ajax promise

ajax.js

function ajax(url, {
  method = 'GET',
  headers = {},
  body = null,
}) {
  return new Promise((resolve, reject) => {
    const xhr = new XMLHttpRequest();

    xhr.open(method, url, true);

    Object.keys(headers).forEach(hn => {
      xhr.setRequestHeader(hn, headers[hn]);
    });

    const cbInner = () => {
      if (xhr.readyState === 4) {
        if (xhr.status >= 200 && xhr.status < 300) {
          const respHeaders = xhr.getAllResponseHeaders().trim().split('\n').reduce((obj, line) => {
            const [key, ...value] = line.split(': ');
            return {
              ...obj,
              [key]: value.join(': '),
            };
          }, {});

          resolve({
            response: xhr.response,
            headers: respHeaders,
          });
        } else {
          reject(xhr);
        }
        return;
      }
    };

    xhr.onload = cbInner;
    xhr.onerror = cbInner;

    xhr.send(body);
  });
}