts[parts.length - 1];

  if( last_one === 'png'  ||
      last_one === 'jpg'  ||
      last_one === 'jpeg' ||
      last_one === 'gif'  ||
      last_one === 'bmp' ){
    return last_one;
  } else {
    return '';
  }
}

function urltob64(url) {
  return new Promise((resolve, rejected) => {
    const type = get_type(url);
    if(type === '') rejected(false);

    return fetch(url)
      .then((res) => res.arrayBuffer())
      .then((buf) => {
        const b64flag = `data:image/${type};base64,`;
        const imgStr = btob64(buf);
        if(imgStr) {
          resolve(`${b64flag}${imgStr}`);
        } else {
          rejected(false);
        }
      });
  });
}

urltob64(
  'https://user-images.githubusercontent.com'+
  '/18028768/27010483-064cee7c-4ed8-11e7-8c5'+
  '7-fac705fc0c50.png'
)
.then(console.log)
.catch(console.log);