dumb_promise_example.js

import fs from 'fs';

class FSError extends Error {}

const readFile = (...args) => 
  new Promise((resolve, reject) => {
    fs.readFile(...args, (err, data) => {
      if (err) {
        return reject(new FSError(err));
      }
      return resolve(data);
    })
  })

readFile('./README.md')
  .then(content => console.log('content'))
  .catch((err) => {
    if (err instanceof FSError) {
      console.log('oops. too bad', err);
      return;
    }

    throw err;
  });

  

output

> readFile('./README.md').then(content => console.log('content')).catch((err) => {
  if (err instanceof FSError) {
    console.log('oops. too bad', err);
    return;
  }

  throw err;
});
Promise {
  <pending>,
  domain:
   Domain {
     domain: null,
     _events: { error: [Function: debugDomainError] },
     _eventsCount: 1,
     _maxListeners: undefined,
     members: [] } }
> oops. too bad Error: Error: ENOENT: no such file or directory, open './README.md'
    at ReadFileContext.fs.readFile [as callback] (repl:5:23)
    at FSReqWrap.readFileAfterOpen [as oncomplete] (fs.js:421:13)

> readFile(null).then(content => console.log('content')).catch((err) => {
  if (err instanceof FSError) {
    console.log('oops. too bad', err);
    return;
  }

  throw err;
});
Promise {
  <pending>,
  domain:
   Domain {
     domain: null,
     _events: { error: [Function: debugDomainError] },
     _eventsCount: 1,
     _maxListeners: undefined,
     members: [] } }
> (node:97) UnhandledPromiseRejectionWarning: Unhandled promise rejection (rejection id: 3): TypeError: path must be a string or Buffe