In this section, we will discuss the “undefined” datatype in MongoDB. This datatype was originally used in the early versions of MongoDB, but now it is deprecated and should not be used in new applications.

What is ‘undefined’?

An ‘undefined’ datatype in MongoDB is a data type that signifies that the value of a field has not been set or has been removed. It represents the absence of a value.

Why should it not be used?

In the newer versions of MongoDB, it is recommended to use the null value for representing missing or undefined values in the database. Although the undefined datatype is still supported for backward compatibility, it is advised to avoid the use of it, as the null value is more widely accepted and understood.

Here is an example to show the difference between null and undefined:

  "field1": null,
  "field2": undefined

In this example, field1 has a null value, while field2 has an undefined value. However, it is recommended to use null instead of undefined to maintain better readability and compatibility.


In summary, while the ‘undefined’ datatype exists in MongoDB, it is now considered deprecated and should be avoided. Instead, it is suggested to use the null value to represent fields with missing or undefined values in your database. This will ensure better compatibility and readability of your code when using MongoDB.