What is ObjectId in MongoDB?

An ObjectId is a 12-byte value that MongoDB uses as the default _id of a document. If you insert a document without an _id, one is generated for you.

It is written as 24 hex characters and has three parts.

• 4 bytes: a timestamp in seconds since the Unix epoch, which is January 1, 1970.
• 5 bytes: a random value generated once per process.
• 3 bytes: a counter that starts at a random value and increments.

Because the timestamp comes first, ObjectIds sort by creation time to the second, and getTimestamp() returns that time. They are likely unique across machines, but uniqueness is not guaranteed.

// ObjectId example
ObjectId("507f1f77bcf86cd799439011")

// Extract timestamp
ObjectId("507f1f77bcf86cd799439011").getTimestamp()
// Returns: ISODate("2012-10-17T21:13:27Z")