用原生JavaScript进行表单验证

代码如下：

    <script>
        let username = document.querySelector("[name=username]");
        let password = document.querySelector("[name=password]");
        let password_again = document.querySelector("[name=password_again]");
        let submit = document.querySelector("#submit");
        let message = document.querySelector("#message");
        document.querySelector("#register-form").addEventListener("submit", (e)=>{
            let event = e || window.event;
            if(username.value == "catfish1921"){
                message.innerHTML = "不好意思，这是我的专用名";
                event.preventDefault();
            }
            if(password.value != password_again.value){
                message.innerHTML = "两次密码请输入相同数值";
                event.preventDefault();
            }
            if(username.value.length < 3 || username.value.length > 10){
                message.innerHTML = "用户名保证在3-10位";
                event.preventDefault();
            }
        })
    </script>