Nestjs schema definition with mongoose: TypeError: Cannot read properties of undefined (reading 'name')

3 min read 04-10-2024

Nestjs schema definition with mongoose: TypeError: Cannot read properties of undefined (reading 'name')

Unraveling the "TypeError: Cannot read properties of undefined (reading 'name')" in NestJS with Mongoose

The dreaded "TypeError: Cannot read properties of undefined (reading 'name')" error can be a real head-scratcher when working with NestJS and Mongoose. It usually pops up when you try to access a property of an object that is unexpectedly undefined, often within a schema definition. This article will dissect the root cause of this error and equip you with solutions to avoid it.

The Scenario:

Imagine you have a NestJS application using Mongoose for database interactions. You define a schema for a User model:

import { Prop, Schema, SchemaFactory } from '@nestjs/mongoose';
import { Document } from 'mongoose';

export class User {
  @Prop()
  name: string;

  @Prop()
  email: string;
}

export const UserSchema = SchemaFactory.createForClass(User);

Now, let's say you're trying to retrieve a user by their ID, and you use the findOne() method in a service:

import { Injectable } from '@nestjs/common';
import { InjectModel } from '@nestjs/mongoose';
import { Model } from 'mongoose';
import { User, UserSchema } from './user.schema';

@Injectable()
export class UserService {
  constructor(@InjectModel(User.name) private userModel: Model<User>) {}

  async findUserById(userId: string) {
    const user = await this.userModel.findOne({ _id: userId });
    return user.name; // The error happens here!
  }
}

When running this code, you might encounter the dreaded "TypeError: Cannot read properties of undefined (reading 'name')" error. This signifies that the user object returned by findOne() is undefined, hence the name property cannot be accessed.

Understanding the Issue:

The error arises when the findOne() method fails to locate a document matching the specified _id. In this case, it returns undefined, and accessing user.name throws the error.

Here's a breakdown of the issue:

Undefined Object: The user variable is set to undefined because no document with the provided _id exists in the database.
Attempting Access: You try to access the name property of user, which is undefined, leading to the error.

Solutions:

There are a few ways to address this issue:

Null Checks:

async findUserById(userId: string) {
  const user = await this.userModel.findOne({ _id: userId });
  if (user) {
    return user.name;
  } else {
    return null; // Or handle the case where the user doesn't exist
  }
}

This approach explicitly checks if the user object is defined before accessing its properties.

Optional Chaining:

async findUserById(userId: string) {
  const user = await this.userModel.findOne({ _id: userId });
  return user?.name; // Optional chaining returns undefined if user is undefined
}

This uses the optional chaining operator (?.) to safely access the name property. If user is undefined, user?.name will also be undefined, avoiding the error.

Throw an Error:

async findUserById(userId: string) {
  const user = await this.userModel.findOne({ _id: userId });
  if (!user) {
    throw new Error(`User with ID ${userId} not found`);
  }
  return user.name;
}

This approach throws an error if the user is not found, preventing further execution and providing more information about the issue.

Best Practices:

Clear Validation: Always validate your input data before making database calls. Ensure the userId is a valid format and exists before calling findOne().
Handle Non-Existent Data: Design your application to gracefully handle scenarios where data is not found. Provide meaningful error messages or default values.
Error Handling: Implement robust error handling mechanisms to catch unexpected issues and provide informative feedback.

Conclusion:

The "TypeError: Cannot read properties of undefined (reading 'name')" is a common error in NestJS applications using Mongoose. Understanding the cause, implementing proper null checks, and adopting best practices for error handling will equip you to handle these situations effectively and build more reliable applications. Remember to embrace the power of optional chaining and error handling to create cleaner and safer code.