fetch中的post报404，找不到路径

只是在vue的学习过程中，用fetch简单的访问一下，本地文件夹

methods:{

        async submitFn(){

            try{

                let form = this.$refs['form1'];

                let dataform = new FormData(form);

                let res = await fetch('./data/1.php',{

                    method:'post', 

                    mode: "cors",

                    headers: {

                        "Content-Type": "application/x-www-form-urlencoded"

                    },

                    body:'a=5&b=2'

                    

                });

                let data = await res.json();

               

                console.log(data);

            }catch(e){

 

            }

            

        }

    }

但是总是报错404，找不到路径

php文件内容是copy来的

<?PHP

echo $_POST['a']+$_POST['b'];

?>