Code Smell 78 - Callback Hell

Processing an algorithm as a sequence of nested callbacks is not clever.

25 June, 2023

Contributors

Maxi Contieri

@mcsee

Processing an algorithm as a sequence of nested callbacks is not clever.

TL;DR: Don't process calls in a callback way. Write a sequence.

Problems

Readability
Hard to debug.
Complexity

Solutions

Change callbacks to sequence calls.
Extract repeated Code
Refactor.

Sample Code

Wrong

var fs = require('fs');

var fileWithData = '/hello.world';  
fs.readFile(fileWithData, 'utf8', function(err, txt) {  
    if (err) return console.log(err);

    txt = txt + '\n' + 'Add Data!';
    fs.writeFile(fileWithData, txt, function(err) {
        if(err) return console.log(err);
        console.log('Information added');
    });
});

Right

var fs = require('fs');

function logTextWasAdded(err) {  
    if(err) return console.log(err);
    console.log('Information added');
};

function addData(error, actualText) {  
    if (error) return console.log(error);

    actualText = actualText + '\n' + 'Add data';
    fs.writeFile(fileWithData, actualText, logTextWasAdded);
}

var fileWithData = 'hello.world';  
fs.readFile(fileWithData, 'utf8', addData);

Detection

This problem shines at the naked eye. Many linters can detect this complexity and warn us.

Conclusion

Callback Hell is a very common problem in programming languages with futures or promises.

Callbacks are added in an incremental way. There's no much mess at the beginning.

Complexity without refactoring makes them hard to read and debug.

Relations

Code Smell 06 - Too Clever Programmer

Code Smell 102 - Arrow Code

There are two ways to write code: write code so simple there are obviously no bugs in it, or write code so complex that there are no obvious bugs in it.

Tony Hoare

Software Engineering Great Quotes

This article is part of the CodeSmell Series.

How to Find the Stinky Parts of your Code