Check only digits in TExtBox using Javascript

//TextBox indicates the ID of the TextBox in the form
function checknum(TextBox)
{
var str=TextBox.value;
for(i=0;i < str.length;i++)
{
var num=str.charAt(i);
if((num>="0")&&(num<="9"))
{
}
else
{
return(false);
}
}
}