Ajax基本原理

function ajax (method, url, data, success) {

var xhr = null ;

if ( window . XMLHttpRequest ){

xhr = new XMLHttpRequest ();

} else {

xhr = new ActiveXObject ( "Microsoft.XMLHTTP" );

}

if (method == "get" && data) {

url += "?" + data;

}

xhr . open (method, url, true );

if (method == "get" ) {

xhr . send ();

} else {

xhr . setRequestHeader ( "content-type" , "application/application/x-www-form-urlencoded" );

xhr . send (data);

}

xhr . onreadystatechange = function () {

if ( xhr . readyState == 4 ) {

if ( xhr . status == 200 ){

success && success ( xhr . responseText );

} else {

}

}

}

}

首先创建

function ajax(method, url, data, success) {
  var xhr = null;
  if(window.XMLHttpRequest){
    xhr = new XMLHttpRequest();
  }else{
    xhr = new ActiveXObject("Microsoft.XMLHTTP");
  }

  if(method == "get" &&data) {
    url += "?" + data;
  }
  xhr.open(method, url,true);

  if(method == "get") {
    xhr.send();
  }else {
    xhr.setRequestHeader("content-type", "application/application/x-www-form-urlencoded");
    xhr.send(data);
  }
  
  xhr.onreadystatechange = function() {
    if(xhr.readyState == 4) {
      if(xhr.status == 200){
        success && success(xhr.responseText);
      }else {

      }
    }
  }
}