Async Routes

Part of the Express Basics section of Coddy's Node.js journey. Lesson 21 of 23.

An array is gone when the server restarts. A file keeps the data.

import { readFile } from 'node:fs/promises';

app.get('/api/forecast', async (req, res) => {
    const text = await readFile('data/forecast.json', 'utf8');
    res.json(JSON.parse(text));
});

A route can be an async function that waits with await, here for readFile from node:fs/promises. Express 5 watches the promise the route returns: if it rejects, because the file is missing or its JSON is broken, the error goes to your error handler, like a throw.

Without await you hold a promise instead of the data, and res.json(promise) sends {}. The new data/recipes.json holds the four recipes.

Challenge

Easy

An array in memory is lost every time the server restarts. Keep the recipes in a file instead: data/recipes.json, which already holds the four recipes.

In routes/recipes.js, import readFile and writeFile from node:fs/promises, and stop importing recipes from ../data.js.
Write one helper, loadRecipes(), an async function that reads data/recipes.json and returns the parsed list. The next lessons use it by that name.
Make every route in the file async, and start each one by awaiting loadRecipes(): /tags, the list, one recipe, the POST and the scale route.
In the POST route, after adding the new recipe to the list, write the whole list back to data/recipes.json with writeFile, as JSON text.
The scale route still throws for a recipe without servings, now from inside an async function. Express 5 passes that error to your error handler as before, so it still responds with 500 and {"error": "Something went wrong"}. Do not catch it in the route.

Try it: Press Run and send /api/recipes: the same four recipes, now read from the file. Pick POST and send {"title": "Gazpacho", "tags": ["vegan", "soup"]} to /api/recipes: status 201 and the recipe with "id": 5. Send /api/recipes/5: Gazpacho. Send /api/recipes/99: status 404. Then POST {"title": "Toast"}, which has no servings and becomes recipe 6, and send /api/recipes/6/scale?by=2: status 500 and {"error":"Something went wrong"}. If you see {} instead of your data, an await is missing.

Try it yourself

import express from 'express';
import { recipes } from '../data.js';

const router = express.Router();

// Every tag, once, in alphabetical order.
router.get('/tags', (req, res) => {
    const tags = new Set(recipes.flatMap((recipe) => recipe.tags));
    res.json([...tags].sort());
});

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

router.get('/:id', (req, res) => {
    const recipe = recipes.find((r) => r.id === Number(req.params.id));
    if (!recipe) {
        res.status(404).json({ error: 'No such recipe' });
        return;
    }
    res.json(recipe);
});

router.post('/', (req, res) => {
    const { title, tags } = req.body;
    if (!title) {
        res.status(400).json({ error: 'A recipe needs a title' });
        return;
    }
    const recipe = { id: recipes.length + 1, title, tags: tags ?? [] };
    recipes.push(recipe);
    res.status(201).json(recipe);
});

router.get('/:id/scale', (req, res) => {
    const recipe = recipes.find((r) => r.id === Number(req.params.id));
    if (!recipe) {
        res.status(404).json({ error: 'No such recipe' });
        return;
    }
    const by = Number(req.query.by);
    if (!Number.isFinite(by) || by <= 0) {
        res.status(400).json({ error: 'Scale by a positive number' });
        return;
    }
    if (!recipe.servings) {
        throw new Error(`Recipe ${recipe.id} has no servings`);
    }
    res.json({ title: recipe.title, servings: recipe.servings * by });
});

export default router;