Schema - Schema.prototype.virtual() - 《Mongoose ODM v5.11.15 Documentation》

name «String»
[options] «Object»
[options.ref] «String|Model» model name or model instance. Marks this as a populate virtual.
[options.localField] «String|Function» Required for populate virtuals. See populate virtual docs for more information.
[options.foreignField] «String|Function» Required for populate virtuals. See populate virtual docs for more information.
[options.justOne=false] «Boolean|Function» Only works with populate virtuals. If truthy, will be a single doc or null. Otherwise, the populate virtual will be an array.
[options.count=false] «Boolean» Only works with populate virtuals. If truthy, this populate virtual will contain the number of documents rather than the documents themselves when you populate().
[options.get=null] «Function|null» Adds a getter to this virtual to transform the populated doc.

Creates a virtual type with the given name.