Creating Sequential Numbers for Table Column Via JavaScript Ascending and Descending

Thanks @Laubender , I forgot to drink enough Coffee!. :), Here is the Complete Code in Short but just two Things to make the code good enough and is how to check if the user select text inside cell instead of selecting the cell it-self! and how to check if the cursor inside table cell (not select anything) and also this shouid be not allowed! 

//User must select the Column need to Count
//Preparing Check List First!
//Check if the User Select One Column Cells
    var Selectedcells = app.selection[0];
    //Check for One Cell Selected
      if (Selectedcells instanceof Cell && Selectedcells.cells.length == 1) {
            alert("One Cell Selection Not Allowed!", "STOP");
            exit();
            }
        else if (Selectedcells instanceof Cell && Selectedcells.cells.length == 0) {
            alert("User Didnt Select any Selection!", "STOP");
            exit();
            }
            else if (Selectedcells instanceof Cell && Selectedcells.cells.length < 1) {
            alert("There is No Selected Table One Column Cells!, Select One Column Cells and Run Script Again!", "STOP");
            exit();
            }
            else if (Selectedcells instanceof Cell && Selectedcells.cells[0].parentColumn == Selectedcells.cells[1].parentColumn == false) {
            alert("Select Only One Column Cells and Run Script Again!", "STOP");
            exit();
          }
         else if(Selectedcells < 0) {
         alert("You Select Nothing!", "STOP");
         exit();
            }
       else if(Selectedcells instanceof TextFrame) {
            alert("Dont Select The Table Frame!", "STOP");
            exit();
            PreRun();
            }

function PreRun() {  //Make all Cell Contents EMPTY at First
var Usercells = app.selection[0];
if(Usercells instanceof Cell && Usercells.columnSpan == 1) {
    for(var i=0 ; i<Usercells.cells.length ; i++) 
	Usercells.cells[i].contents = "";
    $.sleep(3);
    }
}

var Selectedcells = app.selection[0]
var startIdx = 1 //Starting Number of Counting
if(Selectedcells instanceof Cell && Selectedcells.columnSpan == 1)
{
	for(var i = 0; i < Selectedcells.cells.length; i++)
	{
		if(Selectedcells.cells[i].parentRow.rowType == RowTypes.BODY_ROW)
			Selectedcells.cells[i].contents = (startIdx++).toString()
    }
}

Wow @Laubender  Thats Great!, I developed little checker including your advice to check if one column selected or not, thanks a lot, here is the code :

//Check if the User Select One Column Cells
    var Selectedcells = app.selection[0];
           if (Selectedcells instanceof Cell) {
               if (Selectedcells.cells[0].parentColumn == Selectedcells.cells[1].parentColumn == true) {
                   // Returns false // if the first two cells in the selection belong to two different columns
                   // Returns true  // if the first two cells in the same column are selected
        // Delete all Selected Cells content to make it EMPTY Before Continue By Looping
        for(var i=0 ; i<Selectedcells.cells.length ; i++) 
			 Selectedcells.cells[i].contents = ""; //Make all Cell Contents EMPTY
           }
        else {
            alert("Select Only One Column Cells and Run Script Again!", "STOP");
            exit();
    }
}

//Check Again with if no Cells Selected , User didnt Touch a Table
           if (Selectedcells instanceof Cell) {
        // Delete all Selected Cells content to make it EMPTY Before Continue By Looping
        for(var i=0 ; i<Selectedcells.cells.length ; i++) 
			 Selectedcells.cells[i].contents = ""; //Make all Cell Contents EMPTY
           }
        else {
            alert("There is No Selected Table One Column Cells!, Select One Column Cells and Run Script Again!", "STOP");
            exit();
}

ok, thank you @Laubender , i already made a warning alert to the user to show him to select only column cells to numbering, but i wonder can i prevent user from selecting anything execpet one column direction?

Script only run in case user select cells :

Script will stuck if the user select non column cells, i mean not one direction but indesign didnt crash (images from InDesign 2021)

Thank you @Laubender But do you mean i have to alert user to prevent selecting (Merged Cells)? or there are another fix 

Wow, That's Great and simple, Thanks a lot @Manan Joshi , but please can you explain two things in the code? , First thing i wish to understand (columnspan) what does it mean and the benefit of it?  and the second (startidx) , i think the second is the cell ID number(startidx) right?

Thanks a lot again, i really learned a lot, I'm so grateful for your generous efforts.

Dear Uwe Laubender

Thanks a lot for your reply, here is the screen shots in one image, The First Row is Header :