Callbacks

Part of the Node.js Basics section of Coddy's Node.js journey. Lesson 14 of 24.

Reading a large file takes time, and a program that sits waiting for it does nothing else meanwhile.

fs.readFile(path, 'utf8', callback) starts the read and returns at once. When the file is loaded, Node calls your callback with (err, text). err is null on success and an error otherwise, so check it first. Everything that needs the text goes inside the callback.

import fs from 'node:fs';

fs.readFile('poem.txt', 'utf8', (err, text) => {
    if (err) {
        console.log('No poem today');
        return;
    }
    console.log(text.split('\n').length + ' lines');
});
console.log('Asked for the poem');

Asked for the poem prints first: the line after readFile runs while the file is still loading.

Challenge

Easy

count.js prints Reading..., then <n> notes from inside the callback, then Done, also from the callback.
When the file cannot be read, it prints Cannot read <file> instead of the count.

Run node count.js, then node count.js nothere.json.

Try it yourself

import fs from 'node:fs';

const file = process.argv[2] ?? 'notes.json';

console.log('Reading...');
// Read the file with fs.readFile and a callback.
// Print the number of notes, then Done.