Check only characters in Textbox using Javascript

//TextBox indicates the id of the TextBox in the form
{
function checkcharTXT(TextBox)

var str=TextBox.value;
for(i=0;i < str.length;i++)
{
var strchar=str.charAt(i);
if(((strchar>="a")&&(strchar<="z"))||((strchar>="A")&&(strchar<="Z")))
{
}
else
{
return(false);
}
}
}