Sorting

Part of the REST APIs section of Coddy's Node.js journey. Lesson 16 of 25.

A client wants the newest tasks first, or the list from A to Z. A common convention is ?sort=field, with a - in front to reverse:

// GET /api/books?sort=-year
const field = sort.startsWith('-') ? sort.slice(1) : sort;   // 'year'
const direction = sort.startsWith('-') ? -1 : 1;
list = list.toSorted((a, b) => (a[field] - b[field]) * direction);

Check field against the fields you allow first and answer 400 for any other, since a field books do not have gives no useful order. toSorted returns a sorted copy. sort reorders the array itself, and when no filter made a copy, that array is the store: every later request sees the new order. This example subtracts, which works for numbers only. To compare two strings, use a.localeCompare(b): like the subtraction, it returns a negative number, 0 or a positive number. ISO dates are strings that sort in time order.

Challenge

Easy

Replace the title-only sort you wrote in Sorting Text with the field and direction handling below.

Clients want the newest tasks first, or the list from A to Z. Let the list sort itself with ?sort=.

In routes/tasks.js, in the list route, read sort from req.query. Without it, the list keeps its stored order.
?sort=title sorts by title from A to Z. ?sort=createdAt sorts by creation time, oldest first. A - in front reverses the order: ?sort=-title is Z to A, and ?sort=-createdAt is newest first.
Both fields are text. createdAt is an ISO date like "2026-03-03T10:30:00.000Z", which sorts correctly as text. So compare with localeCompare, not by subtracting.

Any other field, like ?sort=color, gets status 400:

{ "error": { "code": "invalid_query", "message": "sort must be title or createdAt, with - in front to reverse" } }

Sort after the filters, so ?done=false&sort=-title returns the open tasks from Z to A.
Sort a copy with toSorted. sort reorders the stored array itself, and every later request would see the new order.

Try it: GET /api/tasks?sort=title returns tasks 2, 3, 1 ("Book...", "Send...", "Write..."). GET /api/tasks?sort=-createdAt returns 3, 2, 1. Then plain GET /api/tasks returns 1, 2, 3 again. GET /api/tasks?sort=color returns 400 with code invalid_query.

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();

// True when another task already has this title, in any case.
const titleTaken = (title, exceptId) =>
    tasks.some((t) => t.id !== exceptId && t.title.toLowerCase() === title.toLowerCase());

router.get('/', (req, res) => {
    const { done, q } = req.query;
    let list = tasks;
    if (done !== undefined) {
        if (done !== 'true' && done !== 'false') {
            return sendError(res, 400, 'invalid_query', 'done must be true or false');
        }
        list = list.filter((t) => t.done === (done === 'true'));
    }
    if (q !== undefined) {
        const words = String(q).toLowerCase();
        list = list.filter((t) => t.title.toLowerCase().includes(words));
    }
    if (req.query.sort === 'title') {
        list = list.toSorted((a, b) => a.title.localeCompare(b.title));
    }
    res.json(list);
});

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;
    if (titleTaken(title)) {
        return sendError(res, 409, 'conflict', 'A task with this title already exists');
    }
    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);
    }
    if (result.data.title !== undefined && titleTaken(result.data.title, task.id)) {
        return sendError(res, 409, 'conflict', 'A task with this title already exists');
    }
    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];
    if (titleTaken(result.data.title, id)) {
        return sendError(res, 409, 'conflict', 'A task with this title already exists');
    }
    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;