Question

我对es6 let和const之间的差异非常熟悉。我不太确定的是在for..of循环的定义范围内。

我知道一个＆＃34;老派＆＃34; for循环签名不接受const 即

for (let i = 0; i < 100; i++) ... // works in chrome

for (const i = 0; i < 100; i++) ... // does not work in chrome (reassignment error)

然而，使用for..of循环，它似乎没有什么区别

const someArray = ['hello', 'hey'];

for (let elem of someArray) ... // works
for (const elem of someArray) ... // also works?

那么这里发生了什么？

为什么const允许for..of而不是旧学校for循环？

循环中的最终结果差异是什么？（除了可能在循环中重新分配elem，我不会期望很多人做的事情）

Answer 1

只重新分配一个变量，因此错误

if (!IsPostBack)
{
    DataTable dt = new DataTable();
    DataRow dr;

    dt.Columns.Add("sno");
    dt.Columns.Add("itemname");
    dt.Columns.Add("quantity");
    dt.Columns.Add("price");
    dt.Columns.Add("totalprice");
    dt.Columns.Add("image");

    if (Request.QueryString["itemname"] != null)
    {
        if (Session["Buyitems"] == null)
        {
            dr = dt.NewRow();

            SqlConnection scon = new SqlConnection(ConfigurationManager.ConnectionStrings["online food orderingConnectionString"].ConnectionString);
            scon.Open();

            String myquery = "select * from food_items where item_name=@items_name";

            SqlCommand cmd = new SqlCommand(myquery, scon);

            cmd.Parameters.AddWithValue("items_name", Request.QueryString["itemname"].ToString());

            SqlDataAdapter da = new SqlDataAdapter(cmd);
            DataSet ds = new DataSet();
            da.Fill(ds);

            dr["sno"] = 1;

            if (ds.Tables[0].Rows.Count > 0)
            {
                dr["itemname"] = ds.Tables[0].Rows[0]["item_name"].ToString();
                dr["image"] = ds.Tables[0].Rows[0]["image"].ToString();
                dr["price"] = ds.Tables[0].Rows[0]["price"].ToString();
                int price = Convert.ToInt16(ds.Tables[0].Rows[0]["price"].ToString());
                int quantity = Convert.ToInt16(Request.QueryString["quantity"].ToString());
                int totalprice = price * quantity; 
                dr["quantity"] = Request.QueryString["quantity"];

                dr["totalprice"] = totalprice;

                SaveCartDetail(ds.Tables[0].Rows[0]["item_name"].ToString(), Request.QueryString["quantity"], ds.Tables[0].Rows[0]["price"].ToString(), totalprice.ToString());
                dt.Rows.Add(dr);

                GridView1.DataSource = dt;
                GridView1.DataBind();

                Session["buyitems"] = dt;

                GridView1.FooterRow.Cells[4].Text = "Total Amount";
                GridView1.FooterRow.Cells[5].Text = grandtotal().ToString();
            }
        }
    }
}

private void SaveCartDetail(String itemname, String quantity, String price, String totalprice)
{
    String query = "insert into cart(item_name, quantity, price, totalprice, username) values ('" + itemname + "','" + quantity + "','" + price + "','" + totalprice + "','" + Session["username"].ToString() + "')";

    SqlConnection scon1 = new SqlConnection(ConfigurationManager.ConnectionStrings["online food orderingConnectionString"].ConnectionString);

    scon1.Open();

    SqlCommand cmd1 = new SqlCommand(query, scon1);
    cmd1.ExecuteNonQuery();
    scon1.Close();

    Response.Write("Items saved in cart");
}

为每个循环创建了一个单独的变量，因此工作正常

for (const i = 0; i < 100; i++)

for和of循环定义中let和const的用法

1 个答案: