手写Ajax

代码如下：

const ajax = (method, url, data, success, fail) => {
    var request = new XMLHttpRequest();
    request.open(method, url); //这里默认用异步请求
    request.onreadystatechange = function () {
        if (request.readyState === 4) {
            if ((request.status >= 200 && request.status < 300) || request.status === 304) {
                success(request);
            } else {
                fail(request);
            }
        }
    };
    request.send();
};

补充一个Promise版本的：

const getJSON = function (url) {
  return new Promise((resolve, reject) => {
    const xhr = new XMLHttpRequest();
    xhr.open("GET", url, false);
    xhr.setRequestHeader("Content-Type", "application/json");
    xhr.onreadystatechange = function () {
      if (xhr.readyState !== 4) return;
      if (xhr.status === 200 || xhr.status === 304) {
        resolve(xhr.responseText);
      } else {
        reject(new Error(xhr.responseText));
      }
    };
    xhr.send();
  });
};