How to use JS code to remember account and password

<!-- 登陆表单 --><form method="post" autocomplete="off" onsubmit="return check()">
  <!-- 用户名输入 -->
  <input type="text"  name="username" id="username" required="required" >
  <!-- 密码输入,禁用自动填充 -->
  <input type="password" autocomplete="new-password" name="password" id="password" required="required">
  <!-- 是否记住密码复选框 -->
  <input type="checkbox" id="isRmbPwd" name="isRmbPwd" checked="checked">记住密码  <!-- 提交按钮 -->
  <input type="submit" value="提交"></form>

function check (){    //获取表单输入:用户名,密码,是否保存密码
    var username = document.getElementById("username").value.trim() ;    var password = document.getElementById("password").value.trim() ;    var isRmbPwd = document.getElementById("isRmbPwd").checked ;    
    //判断用户名,密码是否为空(全空格也算空)
    if ( username.length != 0 && password.length != 0 )
    {        //若复选框勾选,则添加Cookie,记录密码
        if ( isRmbPwd == true )
        {   
            setCookie ( "This is username", username, 7 ) ;
            setCookie ( username, password, 7 ) ;
        }        //否则清除Cookie
        else
        {
            delCookie ( "This is username" ) ;
            delCookie ( username ) ;
        }        return true ;
    }    //非法输入提示
    else
    {
        alert('请输入必填字段!!!')        return false ;
    }   
}

//将function函数赋值给onload对象window.onload = function (){    //从Cookie获取到用户名
    var username = getCookie("This is username") ;    //如果用户名为空,则给表单元素赋空值
    if ( username == "" )
    {        document.getElementById("username").value="" ;        document.getElementById("password").value="" ;        document.getElementById("isRmbPwd").checked=false ;
    }    //获取对应的密码,并把用户名,密码赋值给表单
    else
    {        var password = getCookie(username) ;    
        document.getElementById("username").value = username ;        document.getElementById("password").value = password ;        document.getElementById("isRmbPwd").checked = true ;
    }
}