Lists the indexes that are not used(according to $indexstats) across all the databases.

listUnusedIndex-MongoDBv3-2.js

var dbData = {}
db.adminCommand('listDatabases').databases.forEach(function(dBase){
  if( "admin" == dBase.name || "local" == dBase.name){
	return;
  }
  var currentDb = db.getSiblingDB(dBase.name)
  var collData = {}
  currentDb.getCollectionNames().forEach(function(collName){
      unusedIndexes = currentDb.getCollection(collName).aggregate([{"$indexStats":{}},{"$match":{"accesses.ops" : 0,"name" : {"$ne" : "_id_"}}}]).toArray()
      if(unusedIndexes.length > 0){
	   collData[collName] = unusedIndexes
       }
  })
  if(Object.keys(collData).length > 0) {
	dbData[dBase.name] = collData;
  }

})
printjson(dbData)