CouchDB map/reduce view for getting max integer ID

map.js

function(doc) {
  emit(null, doc._id);
}

reduce.js

function(keys, values) {
  var ids = []

  values.forEach(function(id) {
    if (!isNaN(id))
      ids.push(id);
  });

  return Math.max.apply(Math, ids)
}

rejuice it, with math!