Introduction

In this tutorial, we aim to explore the concept of Error Interfaces in TypeScript. Error handling is an integral part of any programming language, and TypeScript provides a more robust way to handle errors through the use of interfaces.

By the end of this tutorial, you will learn:
- What are Error Interfaces
- How to define and use Error Interfaces
- How Error Interfaces contribute to better type safety
- How to handle exceptions more effectively using Error Interfaces

Prerequisites:
To make the most out of this tutorial, you should have a basic understanding of TypeScript and familiar with interface concept in TypeScript.

Step-by-Step Guide

Understanding Error Interfaces

In TypeScript, an interface is a structure that defines the syntax for classes to follow. It contains only the declaration of the members it is expected to contain. Similarly, error interfaces are used to enforce that a class meets a certain contract for error throwing.

Defining an Error Interface

An Error interface can be defined in TypeScript in a similar way as other interfaces but it usually extends the built-in Error interface.

interface CustomError extends Error {
  status: number;
}

In the above example, CustomError is an error interface that extends the built-in Error interface and adds a status property to it.

Using Error Interfaces

Error Interfaces can be used to create custom error classes that implement the error interface.

class ServerError implements CustomError {
  name: string;
  message: string;
  status: number;

  constructor(message?: string, status?: number) {
    this.name = 'ServerError';
    this.message = message || 'An error occurred on the server';
    this.status = status || 500;
  }
}

Here, the ServerError class implements the CustomError interface and defines the name, message, and status properties.

Code Examples

Let's look at a practical example of how to use Error Interfaces in TypeScript.

interface CustomError extends Error {
  status: number;
}

class ServerError implements CustomError {
  name: string;
  message: string;
  status: number;

  constructor(message?: string, status?: number) {
    this.name = 'ServerError';
    this.message = message || 'An error occurred on the server';
    this.status = status || 500;
  }
}

try {
  throw new ServerError('Server is down', 503);
} catch (error) {
  if (error instanceof ServerError) {
    console.log(`${error.name}: ${error.message}`);
  } else {
    console.log(error);
  }
}

In the above example, we first define a CustomError interface and a ServerError class that implements this interface. Then, we throw a ServerError in a try block and catch it in a catch block. If the error is an instance of ServerError, we log the name and message properties; otherwise, we log the error itself.

Expected output:

ServerError: Server is down

Summary

In this tutorial, we learned about Error Interfaces in TypeScript. We learned how to define and use them for better type safety and more effective exception handling. We also looked at a practical example of using Error Interfaces.

Next, you can explore more about TypeScript's other built-in interfaces and how to use them in your projects. You can also learn more about error handling in TypeScript and other programming languages.

Practice Exercises

1. Define a ClientError interface and a class that implements it. Throw and catch a ClientError in your code.

2. Define a DatabaseError interface with additional properties. Create a class that implements this interface. Throw and catch a DatabaseError in your code.

Solutions

1. Solution for exercise 1:

interface ClientError extends Error {
  status: number;
}

class BadRequestError implements ClientError {
  name: string;
  message: string;
  status: number;

  constructor(message?: string, status?: number) {
    this.name = 'BadRequestError';
    this.message = message || 'Bad request';
    this.status = status || 400;
  }
}

try {
  throw new BadRequestError('Missing parameters', 400);
} catch (error) {
  if (error instanceof BadRequestError) {
    console.log(`${error.name}: ${error.message}`);
  } else {
    console.log(error);
  }
}

1. Solution for exercise 2:

interface DatabaseError extends Error {
  status: number;
  query: string;
}

class QueryError implements DatabaseError {
  name: string;
  message: string;
  status: number;
  query: string;

  constructor(message?: string, status?: number, query?: string) {
    this.name = 'QueryError';
    this.message = message || 'Database query error';
    this.status = status || 500;
    this.query = query || '';
  }
}

try {
  throw new QueryError('Failed to fetch data', 500, 'SELECT * FROM users');
} catch (error) {
  if (error instanceof QueryError) {
    console.log(`${error.name}: ${error.message}`);
    console.log(`Query: ${error.query}`);
  } else {
    console.log(error);
  }
}