var prompt=function(x,y){return'a'};
var alert=function(s){WScript.Echo(s)};
//var alert=print; //for SpiderMonkey, V8, etc
(function(bf){
j=s=[];b=[i=m=0];o=p="";bf=bf.split("");
for(i in bf)if(bf[i]=='[')s.push(i),if(bf[i]==']')j[j[i]=s.pop()]=i;
for(i=0;i<bf.length;++i)
(c=bf[i])=='>'?b[++m]=b[m]||0:
c=='<'?--m:
c=='+'?++b[m]:
c=='-'?--b[m]:
c=='['?b[m]||(i=j[i]):
c==']'?b[m]&&(i=j[i]):
c==','?b[m]=((p=prompt('>',''))&&p.length)?p.charAt(0):0:
c=='.'?o+=String.fromCharCode(b[m]):void(0);

alert(o);
})('++++++++[>++++++++<-]>+.');
//})((S=(D=document).selection)?D.S.createRange().text:D.getSelection());

>>46を勝手に短くしてみた。100文字くらい削れた。