How to check out numeric values in js file?.

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VS2005

C#2.0

C#

IIS

Visual-Studio

How to check out numeric values in js file? I am not able to enter -4,-5 etc for amount field(i mean negative values). My js function as follows:

function IsIntegerFloatDot(sText)
{
   var ValidChars = "0123456789.";
   var IsNumber=true;
   var Char;
   var Count=0;

   for (i = 0; i < sText.length && IsNumber == true; i++)
      {
      Char = sText.charAt(i);
      if (Char == '.' )
        {
            Count++;
        }
      if (ValidChars.indexOf(Char) == -1 || Count > 1)
         {
         IsNumber = false;
         }
      }
   return IsNumber;
and function called as 
if(trimSpaces(document.getElementById("txtAmount").value) != "")
	    {
            var txtTotalEmployees = document.getElementById("txtAmount")
	        if (IsIntegerFloatDot(txtTotalEmployees.value)==false)
	        {
	            alert("Amount should be numeric values.");
		        txtTotalEmployees.focus();
		        return false;
	        }
	    }

   }