[Javascript] Convert a Callback-Based JavaScript Function to a Promise-Based One

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)
})

In nodejs we can use util function:

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);
  }
);