Concurrency

Use Promises, not callbacks

Callbacks aren’t clean, and they cause excessive amounts of nesting. With ES2015/ES6,
Promises are a built-in global type. Use them!

Bad:

import { get } from 'request';
import { writeFile } from 'fs';
get('https://en.wikipedia.org/wiki/Robert_Cecil_Martin', (requestErr, response) => {
  if (requestErr) {
    console.error(requestErr);
  } else {
    writeFile('article.html', response.body, (writeErr) => {
      if (writeErr) {
        console.error(writeErr);
      } else {
        console.log('File written');
      }
    });
  }
});

Good:

import { get } from 'request';
import { writeFile } from 'fs';
get('https://en.wikipedia.org/wiki/Robert_Cecil_Martin')
  .then((response) => {
    return writeFile('article.html', response);
  })
  .then(() => {
    console.log('File written');
  })
  .catch((err) => {
    console.error(err);
  });

Async/Await are even cleaner than Promises

Promises are a very clean alternative to callbacks, but ES2017/ES8 brings async and await
which offer an even cleaner solution. All you need is a function that is prefixed
in an async keyword, and then you can write your logic imperatively without
a then chain of functions. Use this if you can take advantage of ES2017/ES8 features
today!

Bad:

import { get } from 'request-promise';
import { writeFile } from 'fs-promise';
get('https://en.wikipedia.org/wiki/Robert_Cecil_Martin')
  .then((response) => {
    return writeFile('article.html', response);
  })
  .then(() => {
    console.log('File written');
  })
  .catch((err) => {
    console.error(err);
  });

Good:

import { get } from 'request-promise';
import { writeFile } from 'fs-promise';
async function getCleanCodeArticle() {
  try {
    const response = await get('https://en.wikipedia.org/wiki/Robert_Cecil_Martin');
    await writeFile('article.html', response);
    console.log('File written');
  } catch(err) {
    console.error(err);
  }
}