ObjectId is a 12-byte unique identifier used as the default value for the _id field.

Structure (24 hex characters):

• 4 bytes: Unix timestamp
• 5 bytes: Random unique value
• 3 bytes: Incrementing counter

Properties:

• Automatically generated if not provided
• Contains creation timestamp
• Globally unique across machines
• Sortable by creation time

// ObjectId example
ObjectId("507f1f77bcf86cd799439011")

// Extract timestamp
ObjectId("507f1f77bcf86cd799439011").getTimestamp()
// Returns: ISODate("2012-10-17T20:46:47Z")