JAVASCRIPT

fs.writefile

const fs = require('fs');

fs.writeFile("/tmp/test", "Hey there!", function(err) {
    if(err) {
        return console.log(err);
    }
    console.log("The file was saved!");
}); 

// Or
fs.writeFileSync('/tmp/test-sync', 'Hey there!');

nodejs: read and write file: use fs and promise

const { readFile, writeFile} = require('fs').promises;

const start = async () => {
    try {
        const first = await readFile('./content/first.txt', 'utf8');
        const second = await readFile('./content/second.txt', 'utf8');

        // Write file
        await writeFile('./content/result_mind_grenade.txt', `THIS IS AWESOME : ${first}  ${second}`, { flag: 'a' });
        
        console.log(first); 
        console.log(second);
    } catch (error) {
        console.log(error);
    }
}

start();