Filesystem

const fs = require('fs');

fs.readFile('./file.txt', 'utf8', (err, data) => {
  console.log(err ? err : data);
})

Read line by line

const readline = require('readline');
const fs = require('fs');

// creates an object with the input stream
const myInterface = readline.createInterface({
  input: fs.createReadStream('text.txt')
});
 

// reads line by line
myInterface.on('line', (fileLine) => {
  console.log(`The line read: ${fileLine}`);
});

Write

const fs = require('fs')
 
const fileStream = fs.createWriteStream('output.txt');
 
fileStream.write('This is the first line!'); 
fileStream.write('This is the second line!');
fileStream.end();