Async and Await

Part of the Node.js Basics section of Coddy's Node.js journey. Lesson 16 of 24.

A chain of .then calls reads differently from how you think about the steps: read, then parse, then save.

Inside an async function, await pauses until a promise settles and gives you its value. A failed promise throws there, so try and catch handle it. In a module, await also works at the top level.

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

const loadConfig = async () => {
    try {
        return JSON.parse(await readFile('config.json', 'utf8'));
    } catch {
        return {};
    }
};

const config = await loadConfig();
console.log(config.port ?? 3000);

An async function always returns a promise, so its caller awaits it too. Forget an await and you get the promise itself, Promise { <pending> }, instead of its value.

Challenge

Easy

store.js now reads and writes with node:fs/promises. Add the missing awaits so every command works as before.
notes.js awaits store.add and store.remove, which are async now.

Run node notes.js list first to read the error, then add and done 1 once it works.

Try it yourself

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

const file = 'notes.json';

// readFile and writeFile now return promises. Wait for each with await.
const load = async () => {
    if (!existsSync(file)) {
        return [];
    }
    try {
        return JSON.parse(readFile(file, 'utf8'));
    } catch (err) {
        console.error(`notes.json is damaged: ${err.message}`);
        process.exit(1);
    }
};

const store = {
    notes: load(),
    async save() {
        writeFile(file, JSON.stringify(this.notes, null, 2));
    },
    async add(text) {
        this.notes.push(text);
        this.save();
        return this.notes.length;
    },
    async remove(n) {
        const [text] = this.notes.splice(n - 1, 1);
        this.save();
        return text;
    },
};

export default store;