Is this the correct way to wrap readFileSync in a Promise

Question

The reason for the below code is to get rid of callback hell/pyramid of doom. I don't fully understand i/o blocking though yet.

'use strict';

var fs = require('fs');
var co = require('co');

co(function* () {

    var fileName = 'readme.txt';

    var str =
        yield new Promise(function (resolve, reject) {
            var result;
            try {
                result = fs.readFileSync(fileName, 'utf8');
            } catch (err) {
                reject(err);
            }
            resolve(result);
        });
    console.log('result readFileSync: ' + str);

});

All I'm expecting is a yes or no answer to be honest. Hope fully if no could someone give some details as I'm trying to learn properly about JavaScript sync/async and how to harness the power of Promises.

Answer 1

Short answer

No

Useful answer

If you want to wrap a file read operation, try to use the async versions of Node functions as much as possible. Using readFileSync with a promise gives you no advantage over using readFileSync on its own, because readFileSync blocks the process until it is done reading, which readFile does not.

A better solution would therefore be something like this:

'use strict';

var fs = require('fs');

var readFilePromise = function(file) {
  return new Promise(function(ok, notOk) {
    fs.readFile(file, function(err, data) {
        if (err) {
          notOk(err)
        } else {
          ok(data)
        }
    })
  })
}

readFilePromise('/etc/passwd').then(function(data) {
  // do something with the data...
})

Answer 2

The correct way is to use the Node's native util library and promisfy fs.readFile:

const path = require('path');
const fs = require('fs');

const INDEX = path.join(__dirname, 'app', 'index.html');
const readFile = require('util').promisify(fs.readFile);

readFile(INDEX)
  .then(e => console.log(e.toString()))
  .catch(e => console.log('FOOBAR ' + e));

---

result:

one@dolphin:~/github/resume $ node toolchain/inject.js 
FOOBAR Error: ENOENT: no such file or directory, open '/home/one/github/resume/toolchain/app/index.html'