Welcome to this lesson on handling missing Todo items in your NestJS REST API by returning a 404 error. So far, you've learned how to set up basic GET, POST, PUT, and DELETE requests for a Todo application. You've also enhanced your API with filters and specific modifiers. In this lesson, we'll focus on improving user experience by correctly handling "not found" errors, specifically the "404 Not Found" error for missing Todo items. Adding robust error handling ensures that your API is both user-friendly and reliable.

First, let's briefly review what a controller is in NestJS. A controller is a class that handles incoming HTTP requests and returns responses to the client. In the previous lessons, you've already created a TodoController to manage various CRUD operations. Just as a quick reminder, here's how we set up the handlers in the controller:

TypeScript
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1@Controller('todos')
2export class TodoController {
3  constructor(private readonly todoService: TodoService) {}
4
5  @Get()
6  findAll(): TodoDto[] {
7    return this.todoService.findAll();
8  }
9
10  @Get(':id')
11  findOne(@Param('id') id: string): TodoDto {
12    return this.todoService.findOne(id);
13  }
14  
15  // ... other POST, PUT, and DELTE handlers
16}

In this example, the findOne method takes an ID as a parameter and looks up a To-Do item with that ID. If the item exists, it is returned as a TodoDto.

Handling errors properly is crucial for creating a robust API. In HTTP status codes, a "404 Not Found" error indicates that the requested resource could not be found. NestJS makes error handling straightforward by using built-in exceptions.

Before we dive into the specifics, let's quickly go over some common HTTP status codes you might encounter:

• 200 OK: The request succeeded.
• 201 Created: A new resource was created.
• 400 Bad Request: The request was invalid or cannot be served.
• 404 Not Found: The requested resource could not be found.
• 500 Internal Server Error: The server encountered an unexpected condition.

NestJS provides a set of built-in exceptions that you can use to handle errors gracefully. The NotFoundException is particularly useful for our case:

TypeScript
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1import { NotFoundException } from '@nestjs/common';
2
3throw new NotFoundException('Todo not found');

When this exception is thrown, NestJS will automatically send a 404 response to the client.

Let's start by enhancing the findOne method to return a 404 status code if a To-Do item is not found. Here is the updated method:

Next, let's enhance the update method to handle missing items.

Finally, let's address the complete method, which marks a To-Do item as complete.

In this lesson, we focused on enhancing your NestJS REST API to handle missing Todo items by returning a 404 error. We looked at:

• Understanding the role of controllers in handling HTTP requests.
• How to handle errors in NestJS using built-in exceptions.
• Updating the findOne, update, and complete methods to return a 404 status code when the requested item doesn't exist.

By implementing these techniques, your API is now more user-friendly and reliable. Well done on getting to the end of this lesson and this course! You are now equipped with the knowledge to build and handle robust REST APIs using NestJS. Up next are the practice exercises to reinforce what you've learned. Keep exploring and happy coding!