Try It Yourself - SyntaxStudy Editor

HTML / CSS / JS

const fs = require('fs');

// --- Callback style (old way) ---
fs.readFile('data.txt', 'utf8', (err, data) => {
    if (err) {
        console.error('Error:', err);
        return;
    }
    fs.writeFile('output.txt', data.toUpperCase(), (err) => {
        if (err) console.error(err);
        else console.log('Done writing');
    });
});

// --- Promise style ---
function readFileProm(path) {
    return new Promise((resolve, reject) => {
        fs.readFile(path, 'utf8', (err, data) => {
            if (err) reject(err);
            else resolve(data);
        });
    });
}

readFileProm('data.txt')
    .then(data => data.toUpperCase())
    .then(upper => console.log(upper))
    .catch(err => console.error('Promise rejected:', err));

// --- Promise.all — run multiple promises in parallel ---
const p1 = readFileProm('file1.txt');
const p2 = readFileProm('file2.txt');

Promise.all([p1, p2])
    .then(([content1, content2]) => {
        console.log('Both files loaded');
    })
    .catch(err => console.error('One failed:', err));