Martin Chaov chasing bits...

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React Hooks vs FP

With the release of React 16.8 we got the much anticipated hooks! And immediatelly the whole dev community lost their collective mind about how amazing the hooks are! How much more functional we can write our React components/apps... whatever they are writing.

I love functional programming. It makes me feel secure. I know this is not a property of the paradigm ... the code is as good as the developer who creates it. I've seen very bad functional code, and very good OOP code. The functional code I produce is very dense - which may be good or bad depending on your perspective. What makes me feel secure is that I follow some simple rules:

The reason I am putting this rant online is because of conversations I had with various senior developers around the subject. There seems to be a gross misunderstanding about what is FP (at least in the JavaScript community here in BG) and what is React. And how React is functional, and how it is not...

The main thing that got me to write this is a statement from a developer "they want to write only functional code, thus using component implementations with functions and hooks".

TL DR => this is not how functional programming works...

Here follows my take based on my experience with FP and React.

Let's start by evaluating the motivation for adding hooks

Hooks solve a wide variety of seemingly unconnected problems in React that we’ve encountered over five years of writing and maintaining tens of thousands of components. Whether you’re learning React, use it daily, or even prefer a different library with a similar component model, you might recognize some of these problems. - React documentation

Let's check what React docs have to say about it:

It’s hard to reuse stateful logic between components

No it is not... It was never hard... However, it requires thought and design upfront. State as implementation was never mandatory part of the component class or functional based. Most sensible devs I know of, did create the state outside of the React context and passed it as props. And that's all... it is easy to write, easy to maintain and extend, easy to share and test... React is not an application framework - this is not under it's domain of responsibilities.

React doesn’t offer a way to “attach” reusable behavior to a component (for example, connecting it to a store).

Yes, that is not the job of the library to provide... React should remain focused on the presentation. The library should not handle state that is outside of what is related to the component rendering. All the rest should be external and provided as props. And please don't get me started on the context API...

Complex components become hard to understand

Yes - a 300 line class/function/method/component is difficult to understand, so what? How do hooks solve this? How is a 300 line hook easier to understand than anything else with similar LoK.

The authors claim that class methods such as: componentDidMount, componentDidUpdate, componentWillUnmount; can be confusing and scary. So they replaced them with ONE function with one hell of a signature...

The abomination I am talking about is called useEffect and I don't want to copy/paste one hundred sentences so I will just shortlist what this function does:

{SARCASM ALERT} If you are sensitive to sarcasm, you might want to read someone else's blog...

So from the above I see most of the software complexity problems encapsulated into the behavior of one function. So far hooks seem like a hack that someone needed to avoid re-designing their application. And that is fine - I've done similar things out of necessity as well. What I don't get is how can someone still call their components "functional" after inserting all of this stateful, mutable, messy, side-effecty code. Components using hooks are not functional, they are procedural. One of the key differences between a procedure and a function (by definitions) is that the latter can't be affected by it's environment.

Also proceeding with the following note:

If you use this optimization, make sure the array includes all values from the component scope (such as props and state) that change over time and that are used by the effect. Otherwise, your code will reference stale values from previous renders. Learn more about how to deal with functions and what to do when the array values change too often.

This sentence is super strange and points me to a problem in the internal logic of how hooks are executed.

If you want to run an effect and clean it up only once (on mount and unmount), you can pass an empty array ([]) as a second argument. This tells React that your effect doesn’t depend on any values from props or state, so it never needs to re-run. This isn’t handled as a special case — it follows directly from how the dependencies array always works.

Okay, so -> array of values is one thing. Array of no values is another thing... and missing array is third thing... NICE!

If you pass an empty array ([]), the props and state inside the effect will always have their initial values. While passing [] as the second argument is closer to the familiar componentDidMount and componentWillUnmount mental model, there are usually better solutions to avoid re-running effects too often. Also, don’t forget that React defers running useEffect until after the browser has painted, so doing extra work is less of a problem.

So ... reference types behave slightly differently inside useEffect. Cool, good to remember this.

We recommend using the exhaustive-deps rule as part of our eslint-plugin-react-hooks package. It warns when dependencies are specified incorrectly and suggests a fix.

No linter can save you from this, mate!

That seems like a lot to keep in mind for just one of the hooks. Could it be that some SoC is going to clear up things here?

I am not going to list every single hook and strange API decision. The point of this rant is the departure from my beloved FP towards some sadomasochistic melange of declarative and imperative programming.

Classes confuse both people and machines

No they don't ... 17 layers of inheritance confuse people and machines. Using classes as the blueprint for objects without more than one level depth and favouring composition is neither confusing nor complicated.

Classes are a very powerful metaphor and tend to encapsulate a lot of meaning behind a single word. This is why they are so prominent in the UI implementations.

It is hard for me to comprehend what is more complicated about classes compared to having one function that does everything. BTW functions organised in a file with only partial visibility (a module if you will) is also sort of a "class". Or sort of a namespace. Or sort of an object. The only reason we call them different names is because we all aggreed what we mean when we use these words.

A simple example of the Foo.ts:

export class Foo {
    private static state = {}

    constructor() { }

    private static doPrivateStuff() { }

    static doStuff() { }

// is no different than this:

const state = {}

function doPrivateStuff() { }

export function init() { }

export function doStuff() { }

// or that as a matter of fact this:

const state = {}

function doPrivateStuff() { }

export const foo = {
    init() { }
    doStuff() { }

I've used all of these in different situations, but most importantly my decision depends on how the code is going to be imported and used. All of them have subtle differences when you look beyound the import/export and I will probably write a post about it at some point in time.

Rules of hooks

Rules masterfully copy/pasted text from the original documentation.

Hooks are JavaScript functions, but you need to follow two rules when using them. We provide a linter plugin to enforce these rules automatically...

Only Call Hooks at the Top Level

As the documentation states we must not call hooks inside:

Instead, always use Hooks at the top level of your React function. By following this rule, you ensure that Hooks are called in the same order each time a component renders. That’s what allows React to correctly preserve the state of Hooks between multiple useState and useEffect calls. (If you’re curious, we’ll explain this in depth below.)

Wait, wait... WHAT!? Order of execution, limitation of where and how you write them inside the function body? This smells of a bad abstraction. A very leaky one. If the developer needs to understand the low level implementation of the hooks in order to use them correctly, it simply means the whole design of this solution is messed up.

Hm ... maybe if the functions that are returning objects could have a slighlty different syntax that allows for like "construction" logic to be separated from the rest of the function body... So, call them like in a constructor or something?

Only Call Hooks from React Functions

Don’t call Hooks from regular JavaScript functions. Instead, you can:

By following this rule, you ensure that all stateful logic in a component is clearly visible from its source code.

So much for logic reuse, test, inject... This contradicts the earlier statement that "Hooks are JavaScript functions...". Functions in JS can be called inside other functions that are not bound to a specific library.


There is the explanation given for the rules.

My arguments against hooks comes not from missunderstanding of the developers' arguments. Neither they are related to me not writing hooks. Or being unwilling to do so.

This is a solution with obvious drawbacks and limitations. The more you use it, the more your components will become impure. I like the clear separation between functional and class components.

It is easy to understand - when the component is class based it encapsulates state. A functional component on the other hand used to be referentially transparent.

Just as an example - passing the hooks as params is also not going to solve the issues I am talking about.


I do understand hooks value, especially when you need to hack-off a bad design and frankestain something to make it work. I have also hit my fair share of edge cases over the past 3-4 years using React on production with both class and functional components.

However, in my work I am making a clear separation between stateful and pure React components. Stateful = class; Pure = function.

Functional code is deterministic and follows specific rules. You can't just write a function put a ton of side-effects inside and call it "functional".

If you expected me to give a detailed solution - your expectation are not going to be met. I needed to vent my frustration from the direction this library is taking. Making the API more and more complex, instead of focusing on more modular opt-in approach. Confusing FP newbies onto what is FP.

The easiest solution is for your entire state to be constructed outside of the world of React and pass it as props. This requires React to be a part of your application and not your application shell...

The addition of the context and hooks APIs is a warning sign to me. When I work with React I tend to isolate it down to the level of implementation detail responsible for the UI. All my logic and state is extracted into service like software components. I use both classes and functions ... Class/arguments/method/propery decorators are super useful and make the code expressive and easier to read. In short - I do applications that use React as one of many components. I don't do React applications!