JAVASCRIPT

fs.writeFileSync in nodeJs

//to import nodejs module fs
var fs = require('fs');

//first lets check if the file exists in the same folder 
if (fs.existsSync('mynewfile.txt')){ //to check the file in the other directories please change the path of the file from "mynewfile.txt" to the path of your file
  console.log('The file exists.'); //instead of consoling you can also remove or modify the file

}else{ //this will be exevuted if the files doesn't exist
    //this will create a file in the same directory
fs.writeFileSync('mynewfile.txt', 'Hello content!', function (err) { //to create the file in the other directories please change the path of the file from "mynewfile.txt" to the path of your file
    if (err){ throw err;  //if any error happens then this will through the detailed error
    }else{
    console.log('Saved!'); //if no error found then this code will be executed. instead of consoling you can do other stuff like calling any other function etc
    }
  });

}

fs.appendFileSync in nodeJs

var fs = require('fs');

try {
  fs.appendFileSync('message.txt', 'data to append');
  console.log('The "data to append" was appended to file!');
} catch (err) {
  /* Handle the error */
}