Service Layer Foundations

Introduction: Setting the Stage for a Task Manager API

Welcome to the first lesson of our course on building a Task Manager API with Next.js! Unlike earlier courses, where we built projects incrementally — starting with all logic in the API routes, then introducing validation, and finally extracting logic into a service layer — this project reflects a more realistic and production-ready approach from the beginning. Since you've already practiced each of these patterns individually, this time we'll integrate them immediately and treat the project structure as if you're working on a real-world team project. Think of this course as a "capstone" for your previous lessons. We'll skip re-explaining the basics like how API routes work or how to validate inputs from scratch — instead, we'll immediately apply those patterns to build a clean, modular backend. If you ever feel lost, you can always revisit earlier modules where we broke each piece down in more detail.

Our Task Data and Project Structure

Before we dive into the service layer, let’s quickly review the basic setup for our project. Here’s a summary of how we define a task and where we store our data: TypeScript // src/lib/tasks.ts export type Task = {id: number; title: string; content: string; completed: boolean; dueDate?: string;}; // Our in-memory data store. It starts empty. export let tasks: Task[] = []; Task Type: This defines what a task looks like. Each task has an id, title, content, completed status, and an optional dueDate . In-Memory Data Store: We use a simple array called tasks to keep track of all our tasks. This is just for learning — real apps use databases, but this keeps things simple for now. Our files are organized so that all task-related logic lives in the src/lib folder, and our API routes will use this logic.

Understanding the Service Layer

A service layer is a part of your backend code that handles the main logic for your application. Think of it as the “brain” that knows how to create, read, update, and delete tasks. The service layer sits between your data (the tasks array) and your API routes. Why use a service layer? Separation of concerns: Keeps your code organized by separating business logic from request handling. Reusability: You can use the same logic in different parts of your app. Easier to test and maintain: If you need to change how tasks are managed, you only update the service layer. In our project, the service layer will be a set of functions that work with the tasks array.

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Building Task Service Functions (with Examples)

Let’s build the main functions for our service layer. These functions will help us manage tasks in our in-memory store.

TypeScript

// src/lib/services/taskService.ts
import { tasks, type Task } from '@/lib/tasks';

// Get all tasks
export const getAllTasks = (): Task[] => tasks;

// Create a new task
export const createTask = (
  data: { title: string; content: string; dueDate?: string }
): Task => {
  const { title, content, dueDate } = data;
  const newTask: Task = {
    id: tasks.length > 0 ? Math.max(...tasks.map(t => t.id)) + 1 : 1,
    title,
    content,
    completed: false,
    dueDate,
  };
  tasks.push(newTask);
  return newTask;
};

// Get a task by ID
export const getTaskById = (id: number): Task | null => {
  return tasks.find(t => t.id === id) || null;
};

// Update a task by ID
export const updateTaskById = (
  id: number,
  data: any,
  partial = true
): { task?: Task; error?: string } => {
  const index = tasks.findIndex(t => t.id === id);
  if (index === -1) return { error: 'Task not found' };

  if (partial) {
    if ('title' in data) tasks[index].title = data.title;
    if ('content' in data) tasks[index].content = data.content;
    if ('completed' in data) tasks[index].completed = data.completed;
    if ('dueDate' in data) tasks[index].dueDate = data.dueDate;
  } else {
    tasks[index].title = data.title;
    tasks[index].content = data.content;
    tasks[index].completed = data.completed;
    tasks[index].dueDate = data.dueDate;
  }

  return { task: tasks[index] };
};

// Delete a task by ID
export const deleteTaskById = (id: number): boolean => {
  const index = tasks.findIndex(t => t.id === id);
  if (index === -1) return false;
  tasks.splice(index, 1);
  return true;
};

// src/lib/services/taskService.ts
import { tasks, type Task } from '@/lib/tasks';

// Get all tasks
export const getAllTasks = (): Task[] => tasks;

// Create a new task
export const createTask = (
  data: { title: string; content: string; dueDate?: string }
): Task => {
  const { title, content, dueDate } = data;
  const newTask: Task = {
    id: tasks.length > 0 ? Math.max(...tasks.map(t => t.id)) + 1 : 1,
    title,
    content,
    completed: false,
    dueDate,
  };
  tasks.push(newTask);
  return newTask;
};

// Get a task by ID
export const getTaskById = (id: number): Task | null => {
  return tasks.find(t => t.id === id) || null;
};

// Update a task by ID
export const updateTaskById = (
  id: number,
  data: any,
  partial = true
): { task?: Task; error?: string } => {
  const index = tasks.findIndex(t => t.id === id);
  if (index === -1) return { error: 'Task not found' };

  if (partial) {
    if ('title' in data) tasks[index].title = data.title;
    if ('content' in data) tasks[index].content = data.content;
    if ('completed' in data) tasks[index].completed = data.completed;
    if ('dueDate' in data) tasks[index].dueDate = data.dueDate;
  } else {
    tasks[index].title = data.title;
    tasks[index].content = data.content;
    tasks[index].completed = data.completed;
    tasks[index].dueDate = data.dueDate;
  }

  return { task: tasks[index] };
};

// Delete a task by ID
export const deleteTaskById = (id: number): boolean => {
  const index = tasks.findIndex(t => t.id === id);
  if (index === -1) return false;
  tasks.splice(index, 1);
  return true;
};

Let’s break down what each function does:

getAllTasks: Returns the full list of tasks.
createTask: Adds a new task to the list. It automatically assigns a unique id and sets completed to false.
getTaskById: Finds a task by its id. Returns null if not found.
updateTaskById: Updates a task’s details. It can do a full update or a partial update (only the fields you provide).
deleteTaskById: Removes a task from the list by its id. Returns true if successful, false if the task was not found.

The partial flag allows the same function to handle both PUT (full update) and PATCH (partial update). When partial is set to true, only the fields present in the request are updated — this mirrors the semantics of a PATCH request. When it's false, we expect all fields (title, content, completed, dueDate) to be included, mimicking a full PUT operation. This is a useful design pattern that helps avoid duplicating logic for PATCH and PUT operations.

Example Usage:

TypeScript

// Add a new task
const task = createTask({ title: "Buy groceries", content: "Milk, eggs, bread", dueDate: "2025-12-31T23:59:59.000Z" });
console.log(task);
// Output: { 
//   id: 1, 
//   title: "Buy groceries", 
//   content: "Milk, eggs, bread", 
//   completed: false, 
//   dueDate: "2025-12-31T23:59:59.000Z" 
// }

// Get all tasks
console.log(getAllTasks());
// Output: [{ 
//   id: 1, 
//   title: "Buy groceries", 
//   content: "Milk, eggs, bread", 
//   completed: false, 
//   dueDate: "2025-12-31T23:59:59.000Z" 
// }]

// Add a new task
const task = createTask({ title: "Buy groceries", content: "Milk, eggs, bread", dueDate: "2025-12-31T23:59:59.000Z" });
console.log(task);
// Output: { 
//   id: 1, 
//   title: "Buy groceries", 
//   content: "Milk, eggs, bread", 
//   completed: false, 
//   dueDate: "2025-12-31T23:59:59.000Z" 
// }

// Get all tasks
console.log(getAllTasks());
// Output: [{ 
//   id: 1, 
//   title: "Buy groceries", 
//   content: "Milk, eggs, bread", 
//   completed: false, 
//   dueDate: "2025-12-31T23:59:59.000Z" 
// }]

These functions are the core of your service layer. They make it easy to manage tasks without worrying about how the data is stored.

Introduction: Setting the Stage for a Task Manager API

Our Task Data and Project Structure

Understanding the Service Layer

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Building Task Service Functions (with Examples)

How the Service Layer Connects to API Routes

Working with Task Routes by ID (PUT, PATCH, DELETE)

Summary And What’s Next