MongoDB requires each document to contain an _id field. If the _id field is not present when inserting a document, MongoDB generates an ObjectId for you.

ObjectIds have a length of 12 bytes: 4-byte timestamp, 5-byte random value, and 3-byte incrementing counter. An ObjectId can be formatted as a hex string that looks like 61dc2d31bbe643fc32022a5f.

The problem with ObjectIds: you can’t compare an ObjectId string with an ObjectId instance.

This tutorial shows you how to compare ObjectIds in Node.js.

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Compare MongoDB ObjectIds in Node.js

When using ObjectIds in your application, you may use them in the frontend (browser) as well. Requesting data for a given web page may use an ObjectId as an identifier for a given resource. Using identifiers in URLs requires a string representation for the request. In your backend, you need to convert the request parameters back to their original type.

Let’s imagine the following simplified code: you have an ObjectId in string format and a document using an ObjectId for the _id field.

Comparing the string ObjectId against the _id field in the user instance returns false:

const oid = '61dc2d31bbe643fc32022a5f'

const user = {  
  _id: ObjectId("61dc2d31bbe643fc32022a5f"),
  name: { first: "Future", last: "Studio" }
}

const isEqual = user._id === oid // wrong way 🚨  
// false

What you should do instead: use the objectId.equals method. Every ObjectId instance supports the equals method allowing you to provide your comparison value. The MongoDB Node.js driver resolves the given argument value and compares them:

const oid = '61dc2d31bbe643fc32022a5f'

const user = {  
  _id: ObjectId("61dc2d31bbe643fc32022a5f"),
  name: { first: "Future", last: "Studio" }
}

const isEqual = user._id.equals(oid) // ✅ right way  
// true

Enjoy comparing MongoDB ObjectIds!

Mentioned Resources

• MongoDB ObjectId Docs