Composition and React

April 24, 2019

So let's start off with the first item in the title of this topic i.e. Composition.

What is it and why is it important?

It means putting different things together to create something bigger than the individual pieces. A good example of composition are the languages themselves, no not programming languages but our own languages.

How so?

letters put together form words
words put together form sentences
sentences put together to form a paragraph.

Do you get my drift? Just replace put with compose and you will get what I am getting at. Can we just randomly put together words to create sentences? No there are rules governing what makes a sentence i.e. grammar.

So let's try and define the term in the context of programming. So the general idea is basically taking one type of thing and combine them to create other types of things.

In programming languages, we have primitives like integers, strings, functions, objects and we combine them to produce software instead of letters, words, sentences.

So what is the corollary for grammar or rules in case of programming languages? Well at the most basic level its nothing but syntax which compilers enforce, unless you follow the syntax you will not get working software. Similar to how if we don't follow grammar you won't get a proper sentence.

Okay, fair enough but how's it important? Well because as humans there is only so much information one can keep in their heads so we need come up with different ways to logically group things and combine them so that we can build stuff more reliably.

So are there rules for composing software? I mentioned syntax earlier but that's barely enough to guide us to create compositional software.

There are no hardbound rules in software composition. The closest thing we have to rules are design patterns.

Programmers can misuse design patterns since:

  1. They are intuition driven
  2. Compilers can't enforce it
  3. Proper use requires some experience
  4. Can lead to wrong abstractions which are hard to adapt.

Some examples of design patterns:

  • Factory Pattern
  • Facade Pattern
  • Recursion
  • Dependency Injection
  • ...

Turns out logicians/mathematicians have researched this area and come up with laws. It's a topic which deserves more attention (another blog post maybe?) and we need to move on to React.

Composition in React

The best programmers are good at composition. The process of creating effective composition's looks something like this:

  1. Figure out the basic primitives in the problem domain.
  2. Use language primitives and design patterns to combine them to solve a given problem.
  3. Based on usage heuristics and changing requirements iterate on the abstractions.

Let's list out the primitives in React:

  • Perhaps the most important one and the most basic unit: Component
  • Context
  • The Lifecycle methods
  • State and Props
  • Suspense
  • Refs
  • Hooks!
  • Since we write React in good old JS we have at your disposal all that the language provides i.e. loops, arrays, stacks, generators, etc.

So as a React dev our job is basically to use the above in the best way possible to create an app!

The most basic units of composition in React are Component and the new kid on the block Hooks.

Let's look at some basic examples of composition in React.

Title component:

const Title = (props) => <h1>{props.title}</h1>;

Description component:

const Description = (props) => <p>{props.description}</p>;

We can combine the above two to create a new component:

const Card = props => (
    <Title title={props.title} />
    <Description description={props.description} />

// Usage

<Card title="Composition" description="jibber jabber" />

I think the above is a pretty straight forward way we use React day today and is a basic example of a composition in React.

Now the main part of the blog:

Effective Composition in React

I will do this in a before/after kind of pattern i.e. I will show one way of doing things and show why it's bad and show a better way of achieving the same thing:

The Simple Button :

function BadButton(props) {
  if (props.primary) {
    return <button className={`btn btn-primary`}>{props.children}</button>;

  if (props.secondary) {
    return <button className={`btn btn-secondary`}>{props.children}</button>;

  return null;

The above component which seems fairly simple and innocent can get bad very quickly, let's see how it could be used in practice:

    <BadButton primary /> // gives a primary button
    <BadButton secondary /> // gives a secondary button
    <BadButton primary secondary /> // gives what???

See what I mean, here the root cause is due to the fact that we are modeling the type of a button as a boolean and that quickly led to an invalid state.

No worries we can do better by doing this:

function GoodButton(props) {
  if (props.buttonType === "primary") {
    return <button className={`btn btn-primary`}>{props.children}</button>;
  if (props.buttonType === "secondary") {
    return <button className={`btn btn-secondary`}>{props.children}</button>;

  return null;

GoodButton.propTypes = {
  buttonType: PropTypes.oneOf(["primary", "secondary"]),

See? Just changing it to a simple enum removed that invalid state altogether (JavaScript doesn't have enums but by using a simple string and prop-types we can emulate it)

Let's take it a step forward using the above component :

function PrimaryButton(props) {
  const { buttonType, } = props;
  return <GoodButton buttonType="primary" {} />;

function SecondaryButton(props) {
  const { buttonType, } = props;
  return <GoodButton buttonType="secondary" {} />;

See what I did there? I used props to create new components! So what is so great about this? It hides away the implementation details of how PrimaryButton is created and the consumers don't have to worry about which props to pass to make it a PrimaryButton in the first place.

Say tomorrow your designer comes in and says that the PrimaryButton needs to have an italic text you can just go ahead add modify the code like this:

function PrimaryButton(props) {
  const { buttonType, } = props;
  return <GoodButton buttonType="primary" textStyle="itallic" {} />;

Thats it, the conumsers don't have to change anything!

Here's the codesanbox link with the full code:

Let's look at another nontrivial example and with some other primitives.

A DropDown Component

Now usually a component like this is implemented in an idiomatic way wherein we expect the shape of the input in a certain fashion and we pass it to the component which renders the required component with behavior encapsulated within it.

Something like this:

function DropDown(props) {
  const [selectedItem, handleSelectedItem] = useState(props.initialValue);
  return (
      onChange={(e) => handleSelectedItem(}
      { => (
        <option value={eachOption.value}>{eachOption.label}</option>

So the component expects two props i.e. an initialValue and second list options which looks something like this:

  const options = [
      {option: 'One', value '1'},
      {option: 'Two', value '2'}
  // Usage
  <DropDown initialValue="1" options={options} />

For most basic use cases this works fine but it quickly becomes hard to adapt it to different requirements:

  1. We are constraining the options to be passed in a particular fashion which puts a constraint on the consumers to adapt all their data to this structure which is an additional thing which one must do.
  2. Let's say we want the third option to be disabled what do we do? Add another prop which takes the index or an id, fine but let's say you want to add a search bar to filter through your options another prop? or if we now want to add the ability to select multiple options - the component becomes increasingly complex and bug-prone.
  3. What to do if we want to render the options in some other place other than at the bottom?
  4. In a particular scenario, I want the options to be displayed in a reverse fashion or sorted by some logic!

See how things grow and if we do it the usual way of adding more and more props we end up introducing loads of complexity and likely introduce lots of bugs.

Composition to the rescue!

Let's refactor the above to be a bit more compositional. First off let's break down the pieces and create components out of it:

function DropDown(props) {
  const [selectedItem, handleSelectedItem] = useState(props.initialValue);
  return <select>{props.children}</select>;

function Option(props) {
  const { value, label, } = props;
  return (
    <option value={value} {}>

I know that the above implementation won't work yet, but this is the API I would be aiming for. Right out of the box this solves most problems i.e. if you want to disable a particular option the consumer would just have to pass a disabled flag to the Option component and that's it! and if you think about it it's the Option component which should be aware of that piece of information, not the parent DropDown component. And it doesn't set any constraints on the consumer as to how the options structure needs to be it can be anything!

Also if you want to add search based on some term we don't need to do anything consumers can implement it fairly easily since it's now composable :

    .filter((option === option.label.indexOf(searchTerm)) !== -1)
    .map((option) => (
      <Option {...option} />

That's it! I hope it's clear how composition reduces complexity? It does it by basically giving the consumers the pieces and letting them put it together in the fashion they need and while taking care of the core piece of logic i.e. in this case selecting an item in the dropdown. The fancy terminology used for this is called inversion of control.

We now know the API we need to go for, let's fill in the pieces we need to get this working as we want. So far we have used the Hooks primitive and of course the Component now we will use Context to connect the now separate DropDown and Option components.

const DropDownContext = React.createContext("");

function DropDown(props) {
  const [selectedItem, handleSelectedItem] = useState(props.initialValue);
  return (
    <ul className="custom_dropdown">
      <DropDownContext.Provider value={{ selectedItem, handleSelectedItem }}>

function Option(props) {
  const { selectedItem, handleSelectedItem } = useContext(DropDownContext);
  return (
      selected={selectedItem === value}
      onClick={() => handleSelectedItem(value)}

Now, this should work! Notice I have changed the native select to ul and li which doesn't matter anymore because the consumer would only see DropDown and an Option how its implemented is not their concern!

And the great benefit of using Context is you are not constrained by where it needs to be rendered the logic would still work i.e. as long you are a descendant of the provider, so in theory, you could do something like this:

    <Option value={1} label="One"></Option>
    <Option value={2} label="Two"></Option>
    <Option value={3} label="Three"></Option>

That's it, here I have assumed I have a Modal component which renders the children in a modal popup and by combining DropDown, Modal, Option we have created a new component which renders the options inside a modal! without needing to much additional work at all.

Now imagine doing the above in the first implementation :-), it would have added loads of complexity and probably only for a few cases where this kind of behavior is needed.

The essence of all this is that when creating any component we cannot predict the number of ways it can be used and optimizing for composition helps in not having to worry about that as much as possible since we give the pieces and consumers use them as they want without having to worry about the core mechanics of it, in this case, selecting an item and leaving it to the consumers where and how they want to render it as we just demonstrated.

This is what makes composition such a useful proposition and whichever framework/library designs their API's keeping this in my mind will stay for long is what I believe (Obviously while giving a decent performance!).

And the Hooks API is just another step in that direction which gives us lot more primitives to compose with and my mental models around it have not been developed yet to a point to create such effective compositions, probably after I use it a while I will come up with something or the community will (probably the latter!)

All of this is not my own thinking its something derived from talks/blogs and other materials shared by the awesome folks in the community. So here are some references:

Ryan's Talk on Compound Components
Kent C Dodd's talk titled 'Simply React'
Fun with React Hooks

Codesandbox for the full working implementation of the DropDown component:

I hope this was helpful, thanks for reading!