Question

Do you have to do anything special while passing in a dynamically created string as a clientID for document.getElementById?

I have a asp:gridview control that has a textbox column and a checkbox column. I added an onclick event to the checkboxes to set the textbox value of that row to the max value of all checked rows +1. I pass in the IDs of the grid and the controls of the row that was selected. I can getElementByID fine for these controls, but When I dynamically build the IDs of the other controls, I keep getting null, even though I know that the IDs are correct. My code is bellow.

function SetPriority(cbID, tbID, gridID) {
    var cb = document.getElementById(cbID);
    if (cb.checked) {
        var tb = document.getElementById(tbID);
        var grid = document.getElementById(gridID);

        var maxv = 0;

        for (var i = 0; i < grid.rows.length; i++) {
            var indexID = 102 + i;
            var cbClientID = 'LeaveInfo_pnlMain_wgbLeaveSummary_gridSubmitted_ct' + indexID + '_chkGroup';
            var tbClientID = 'LeaveInfo_pnlMain_wgbLeaveSummary_gridSubmitted_ct' + indexID + '_txtPriority';

            console.log("row" + i);

            //just for example of how it should be working
            console.log(cbID);
            var cbx = document.getElementById(cbID);
            console.log(cbx);

            //get row checkbox
            console.log(cbClientID);
            var thisCB = document.getElementById(cbClientID);
            console.log(thisCB);

            //get row textbox
            var thisTB = document.getElementById(tbClientID);
            console.log(thisTB);

            if (thisCB) {
                if (thisCB.type == "checkbox") {
                    if (thisCB.checked) {
                        if (thisTB.value > maxv)
                            maxv = thisTB.value;
                    }
                }
            }

        }
        tb.value = parseInt(maxv) + 1;
    }
}

Here is how its showing up in the console, where you can see the IDs for the first row are the same

For Those wondering about How I am calling the function, I am adding it on to a checkbox in a .net gridview control on row databind. It renders as follows:

<input id="LeaveInfo_pnlMain_wgbLeaveSummary_gridSubmitted_ctl02_chkGroup" type="checkbox" name="LeaveInfo$pnlMain$wgbLeaveSummary$gridSubmitted$ctl02$chkGroup" onclick="javascript:SetPriority('LeaveInfo_pnlMain_wgbLeaveSummary_gridSubmitted_ctl02_chkGroup','LeaveInfo_pnlMain_wgbLeaveSummary_gridSubmitted_ctl02_txtPriority','LeaveInfo_pnlMain_wgbLeaveSummary_gridSubmitted');">

The vb .net code to add the function is this…(on-_RowDataBound)

        Dim chk As CheckBox = CType(e.Row.FindControl("chkGroup"), CheckBox)
        Dim tb As TextBox = CType(e.Row.FindControl("txtPriority"), TextBox)

        chk.Attributes.Add("onclick", String.Format("javascript:SetPriority('{0}','{1}','{2}');", chk.ClientID, tb.ClientID, gridSubmitted.ClientID))

Answer 1

No, you don’t have to do anything special when dynamically building a string. A string in javascript is the same string whether it was built dynamically or specified directly in your code. If document.getElementById() is not working, then one of the following is likely the cause:

1. Your string isn’t what you think it is so it doesn’t match the target id.
2. Your DOM id isn’t what you think it is.
3. You have multiple elements with the same id (not likely here because you won’t get null)
4. You are calling getElementById() before the DOM is ready or before the desired elements have been added to the DOM.

In this case, it seems more likely that 1) or 2) are the issues here, but you don’t show us any context to know whether 4) could be the problem.