Why structured errors improve API quality in NestJS - See It in Action

Why structured errors improve API quality

📖 Scenario: You are building a simple NestJS API that returns user data. You want to improve the API quality by sending structured error responses when something goes wrong.

🎯 Goal: Create a NestJS controller that returns user data. Add structured error handling so the API returns clear, consistent error messages with status codes and error details.

📋 What You'll Learn

Create a users array with exact user objects

Add a variable to hold the user ID to find

Use a method to find the user by ID and throw a structured error if not found

Add a controller method that returns the user or the structured error response

💡 Why This Matters

🌍 Real World

APIs often need to tell clients exactly what went wrong. Structured errors make this clear and easy to handle.

💼 Career

Backend developers use structured error handling to build reliable, user-friendly APIs that frontend and mobile apps depend on.

1

Create the users data array

Create a constant array called users with these exact objects: { id: 1, name: 'Alice' }, { id: 2, name: 'Bob' }, and { id: 3, name: 'Charlie' }.

NestJS

// Create the users array with exact user objects
// Your code here

Need a hint?

Use const users = [ ... ] with the exact objects inside.

2

Add a variable for the user ID to find

Create a constant called userId and set it to 4 to simulate looking for a user that does not exist.

NestJS

const users = [
  { id: 1, name: 'Alice' },
  { id: 2, name: 'Bob' },
  { id: 3, name: 'Charlie' }
];

// Create userId variable set to 4
// Your code here

Need a hint?

Use const userId = 4; exactly.

3

Find user by ID and throw structured error if not found

Create a function called findUserById that takes id as a parameter. Inside, find the user in users with matching id. If no user is found, throw a HttpException with status 404 and a JSON object containing statusCode, error, and message fields.

NestJS

const users = [
  { id: 1, name: 'Alice' },
  { id: 2, name: 'Bob' },
  { id: 3, name: 'Charlie' }
];

const userId = 4;

// Create findUserById function with structured error throw
// Your code here

Need a hint?

Use HttpException from @nestjs/common to throw a structured error with status 404.

4

Add controller method to return user or structured error

Create a NestJS controller class called UserController with a method getUser that calls findUserById(userId) inside a try-catch block. Return the user if found. If an error is caught, rethrow it to send the structured error response.

NestJS

import { HttpException, HttpStatus } from '@nestjs/common';

const users = [
  { id: 1, name: 'Alice' },
  { id: 2, name: 'Bob' },
  { id: 3, name: 'Charlie' }
];

const userId = 4;

function findUserById(id: number) {
  const user = users.find(user => user.id === id);
  if (!user) {
    throw new HttpException({
      statusCode: 404,
      error: 'Not Found',
      message: `User with id ${id} not found`
    }, HttpStatus.NOT_FOUND);
  }
  return user;
}

// Create UserController class with getUser method using try-catch
// Your code here

Need a hint?

Use @Controller and @Get decorators from @nestjs/common. Use try-catch to handle errors.