some progress

node_modules/mongoose/lib/options/SchemaMapOptions.js

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+'use strict';
+
+const SchemaTypeOptions = require('./SchemaTypeOptions');
+
+/**
+ * The options defined on a Map schematype.
+ *
+ * #### Example:
+ *
+ *     const schema = new Schema({ socialMediaHandles: { type: Map, of: String } });
+ *     schema.path('socialMediaHandles').options; // SchemaMapOptions instance
+ *
+ * @api public
+ * @inherits SchemaTypeOptions
+ * @constructor SchemaMapOptions
+ */
+
+class SchemaMapOptions extends SchemaTypeOptions {}
+
+const opts = require('./propertyOptions');
+
+/**
+ * If set, specifies the type of this map's values. Mongoose will cast
+ * this map's values to the given type.
+ *
+ * If not set, Mongoose will not cast the map's values.
+ *
+ * #### Example:
+ *
+ *     // Mongoose will cast `socialMediaHandles` values to strings
+ *     const schema = new Schema({ socialMediaHandles: { type: Map, of: String } });
+ *     schema.path('socialMediaHandles').options.of; // String
+ *
+ * @api public
+ * @property of
+ * @memberOf SchemaMapOptions
+ * @type {Function|string}
+ * @instance
+ */
+
+Object.defineProperty(SchemaMapOptions.prototype, 'of', opts);
+
+module.exports = SchemaMapOptions;