Detailed steps for using javascript to update data without refreshing asp

In program design, you often encounter a situation where you cannot know in advance what data the user needs. You must extract the data from the server and then feed it back to the user based on the user's selection. For example, if you select a province, we will immediately display all the cities in the province. In this case, you need to refresh the entire page before reading it again. However, this is not only inefficient, but also not elegant. In fact, we can use javascript to combine Microsoft's XMLHTTP object. We don't need to refresh it. We can read the data from the server just a little bit, which is both professional and efficient.
Next we will demonstrate this technology by verifying whether a user is registered.
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1. First, create a CheckUser. asp file on the server to check whether the user exists. Based on whether the user exists, report 0 and 1 respectively.
U_name = Request. QueryString ("u_name ")
If u_name has then
Response. write "0"
Else
Response. write "1"
End if
2. Client HTML design:
I. JavaScript code
<Script language = javascript>
Function check_user_exists (form ){
U_name = form. u_name.value;
If (u_name = null | u_name = ''){
Alert ("Enter the user name ");
Return false;
}
InfoBoard = document. getElementById ("checkInfo ");
InfoBoard. innerText = 'querying ...';
Myurl = location. protocol + "//" + location. hostname + "/CheckUser. asp? U_name = "+ u_name;
RetCode = openUrl (myurl );
Switch (retCode ){
Case "-2 ":
InfoBoard. innerHTML = '<font color = red> sorry </font>, query failed'; break;
Case "1 ":
InfoBoard. innerHTML = '<font color = red> congratulations </font>.' + u_name + 'Can Be Used'; break;
Case "0 ":
InfoBoard. innerHTML = '<font color = red> sorry </font>, Username' + u_name + 'is in use ';
}
Return;
}

Function openurl (/url ){
Var objxml = new ActiveXObject ("Microsoft. XMLHttp ")
Objxml. open ("GET", url, false );
Objxml. send ();
RetInfo = objxml. responseText;
If (objxml. status = "200 "){
Return retInfo;
}
Else {
Return "-2 ";
}
}
</Script>
Ii. HTML form design:
<Form name = form1 action = "XXXX. asp" method = "post">
<Input type = text name = u_name> <span id = "checkInfo"> </span> <input type = button name = checkuser value = "check whether a user exists" onClick =" check_user_exists (this. form); ">
</Form>

After the above three steps, a page does not need to refresh the data update program completed (Demo address: http://www.web9898.cn/reg), according to this method, you can achieve a lot of will be cool application :)