Sometimes, you might want to convert a JavaScript function that accepts a callback to one that returns a Promiseobject. This lesson shows how to manually wrap a promise-based API around the fs.readFile() function. It also explains how to use the util.promisify() method that is built into the Node.js standard library.

const fs = require('fs')

function readFile(path, encoding) {

  return new Promise((resolve, reject) => {

    fs.readFile(path, encoding, (error, contents) => {

      if (error) {

        reject(error)

      } else {

        resolve(contents)

      }

    })

  })

}

readFile(__filename, "utf8")

.then((contents) => {

  console.log(contents)

}, error => {

  console.error(error)

})

const fs = require("fs");

const util = require("util");

const readFile = util.promisify(fs.readFile);

readFile(__filename, "utf8").then(

  contents => {

    console.log(contents);

  },

  error => {

    console.error(error);

  }

);