Creating a Custom Error Class in an Express App

Introduction

Error handling is a crucial aspect of developing robust applications. JavaScript provides a built-in "Error" class to throw exceptions, but it lacks the flexibility to include additional details like HTTP status codes. This blog post will guide you through creating a custom error class by extending the "Error" class, allowing you to provide more informative error responses.

• This is the tenth blog of my series where I am writing how to write code for an industry-grade project so that you can manage and scale the project.

• The first nine blogs of the series were about "How to set up eslint and prettier in an express and typescript project", "Folder structure in an industry-standard project", "How to create API in an industry-standard app", "Setting up global error handler using next function provided by express", "How to handle not found route in express app", "Creating a Custom Send Response Utility Function in Express", "How to Set Up Routes in an Express App: A Step-by-Step Guide", "Simplifying Error Handling in Express Controllers: Introducing catchAsync Utility Function" and "Understanding Populating Referencing Fields in Mongoose". You can check them in the following link.

https://dev.to/md_enayeturrahman_2560e3/how-to-set-up-eslint-and-prettier-1nk6

https://dev.to/md_enayeturrahman_2560e3/folder-structure-in-an-industry-standard-project-271b

https://dev.to/md_enayeturrahman_2560e3/how-to-create-api-in-an-industry-standard-app-44ck

https://dev.to/md_enayeturrahman_2560e3/setting-up-global-error-handler-using-next-function-provided-by-express-96c

https://dev.to/md_enayeturrahman_2560e3/how-to-handle-not-found-route-in-express-app-1d26

https://dev.to/md_enayeturrahman_2560e3/creating-a-custom-send-response-utility-function-in-express-2fg9

https://dev.to/md_enayeturrahman_2560e3/how-to-set-up-routes-in-an-express-app-a-step-by-step-guide-177j

https://dev.to/md_enayeturrahman_2560e3/simplifying-error-handling-in-express-controllers-introducing-catchasync-utility-function-2f3l

https://dev.to/md_enayeturrahman_2560e3/understanding-populating-referencing-fields-in-mongoose-jhg

Basic Error Handling

In JavaScript, throwing an error is straightforward:

throw new Error('This department already exists!');

This method only allows sending a simple error message. However, in many applications, especially web applications, it's essential to include more information, such as HTTP status codes, to make error handling more informative and user-friendly.

Creating a Custom Error Class

To overcome the limitations of the basic "Error" class, we can create a custom error class that extends "Error". This custom class will allow us to add properties like statusCode to provide more context for the error.

Let's create a file named "AppError.ts" and define our custom error class:

class AppError extends Error {
  public statusCode: number;

  constructor(statusCode: number, message: string, stack = '') {
    super(message);
    this.statusCode = statusCode;

    if (stack) {
      this.stack = stack;
    } else {
      Error.captureStackTrace(this, this.constructor);
    }
  }
}

export default AppError;

In this "AppError" class:

• We extend the built-in Error class.
• We add a statusCode property to hold the HTTP status code.
• We capture the stack trace, making debugging easier.

Using the Custom Error Class

Now, let's use the AppError class in our academicDepartmentModel.ts file to throw more informative errors.

import httpStatus from 'http-status';
import { Schema, model } from 'mongoose';
import AppError from '../../errors/AppError';
import { TAcademicDepartment } from './academicDepartment.interface';

const academicDepartmentSchema = new Schema<TAcademicDepartment>(
  {
    name: {
      type: String,
      required: true,
      unique: true,
    },
    academicFaculty: {
      type: Schema.Types.ObjectId,
      ref: 'AcademicFaculty',
    },
  },
  {
    timestamps: true,
  },
);

academicDepartmentSchema.pre('save', async function (next) {
  const isDepartmentExist = await AcademicDepartment.findOne({
    name: this.name,
  });

  if (isDepartmentExist) {
    throw new AppError(
      httpStatus.NOT_FOUND,
      'This department already exists!',
    );
  }

  next();
});

academicDepartmentSchema.pre('findOneAndUpdate', async function (next) {
  const query = this.getQuery();
  const isDepartmentExist = await AcademicDepartment.findOne(query);

  if (!isDepartmentExist) {
    throw new AppError(
      httpStatus.NOT_FOUND,
      'This department does not exist!',
    );
  }

  next();
});

export const AcademicDepartment = model<TAcademicDepartment>(
  'AcademicDepartment',
  academicDepartmentSchema,
);

Recap

To implement a custom error class in JavaScript, follow these steps:

• Create a Custom Error Class:

  • Extend the built-in Error class.
  • Add any additional properties needed (e.g., statusCode).

• Use the Custom Error Class:

  • Import and throw the custom error in your application to provide more informative error messages.

• Handle Errors Appropriately:

  • Catch these custom errors in your error-handling middleware or functions to send proper responses to the client.

Conclusion

By extending the Error class, you can create more detailed and informative error messages, improving the overall robustness and user experience of your application.