Conflicts

Part of the REST APIs section of Coddy's Node.js journey. Lesson 13 of 25.

A second task called "Send the invoice" makes the list confusing, but its body is valid. That is a 409 Conflict: the request clashes with what is already stored. A 400 means the body itself is wrong.

const taken = users.some((u) => u.name.toLowerCase() === name.toLowerCase());
if (taken) {
    return sendError(res, 409, 'conflict', 'That username is taken');
}

Compare in lower case, since people read "Ada" and "ada" as the same name. When a request renames something, skip the item itself in the search. Otherwise a user who saves the form without changing their name gets a conflict with themselves.

Challenge

Easy

Two tasks called "Send the invoice" make the list confusing. Refuse a title that another task already has.

In routes/tasks.js, compare titles ignoring case: "Book the meeting room" and "book the MEETING room" count as the same title.
In POST, after the schema check passes: if any task already has the title, use sendError to respond with status 409, and add nothing:
```
{ "error": { "code": "conflict", "message": "A task with this title already exists" } }
```
Do the same in PATCH (only when its body sends a title) and in PUT. In these two routes, leave out the task being changed when you search, so a task may keep its own title or change only its capitals. A refused PATCH or PUT changes nothing.

Try it: POST { "title": "book the meeting room" } to /api/tasks: you get 409 with code conflict, because task 2 is called "Book the meeting room". PATCH /api/tasks/3 with { "title": "Write the Project Brief" }: 409, and GET /api/tasks/3 still shows "Send the invoice". PUT /api/tasks/2 with { "title": "Book the meeting room", "done": true }: 200, because that is task 2's own title. POST { "title": "Order new chairs" }: 201.

Try it yourself

import { Router } from 'express';
import { z } from 'zod';
import { sendError, sendInvalid } from '../errors.js';
import { nextId, tasks } from '../store.js';

const router = Router();

// A whole task, as PUT sends it. .strict() refuses fields it does not list.
const Task = z
    .object({
        title: z.string().trim().min(1).max(100),
        done: z.boolean(),
    })
    .strict();
const NewTask = Task.partial({ done: true });
const TaskChanges = Task.partial();

router.get('/', (req, res) => {
    res.json(tasks);
});

router.get('/:id', (req, res) => {
    const task = tasks.find((t) => t.id === Number(req.params.id));
    if (!task) {
        return sendError(res, 404, 'not_found', 'Task not found');
    }
    res.json(task);
});

router.post('/', (req, res) => {
    const result = NewTask.safeParse(req.body);
    if (!result.success) {
        return sendInvalid(res, result.error);
    }
    const { title, done = false } = result.data;
    const task = { id: nextId(), title, done, createdAt: new Date().toISOString() };
    tasks.push(task);
    res.status(201).location(`/api/tasks/${task.id}`).json(task);
});

router.patch('/:id', (req, res) => {
    const task = tasks.find((t) => t.id === Number(req.params.id));
    if (!task) {
        return sendError(res, 404, 'not_found', 'Task not found');
    }
    const result = TaskChanges.safeParse(req.body);
    if (!result.success) {
        return sendInvalid(res, result.error);
    }
    Object.assign(task, result.data);
    res.json(task);
});

router.put('/:id', (req, res) => {
    const index = tasks.findIndex((t) => t.id === Number(req.params.id));
    if (index === -1) {
        return sendError(res, 404, 'not_found', 'Task not found');
    }
    const result = Task.safeParse(req.body);
    if (!result.success) {
        return sendInvalid(res, result.error);
    }
    const { id, createdAt } = tasks[index];
    tasks[index] = { id, ...result.data, createdAt };
    res.json(tasks[index]);
});

router.delete('/:id', (req, res) => {
    const index = tasks.findIndex((t) => t.id === Number(req.params.id));
    if (index === -1) {
        return sendError(res, 404, 'not_found', 'Task not found');
    }
    tasks.splice(index, 1);
    res.status(204).end();
});

export default router;