A utility method to get the ObjectID from a variety of input types. In working with a Mongoose REST API, I encounter mongoose subdocuments that may or may not be populated, may or may not be hydrated Mongoose models, and _id's that may be coming from the front-end that are just strings. This method allows you to pass in those 3 possibilities and…

getObjectId.js

const ObjectID = require('mongodb').ObjectID;

/**
 * Returns the ObjectId of the object, taking into account
 * whether the object is a populated Model, an ObjectId, or
 * a string representation of an ObjectID.
 * 
 * @param {MongooseModel|Object|String} input
 * @return ObjectId
 */
module.exports = function(input){
  const constructor = input.constructor.name.toLowerCase();
  if (constructor === 'objectid'){ return input; }
  if (constructor === 'string'){ return new ObjectID(input); } 
  // else, assume it's a populated model and has an _id property
  const _idConstructor = input._id.constructor.name.toLowerCase();
  if (_idConstructor === 'objectid') { return input._id; } 
  // might be a POJO so coerce it to an ObjectID object
  return new ObjectID(input._id.toString());
}