Use callback function from .then

index.js

const {promisify} = require('util');
const fs = require('fs');
const readFileAsync = promisify(fs.readFile);

const filePath = 'index.js';
readFileAsync(filePath, {encoding: 'utf8'}).then(function (text) {
  console.log(text);
}).catch(function (error) {
  console.log(error);
});