step1.js

var crimes = [
  {"crime": "Assault", "property": "States Avenue"},
  {"crime": "Larceny", "property": "Marvin Gardens"},
  {"crime": "Assault", "property": "States Avenue"},
  {"crime": "Motor Vehicle Theft", "property": "Boardwalk"},
  {"crime": "Assault", "property": "States Avenue"},
  {"crime": "Larceny", "property": "Marvin Gardens"},
  {"crime": "Assault", "property": "States Avenue"},
  {"crime": "Drugs", "property": "Marvin Gardens"},
  {"crime": "Drugs", "property": "Marvin Gardens"},
  {"crime": "Larceny", "property": "Marvin Gardens"},
  {"crime": "Larceny", "property": "Marvin Gardens"},
  {"crime": "Motor Vehicle Theft", "property": "Boardwalk"},
  {"crime": "Motor Vehicle Theft", "property": "Boardwalk"},
  {"crime": "Drugs", "property": "Marvin Gardens"}
]

if(db.crimes.count() == 0) {
  crimes.forEach(function(c) {
    db.crimes.insert(c)
  })
}


function map() {
  emit(this.property, 1)
}

function reduce(propKey, vals) {
  var sum = 0
  vals.forEach(function(v) {
    sum += v
  })
  return sum
}

var mr = db.crimes.mapReduce(map, reduce),
    results = db[mr.result].find()
    
printjson(result.toArray())