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🗓️ Apr 16th, 2022

Single Level of Abstraction applied to React code

Principle statement

Single Level of Abstraction or SLA is a clean code principle. It states that all code inside a function or a method must be at the same level of abstraction.

When a method or a function mixes different levels of abstraction, the code is harder to read and understand.

function signUp(email: string, password: string) {
const user = db.user.findOne({ where: { email } });
if (user) throw new Error("User already exists");
const hashedPassword = hashPassword(password);
const newUser = db.user.createOne({
hashedPassword,
email,
});
return newUser;
}

In the example above, the signUp function mixes

When a reviewer read this code, he or she has to mentally group the statements that belong together. It is called mental grouping.

To avoid mental grouping, we use the principle of single level of abstraction.

function doesUserAlreadyExist(email: string) {
const user = db.user.findOne({ where: { email } });
return user ? true : false;
}
function createUser(email: string, hashedPassword: string) {
const user = db.user.createOne({
hashedPassword,
email,
});
return user;
}
function signUp({ email, password }) {
if (doesUserAlreadyExist(email)) {
throw new Error("User already exists");
}
const hashedPassword = hashPassword(password);
const user = createUser(email, hashedPassword);
return user;
}

Now, signUp function is composed of functions of the same level of abstraction. A potential reviewer does not need to know the details of each function to understand the signUp function.

How to apply SLA to React code

We can apply this principle to React too, especially to the JSX part. Let’s consider this example

const BlogHomePage = ({ articles }) => {
return (
<>
<SEO />
<main>
<header>
<h1>My blog</h1>
<p>Welcome to my blog</p>
</header>
<ul>
{articles.map((article) => (
<ArticlePreview key={title} article={article} />
))}
</ul>
</main>
</>
);
};

In this simple example, the component BlogHomePage is somewhat readable. But still it requires extra effort to mentally visualize to what elements this component renders.

We could refactor this component like so

const Header = () => (
<header>
<h1>My blog</h1>
<p>Welcome to my blog</p>
</header>
);
const Articles = ({ articles }) => (
<ul>
{articles.map((article) => (
<ArticlePreview key={title} article={article} />
))}
</ul>
);
const BlogHomePage = ({ articles }) => {
return (
<>
<SEO />
<main>
<Header />
<Articles articles={articles} />
</main>
</>
);
};

This way, we can easily visualize all the different parts of BlogHomePage at once.

Caveats

The only caveat of the SLA principle is that it can result in the creation of multiple functions, or mulitple components in React.

And there is also the risk of creating the wrong abstraction. Thus resulting in mental inlining : the opposite of mental grouping.