fs write file on complete code example

Example 1: data not write in file node js

const fs = require('fs');

fs.readFile('file.txt', 'utf-8', (err, data) => {
    if(err) {
        throw err;
    }
    console.log(data);
});

Example 2: how to right plain text format file in node js

var fs = require('fs')
fs.appendFile('log.txt', 'new data', function (err) {
  if (err) {
    // append failed
  } else {
    // done
  }
})