You can easily implement yourself, for instance
var exists = [];
var s = "abcdefoo";
for (var i=0; i < s.length; ++i)
{
exists[s[i]] = 1;
}
for ( var i = 97; i<=122; ++i)
{
c = String.fromCharCode(i);
if ( !exists[c])
{
alert( c + " is missing");
break;
}
}