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Learn how to manage return types in TypeORM with NestJS when working with Postgres databases. This guide will help you clarify return values effectively.
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How to Correctly Type the Return Value from a Postgres Database in TypeORM with NestJS

As you embark on your journey to create a CRUD application using Postgres, NestJS, and TypeORM, one common issue that developers encounter is correctly typing the return values of queries. This is particularly important to ensure that your application's data flow is both predictable and type-safe. Let’s dive into understanding this problem and provide you with a clear solution.

The Problem

Many developers new to TypeORM often face the following dilemma: when querying for data, especially for specific fields or conditions, the inferred return type may not accurately reflect the nature of what is being returned.

For instance, in a scenario with the UserEntity defined as:

[[See Video to Reveal this Text or Code Snippet]]

Example Methods in UsersService

findUserById: This method fetches a user by ID, omitting sensitive data such as the password or refresh token.

findUserByEmail: Conversely, this method retrieves the complete user details, including sensitive information.

While both return types are inferred as Promise<UserEntity>, this lacks clarity, especially since findUserById is designed to return a more restricted data set.

Understanding the Return Types

Why Promise<UserEntity | null>?

The primary concern in this case is understanding why the inferred return type does not account for cases where a user is not found. It's essential to acknowledge that:

If a record is found, it resolves with the full UserEntity.

If not found, it should return null.

Thus, you should indeed specify the return type as Promise<UserEntity | null> to make it clear that the user might not exist.

Can TypeORM Infer the Correct Return Types Automatically?

TypeORM, by default, assumes that you are retrieving the full entity and does not differentiate based on whether you're using methods like .select(). Therefore, it is up to you as the developer to specify the expected return type explicitly.

Solutions for Correct Typing

Here are a few strategies for correctly typing return values in your service methods.

1. Using Partial, Omit, and Pick

Omit: You can create return types excluding the sensitive fields. For example:

[[See Video to Reveal this Text or Code Snippet]]

Pick: If you want only specific fields available in the return type, you can use:

[[See Video to Reveal this Text or Code Snippet]]

2. Explicit Return Type Annotations

Specify the return type explicitly in your methods:

[[See Video to Reveal this Text or Code Snippet]]

Conclusion

Accurate typing in TypeORM with NestJS is crucial for maintaining the integrity of your application. By understanding the return types, particularly when using selective queries, you can improve your code's clarity and robustness. Don’t rely on default inferences; take charge by using Omit, Pick, or creating custom types to tailor your return values to the needs of your application.

Get Started

Now that you're equipped with the knowledge to correctly type your return values, you can confidently proceed with your CRUD application using Postgres, NestJS, and TypeORM.