Javascript combinations enumeration (not fast)

comb=function(a,n){
  var ids=new Array(n).fill(0).map((i,ind)=>ind);
  var res=[];
  //console.log(indexes);
  var evs="";
  for(i in ids)
    evs+=`for(ids[${i}]=((ids[${i}-1]==null)?(-1):ids[${i}-1])+1;ids[${i}]<=a.length-n+${i};ids[${i}]++){\n`;
  evs+=`res.push(a.filter(    (ii,ind)=>(ids.indexOf(ind)!=-1)    ));\n`;
  evs+="}\n".repeat(n);
  console.log(evs);
  eval(evs);
  return (res);
}



alert(JSON.stringify(comb([1,2,3],2)));