原生js实现ajax

function ajax(){ 
  var ajaxData = { 
    type:arguments[0].type || "GET", 
    url:arguments[0].url || "", 
    async:arguments[0].async || "true", 
    data:arguments[0].data || null, 
    dataType:arguments[0].dataType || "text", 
    contentType:arguments[0].contentType || "application/x-www-form-urlencoded", 
    beforeSend:arguments[0].beforeSend || function(){}, 
    success:arguments[0].success || function(){}, 
    error:arguments[0].error || function(){} 
  } 
  ajaxData.beforeSend() 
  var xhr = createxmlHttpRequest();  
  xhr.responseType=ajaxData.dataType; 
  xhr.open(ajaxData.type,ajaxData.url,ajaxData.async);  
  xhr.setRequestHeader("Content-Type",ajaxData.contentType);  
  xhr.send(convertData(ajaxData.data));  
  xhr.onreadystatechange = function() {  
    if (xhr.readyState == 4) {  
      if(xhr.status == 200){ 
        ajaxData.success(xhr.response) 
      }else{ 
        ajaxData.error() 
      }  
    } 
  }  
} 
  
function createxmlHttpRequest() {  
  if (window.ActiveXObject) {  
    return new ActiveXObject("Microsoft.XMLHTTP");  
  } else if (window.XMLHttpRequest) {  
    return new XMLHttpRequest();  
  }  
} 
  
function convertData(data){ 
  if( typeof data === 'object' ){ 
    var convertResult = "" ;  
    for(var c in data){  
      convertResult+= c + "=" + data[c] + "&";  
    }  
    convertResult=convertResult.substring(0,convertResult.length-1) 
    return convertResult; 
  }else{ 
    return data; 
  } 
}
用法：
ajax({ 
  type:"POST", 
  url:"ajax.php", 
  dataType:"json", 
  data:{"val1":"abc","val2":123,"val3":"456"}, 
  beforeSend:function(){ 
    //some js code 
  }, 
  success:function(msg){ 
    console.log(msg) 
  }, 
  error:function(){ 
    console.log("error") 
  } 
})