es2015/migration/DatabaseMigrator.js
function normalizeFetchBeforeObjectStores(objectStores) {
  return objectStores.map((objectStore) => {
    if (typeof objectStore === "string") {
      return {
        objectStore,
        preprocessor: record => record
      }
    } else if (!objectStore.preprocessor) {
      return {
        objectStore: objectStore.objectStore,
        preprocessor: record => record
      }
    } else {
      return objectStore
    }
  })
}
function normalizeFetchBeforeObjectStores(objectStores) {
  return objectStores.map((objectStore) => {
    if (typeof objectStore === "string") {
      return {
        objectStore,
        preprocessor: record => record
      }
    } else if (!objectStore.preprocessor) {
      return {
        objectStore: objectStore.objectStore,
        preprocessor: record => record
      }
    } else {
      return objectStore
    }
  })
}
Normalizes the provided array of object store fetch descriptors to process
before upgrading the database schema.

@param {(string|{objectStore: string, preprocessor: function(*, (number|string|Date|Array)): (*|UpgradedDatabaseSchema.SKIP_RECORD|UpgradedDatabaseSchema.DELETE_RECORD)=})[]} objectStores
The names of object stores that should have their records fetch or
(possibly partially filled) object store fetch descriptors, mixed in
an array.
@return {{objectStore: string, preprocessor: function(*, (number|string|Date|Array)): (*|UpgradedDatabaseSchema.SKIP_RECORD|UpgradedDatabaseSchema.DELETE_RECORD)}[]}
Normalized object store fetch descriptors.