Comparing mongoose _id and strings

The three possible solutions suggested here have different use cases.

1. Use .equals when comparing ObjectId on two mongoDocuments like this

results.userId.equals(AnotherMongoDocument._id)

2. Use .toString() when comparing a string representation of ObjectId to an ObjectId of a mongoDocument. like this

results.userId === AnotherMongoDocument._id.toString()

converting object id to string(using toString() method) will do the job.

ObjectIDs are objects so if you just compare them with == you're comparing their references. If you want to compare their values you need to use the ObjectID.equals method:

if (results.userId.equals(AnotherMongoDocument._id)) {
    ...
}

Mongoose uses the mongodb-native driver, which uses the custom ObjectID type. You can compare ObjectIDs with the .equals() method. With your example, results.userId.equals(AnotherMongoDocument._id). The ObjectID type also has a toString() method, if you wish to store a stringified version of the ObjectID in JSON format, or a cookie.

If you use ObjectID = require("mongodb").ObjectID (requires the mongodb-native library) you can check if results.userId is a valid identifier with results.userId instanceof ObjectID.

Etc.