SOURCE

$.ajaxDownload=function(opt){
	var defaults = {
			url:'',
			type:'GET',
			parameters:null,
			onloadstart:null,
			ontimeout:null,
			onerror:null,
			onloadend:null
		}
	var settings = $.extend(true,{}, defaults, opt);
	var xhr = new XMLHttpRequest();
	xhr.open(settings.type, settings.url, true);
	xhr.responseType = "blob";  // 返回类型blob
	if(typeof FormData == "undefined") {
	    xhr.setRequestHeader("Content-Type", "application/x-www-form-urlencoded");
	}
	if(Object.prototype.toString.call(settings.onloadstart)==="[object Function]"){
		xhr.onloadstart=settings.onloadstart;
	}
	if(Object.prototype.toString.call(settings.onerror)==="[object Function]"){
		xhr.onerror=settings.onerror;
	}
	if(Object.prototype.toString.call(settings.onloadend)==="[object Function]"){
		xhr.onloadend=settings.onloadend;
	}
	xhr.onload = function () {
	    // 请求完成
	    if (this.status === 200) {
	      // 返回200
	      var blob = this.response;
	      var reader = new FileReader();
	      reader.readAsDataURL(blob);  // 转换为base64，可以直接放入a表情href
	      reader.onload = function (e) {
	        // 转换完成，创建一个a标签用于下载
	        var a = document.createElement('a');
	        a.download = settings.parameters.fileName;
	        a.href = e.target.result;
	        $("body").append(a);  // 修复firefox中无法触发click
	        a.click();
	        $(a).remove();
	      }
	    }
	 };
	xhr.send(postDataFormat(settings.parameters))// 发送ajax请求
	 function postDataFormat(obj){
	      if(typeof obj != "object" ) {
	         alert("输入的参数必须是对象");
	         return;
	     }
	      // 支持有FormData的浏览器（Firefox 4+ , Safari 5+, Chrome和Android 3+版的Webkit）
	      if(typeof FormData == "function") {
	          var data = new FormData();
	         for(var attr in obj) {
	             data.append(attr,obj[attr]);
	          }
	          return data;
	      }else {
	          // 不支持FormData的浏览器的处理 
	          var arr = new Array();
	          var i = 0;
	          for(var attr in obj) {
	              arr[i] = encodeURIComponent(attr) + "=" + encodeURIComponent(obj[attr]);
	              i++;
	          }
	          return arr.join("&");
	      }
	  }
}