AJAX

Async JavaScript And XML

关键词：异步、JS、XML

实质：用JS发请求和收响应

四个步骤

1. 创建 HttpRequest 对象（全称是 XMLHttpRequest）
2. 调用对象的 open 方法
3. 监听对象的 onreadystatechange 事件
4. 调用对象的 send 方法（发送请求）

代码示例：

getCSS.onclick = () => {
  const request = new XMLHttpRequest(); //创建一个新对象
  request.open("GET", "/style.css"); //调用open方法
  request.onreadystatechange = () => {
    if (request.readyState === 4) {
      if (request.status >= 200 && request.status < 300) {
        const style = document.createElement("style"); //创建style标签
        style.innerHTML = request.response; //填写标签内容
        document.head.appendChild(style); //插入head
      } else {
        alert("加载CSS失败");
      }
    }
  };
  request.send();
};

getJS.onclick = () => {
  const request = new XMLHttpRequest();
  request.open("GET", "/2.js");
  request.onreadystatechange = () => {
    if (request.readyState === 4) {
      if (request.status >= 200 && request.status < 300) {
        const script = document.createElement("script");
        script.innerHTML = request.response;
        document.body.appendChild(script);
      } else {
        alert("加载JS失败");
      }
    }
  };
  request.send();
};

getHTML.onclick = () => {
  const request = new XMLHttpRequest();
  request.open("GET", "/3.html");
  request.onreadystatechange = () => {
    if (request.readyState === 4) {
      if (request.status >= 200 && request.status < 300) {
        const div = document.createElement("div");
        div.innerHTML = request.response;
        document.body.appendChild(div);
      } else {
        alert("加载HTML失败");
      }
    }
  };
  request.send();
};

getXML.onclick = () => {
  const request = new XMLHttpRequest();
  request.open("GET", "/4.xml");
  request.onreadystatechange = () => {
    if (request.readyState === 4) {
      if (request.status >= 200 && request.status < 300) {
        const dom = request.responseXML;
        const text = dom.getElementsByTagName("warning")[0].textContent;
        console.log(text.trim());
      } else {
        alert("加载XML失败");
      }
    }
  };
  request.send();
};

getJSON.onclick = () => {
  const request = new XMLHttpRequest();
  request.open("GET", "/5.json");
  request.onreadystatechange = () => {
    if (request.readyState === 4) {
      if (request.status >= 200 && request.status < 300) {
        const object = JSON.parse(request.response);
        myName.textContent = object.name;
        console.log(object);
      } else {
        alert("加载JSON失败");
      }
    }
  };
  request.send();
};


//加载分页
let n = 1;
getPage.onclick = () => {
  const request = new XMLHttpRequest();
  request.open("GET", `/page${n + 1}`);
  request.onreadystatechange = () => {
    if (request.readyState === 4) {
      if (request.status >= 200 && request.status < 300) {
        const array = JSON.parse(request.response);
        array.forEach((item) => {
          const li = document.createElement("li");
          li.textContent = item.id;
          xxx.appendChild(li);
        });
        n += 1;
      } else {
        alert("加载下一页失败");
      }
    }
  };
  request.send();
};