can'；使用Javascript无法获得ajax的答案

我有这个：

var something = makeRequest(url);

function makeRequest(url) {

  var http_request = false;

  if (window.XMLHttpRequest) {
    http_request = new XMLHttpRequest();
    if (http_request.overrideMimeType) {
      http_request.overrideMimeType('text/xml');
    }
  } else if (window.ActiveXObject) {
    try {
      http_request = new ActiveXObject("Msxml2.XMLHTTP");
    } catch (e) {
      try {
        http_request = new ActiveXObject("Microsoft.XMLHTTP");
      } catch (e) {}
    }
  }

  if (!http_request) {
    console.log('Falla :( No es posible crear una instancia XMLHTTP');
    return false;
  }

  http_request.onreadystatechange = alertContents;
  http_request.open('GET', url, true);
  http_request.send(null);


  function alertContents() {

    if (http_request.readyState == 4) {
      if (http_request.status == 200) {
        //console.log(http_request.responseText); //this show a string result
        //return http_request.responseText; //this not return nothing
        //var something = http_request.responseText; // this return a undefined variable
      } else {
        console.log('Hubo problemas con la petición.');
      }
    }
  }
}

看看“if（http_request.status==200）”下面我证明的三件事，什么都没有

我不知道如何从函数中得到调用的结果。请帮助我，您的alertContents（）函数是异步执行的。不能从函数返回值。您可以声明一个全局变量，并将reponseText值赋给它，然后对其进行处理

var result ;
function alertContents() {
    if (http_request.readyState == 4) {
        if (http_request.status == 200) {
            result = http_request.responseText ;
            process(result) ; // guaranteed that there will be some response
        }
    }
}

process(result); // Does not guarantee that result will have the 
                 // returned value (response) and may result in failure

看到和