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Rediscovering JavaScript: ES6, ES7 & ES8

No Thanks to Callback Hell

Understand the core problems of callback-based asynchronous programming in JavaScript, such as inconsistent error handling, parameter order confusion, and difficult code extension. This lesson explains why callbacks lead to callback hell and prepares you to discover how promises can offer a cleaner solution.

We'll cover the following...

Any method that does not return instantaneously should be asynchronous.

Callbacks in asynchronous programming

Traditionally, designing asynchronous functions relied on callbacks. Let’s take a look at a small example and discuss the issues with this approach.

Node.js
'use strict';
  
//START:DEFINE
const fs = require('fs');
const displayFileContent = function(pathToFile) {
  const handleFile = function(err, contents) {
    if(err) {
      console.log(err.message);
    } else {
      console.log(contents.toString());
    }
  };
  try {
    fs.readFile(pathToFile, handleFile);    
  } catch(ex) {
    console.log(ex.message);
  }
};
//END:DEFINE
 
//START:GOODCALL
displayFileContent('readfile.js');
//END:GOODCALL
//START:INVALID_FILE
displayFileContent('does-not-exits');
//END:INVALID_FILE
//START:NOFILE
displayFileContent();
//END:NOFILE

Explanation

First, we bring in the fs library, which provides both synchronous and asynchronous functions to read files.

📍 The require() method is used to load JavaScript modules. ...

Issues with