Callback Hell example

main.js

const fs = require('fs');
const file = 'sample.txt';

// Execution Starts here
// We just want to read some data from a file, update it, write it back and read it again.
// This is what it will look like with callbacks - welcome to the Callback Hell!
read(file, data => {
    console.log('READ-1:', data);

    // Now we'll change the text.
    const updatedData = `${data} - This has been updated!`;

    // ...and update the same file.
    write(file, updatedData, () => {
        console.log('WRITE-1:', updatedData);

        read(file, data => {
            // Assume we'll be doing something different with the data this time.
            // For now, let's just console.log it again.
            console.log('READ-2:', data);
        });
    });
});

// This function writes the 'content' to the FILE
// and executes a callback after the write is completed.
function write(file, content, callback) {
  fs.writeFile(file, content, err => {
    if (err) console.log('Error:', err);
    callback();
  });
}

// This function reads from the file and
// passes the data into a callback function
function read(file, callback) {
  fs.readFile(file, 'utf8', (err, data) => {
    if (err) console.log('Error:', err);
    callback(data);
  })
}