find unique letters in string

dear teacher, how to find the unique letters in string.. i tried a lot but not able to solve..please help me.

Thank you.

hi Sarita,
below is function for unique letter in string.

function check()
{
var string=document.form1.txt1.value;
if(Number(string)||string=="")
{
alert("Enter a string");
document.form1.txt1.value="";
}
else
{
var count=0;
string=string.split('');
var len=string.length;
alert("The entered string has "+len+" letters.");
for(i=0;i<len;i++)
{
for(j=i+1;j<=len;j++)
{
if(string[i]==string[j])
{
count++;
break;
}
}
}
var unique=len-count;
alert("The number of unique characters the string "+string+" has is "+unique);