Debug Driven Development

Published on Jun 14, 2022

You've likely heard of Test Driven Development, or maybe even Behavrioal Driven Development. I have a new phrase: Debug Driven Development(™).

What is Debug Driven Development

Consider the following two code samples. Which would you rather face when locating a bug or making an update for a new feature. Which would you prefer to leave for your future self or coworker.

// Option 1.

export const getNameFrequency = (myObj) => {
    return myObj["prop"].map(res =>, curr) => {

        const value = acc[curr];

        return {
                [curr]: value !== undefined ?
                    value + 1 :

    }, {});

// --------------------------------------------------------

// Option 2.

export const getNameFrequency = (myObj) => {

    const users = myObj["prop"]
    const userNames = =>;
    const userNamesToFrequency = userNames.reduce((acc, curr) => {

        const value = acc[curr];
        const currentCount = value !== undefined ?
            value + 1 :

        const entry = {
            [curr]: currentCount

        const updatedAccumulator = {

        return updatedAccumulator;

    }, {});

    return userNamesToFrequency;

The first snippet is readable but I wouldn't call it simple. The logic is straightforward and you figure out we're counting the frequency of the keys of a given object pretty quickly but I think you'll agree one is far easier to read (and work in) than the other.

For any work in one of the snippets, you'll likely use some if not all of the following techniques.

Regardless of which method we use, we'll want to inspect the intermediate state of our data at various points. Seeing the intermediate transformations of our data is essential to narrow in on a bug or validate a change. Debug Driven Development optimizes writing code to provide opportunities for future debugging and maintenance.

Hopefully you're building up an intuition for debug driven development.

Is Debug Driven Development Really a New Idea?

I don't know but I haven't heard of it before though I'm sure some of the ideas in here have been codified and are part of other strategies for writing maintainable code (perhaps in Clean Code or similar places).

Debug Driven Development might sound similar to KISS: Keep It Simple Stupid. The two are related, but Debug Driven Development goes further. Phrases like KISS (and I'd argue Clean Code too) focus on readability whereas Debug Driven Development focuses on writing code so that it is as easy to debug as possible.

This principle manifests most concretely by storing intermediate states of computation in their own variables, avoiding nested logic, and avoiding inlining function calls. Avoiding these styles makes it easy to toss in a console.log, add a breakpoint between two function calls and modify existing code as part of new requirements.

// Nesting of logic:

const data = myObj[!!key ? key : 'fallbackKey'];

const config = {
	name: name > 5 ? name.slice(0, 5) : name,

// Inlining Function Calls

const config = {,
	name: userDataIsAvailableFromSession(session) ?
		getUserNameFromSession(session) :

These examples above are mostly obvious but I'd be surprised if you couldn't find similar ones in any project you work in regularly (and please, these are for illustration - writing fake code is hard!).

Another obvious example where this can manifest is Array method chains (and broadly chainable method calls on any object). Of the two examples below, which would be easier to debug or modify? If you say the 2nd one, well, I'm at least thankful you're still reading.

You can imagine the filter predicates contain more gnarly conditionals than these trivial examples but these examples are enough to convey the broader point.

// Example 1

const myArr = getArrayOfData();

const nonEmptyValues = myArr.filter(entry => {
	const isNotEmpty = entry !== null && entry !== undefined;
	return isNotEmpty;

const lengthNames = nonEmptyValues.filter(entry => {
	const entryIsLongEnough = > 5;
	return entryIsLongEnough;

const over21 = lengthNames.filter(entry => {
	const entryIsOldEnought = entry.age > 21
	return entryIsOldEnought;

// --------------------------------------------------------

// Example 2

const filteredResults = getArrayOfData().filter(entry => {
    return entry !== null && entry !== undefined && > 5 &&
		entry.age 21

You're probably bulking at the 3 repeated loops in the first example and just begging to refactor it to be closer to the 2nd example. While the 2 extra loops is certainly not great, the first is more debuggable than the second. If there were an issue with the end result, it would be due to one of the conditions being off. Having three separate filter calls allows us to easily go in and inspect which predicates(s) are wrong at any intermediate step.

There is a middle ground of course, though it still fails to allow us to inspect the value of the entire array between each predicate.

const myArr = getArrayOfData();

const filteredResults = myArr.filter(entry => {
    const isNotEmpty = entry !== null && entry !== undefined
    const entryIsLongEnough = > 5
    const isOver21 = entry.age 21

    return isNotEmpty && entryIsLongEnough && isOver21;

So which do we use? The middle ground option or the triple loop. Does the array have under 1,000 values? If so I'd probably opt for the 3 filter calls, otherwise I may select the more optimized approach.

It doesn't matter which you choose (to a degree - measure performance and adapt code as needed), so long as you make your choice debuggable.

Other Forms of Debug Driven Development

Depending on the language or tools you're using another form of debug driven development manifests in a type system! Types help make your code debuggable especially when abiding by the make impossible states impossible principle by designing your types so that illegal states are not representable in your type system.

Fundamentally types are powerful as they let us embed logic that is verified and provable by a compiler. Debugging an error when working in a typed project is significantly easier and can save you a few console.log and debugger; cycles to try and understand the exact shape of your data, a function call's arguments, or a class instance.

And that's Debug Driven Development!

Wrapping Up

Debug Driven Development is compatible with other methodologies like TDD, BDD, or Clean Code. We do aim for composition oriented solutions as engineers after all.

Focusing on any code writing strategy will often include partly focusing on the others. Debug Driven Development is just another tool to help write better code.