用原生JS判断用户输入的邮箱是否正确

JS操作：

var x = prompt("输入格式");
if(x.charAt(x.length-4) == "." && x.indexOf(".") == x.length-4 && x.lastIndexOf(".") == x.length-4){
if(x.indexOf("@") == x.length-7 || x.indexOf("@") == x.length-8 && x.lastIndexOf("@") == x.length-7 ||  x.lastIndexOf("@") == x.length-8){
alert("输入正确");
}else{
alert("输入错误");
}
}else{
alert("输入错误");
}