跨域请求

后端设置返回头

res.setHeader("Access-Control-Allow-Origin", "*");
res.setHeader(
  "Access-Control-Allow-Headers",
  "Content-type,Content-Length,Authorization,Accept,X-Requested-Width"
);
res.setHeader("Access-Control-Allow-Methods", "PUT,POST,GET,DELETE,OPTIONS");

示例

function crossDomain(url, fn) {
  iframe = document.createElement("iframe");
  iframe.style.display = "none";
  var state = 0;

  iframe.onload = function () {
    if (state === 1) {
      fn(iframe.contentWindow.name);
      iframe.contentWindow.document.write("");
      iframe.contentWindow.close();
      document.body.removeChild(iframe);
    } else if (state === 0) {
      state = 1;
      iframe.contentWindow.location = "www.baidu.com/proxy.html";
    }
  };

  iframe.src = url;
  document.body.appendChild(iframe);
}

// 调用
// 服务器地址
var url = "www.baidu.com/data.php";
crossDomain(url, function (data) {
  // 处理数据 data就是window.name的值(string)
  var data = JSON.parse(iframe.contentWindow.name);
  console.log(data);
});

window.onload = function () {
  var xhr = new XMLHttpRequest();
  xhr.open("get", "https://a1.cnblogs.com/units/image/C2/creative");
  xhr.send();
  xhr.onreadystatechange = function () {
    console.log(xhr.response);
  };

  var proxy = function (url, callback) {
    var state = 0;
    var iframe = document.createElement("iframe");
    var img = document.createElement("span");
    document.body.appendChild(img);
    // 加载跨域页面
    iframe.src = url;

    // onload事件会触发2次，第1次加载跨域页，并留存数据于window.name
    iframe.onload = function () {
      console.log(1);
    };
    document.body.appendChild(iframe);

    // 获取数据以后销毁这个iframe，释放内存；这也保证了安全（不被其他域frame js访问）
    function destoryFrame() {
      iframe.contentWindow.document.write("");
      iframe.contentWindow.close();
      document.body.removeChild(iframe);
    }
  };

  // 请求跨域b页面数据
  proxy("https://a1.cnblogs.com", function (data) {
    alert(data);
  });
};