MongoDB - show space available to reuse per collection

mongo_space_available_for_reuse.js

// mongo  --quiet 127.0.0.1:27017/admin mongo_space_available_for_reuse.js
db.getMongo().setSlaveOk()
var dbs = db.runCommand({ "listDatabases": 1 }).databases
var total_available_space_to_reuse = 0

print("Space available to reuse per collection:\n")

dbs.forEach(function(database) {

  db = db.getSiblingDB(database.name)

  print("  " + db + ":")

  var available_size = 0

  db.getCollectionNames().forEach(function(col) {
    var available_size_in_collection = db[col].stats().wiredTiger['block-manager']['file bytes available for reuse']
    if (available_size_in_collection > 0) {
      print("    " + col + ": " + available_size_in_collection / 1024 / 1024 + " MB")
    }
    available_size = available_size + available_size_in_collection
  })

  var available_size_mb = available_size / 1024 / 1024
  total_available_space_to_reuse = total_available_space_to_reuse + available_size_mb

})

print("\n\ntotal available space to reuse: " + total_available_space_to_reuse + " MB")