ajax封装：get封装方式、post封装方式

function getRequest(url, callback) {
  const xhr = new XMLHttpRequest();
  xhr.open('GET', url, true);
  xhr.onreadystatechange = function() {
    if (xhr.readyState === 4 && xhr.status === 200) {
      callback(xhr.responseText);
    }
  };
  xhr.send();
}

function postRequest(url, data, callback) {
  const xhr = new XMLHttpRequest();
  xhr.open('POST', url, true);
  xhr.setRequestHeader('Content-Type', 'application/x-www-form-urlencoded');
  xhr.onreadystatechange = function() {
    if (xhr.readyState === 4 && xhr.status === 200) {
      callback(xhr.responseText);
    }
  };
  xhr.send(data);
}

function getRequest(url) {
  return fetch(url).then(response => response.text());
}

function postRequest(url, data) {
  return fetch(url, {
    method: 'POST',
    headers: {
      'Content-Type': 'application/x-www-form-urlencoded'
    },
    body: data
  }).then(response => response.text());
}