db.collection('users').ensureIndex({username:1}, {unique:true, background:true, dropDups:true, safe:true}, function(err){
    if(err) {console.log(err.stack)};
});

顺便推广一下cclog

db.collection('users').ensureIndex({username:1}, {unique:true, background:true, dropDups:true, safe:true}, cclog.ifError);

1 不需要每次.

2 省事 行号