Use the node v8 promisify API to remove unnecessary code

[addityasingh]

We can remove some unnecessary explicit promisify code by using the v8 utils.promisify API (https://nodejs.org/en/blog/release/v8.0.0/#improved-support-for-promises).

E.g.: This function can be changed to a much elegant and declarative implementation.

const readFile = (path) => 
    new Promise((resolve, reject) => {
        fs.readFile(path, 'utf-8', (err, data) => {
            if (err) {
                reject(new TemplateError(err));
                return;
            } 
            resolve(data);
        });
    });

to

const util = require('util');

const readFile = (path) => 
    util.promisify(fs.readFile)(path)
        .catch(err => new TemplateError(err));

[vigneshshanmugam]

Yup nice idea, but we need to polyfill it in lower versions. https://github.com/ljharb/util.promisify

[addityasingh]

Agreed. I will use this to have a polyfill for node 6.0 - node < v8.0

[dazld]

should promisfy once and use the returned function instead of doing it on every invocation.

[dazld]

ie:

const readFileP = util.promisify(fs.readFile); // or whatever

const readFile = (path) => readFileP(path).catch(err => new TemplateError(err));