Build the In Memory Todo API

FULL_STACK_DEVELOPER_2026_COMPLETE - Phase 05 Build the In-Memory Todo API

VOLUME 02 - FRONTEND + BACKEND FOUNDATIONS

Build the In-Memory Todo API Turn HTTP semantics into observable GET, POST, PATCH and DELETE behavior

HTTP is the contract between independent programs.

REQUEST HTTP Client TaskFlow API

PowerShell / browser / later Next.js NestJS + Fastify RESPONSE

method + target + headers + optional body

status + headers + optional body

Figure - HTTP request/response exchange. Phase 05 studies the protocol before Phase 06 introduces a browser cross-origin call.

Book 5 of Phase 05

FULL_STACK_DEVELOPER_2026_COMPLETE - Phase 05 Build the In-Memory Todo API

Build a temporary in-memory Todo API We now need a concrete protocol surface to exercise. The data deliberately lives in memory so a server restart resets it. That limitation is not a bug in this phase - it isolates HTTP behavior from database persistence. Volume 03 will solve persistence properly.

PowerShell Client Invoke-RestMethod

Fastify receives HTTP request

TodosController method + route + HTTP boundary

TodosService temporary in-memory behavior

HTTP Response status + JSON / no body

Figure - The Phase 05 Todo API is intentionally in-memory. PostgreSQL and Prisma arrive later; this phase isolates HTTP semantics.

Step 1 - Generate the Todo feature From: taskflow\apps\api

pnpm exec nest generate module todos
pnpm exec nest generate controller todos --no-spec
pnpm exec nest generate service todos --no-spec

Step 2 - Give the service temporary state apps/api/src/todos/todos.service.ts - first read operations

FULL_STACK_DEVELOPER_2026_COMPLETE - Phase 05 Build the In-Memory Todo API

import { Injectable, NotFoundException } from '@nestjs/common';

interface Todo { id: number; title: string; completed: boolean; }

@Injectable()
export class TodosService {
private nextId = 3;
private readonly todos: Todo[] = [
{ id: 1, title: 'Learn HTTP', completed: false },
{ id: 2, title: 'Build the API contract', completed: false },
];

findAll() { return this.todos; }

findOne(id: number) { const todo = this.todos.find((item) => item.id === id); if (!todo) throw new NotFoundException('Todo not found'); return todo; } }

The service owns temporary application state. The controller should not own an array just because it receives HTTP requests. Later, Prisma replaces this in-memory persistence without changing the conceptual controller responsibility.

Step 3 - Add read routes apps/api/src/todos/todos.controller.ts - read routes

@Controller('todos')
export class TodosController {
constructor(private readonly todosService: TodosService) {}

@Get() findAll() { return this.todosService.findAll(); }

@Get(':id')
findOne(@Param('id') id: string) {
return this.todosService.findOne(Number(id));
}
}

We convert the route parameter to a number for this protocol exercise. Formal validation and typed DTO boundaries are deliberately deferred. The book should make that technical debt visible rather than pretend raw input is trustworthy.

Step 4 - Add creation todos.service.ts - temporary creation behavior

create(title: string) { const todo = { id: this.nextId++, title, completed: false, }; this.todos.push(todo); return todo; }

todos.controller.ts - POST /todos

FULL_STACK_DEVELOPER_2026_COMPLETE - Phase 05 Build the In-Memory Todo API

@Post() create(@Body() body: { title?: string }) { if (!body.title?.trim()) { throw new BadRequestException('title is required'); } return this.todosService.create(body.title.trim()); }

Nest defaults successful POST handlers to 201 Created. We use only a tiny manual guard here so the protocol example is usable; this is not the validation architecture taught later.

Step 5 - Add partial update todos.service.ts - patch behavior

update(id: number, changes: { title?: string; completed?: boolean }) { const todo = this.findOne(id); if (changes.title !== undefined) todo.title = changes.title; if (changes.completed !== undefined) todo.completed = changes.completed; return todo; }

todos.controller.ts - PATCH /todos/:id

@Patch(':id')
update(
@Param('id') id: string,
@Body() body: { title?: string; completed?: boolean },
) {
return this.todosService.update(Number(id), body);
}

Step 6 - Add deletion with 204 todos.service.ts - removal behavior

remove(id: number) {
const todo = this.findOne(id);
const index = this.todos.indexOf(todo);
this.todos.splice(index, 1);
}

todos.controller.ts - DELETE /todos/:id

@Delete(':id')
@HttpCode(204)
remove(@Param('id') id: string) {
this.todosService.remove(Number(id));
}

A 204 response intentionally has no content. Returning a JSON success object would conflict with the meaning we are teaching. Nest allows explicit status selection through @HttpCode(204).

Temporary limitation

The array disappears when the server stops. That is useful right now because the learner can focus on request/response semantics. Do not add Prisma early to "fix" it. Persistence is a separate architectural problem and gets its own phase.

Official source notes Primary protocol reference: RFC 9110 - HTTP Semantics (RFC Editor). Supplemental developer reference: MDN HTTP Reference and HTTP Request Methods. Nest-specific routing and status behavior reference: NestJS Controllers documentation.

FULL_STACK_DEVELOPER_2026_COMPLETE - Phase 05 Build the In-Memory Todo API

This phase teaches HTTP semantics independent of a specific transport version. HTTP/2 and HTTP/3 change framing and transport details, while request methods, target resources, headers, status codes and representation semantics remain the important application-level concepts for this course.