Introduction
The release of Vue 3 ushered in a transformational change, moving from the Options API to the Composition API. At the heart of this transition lies the concept of "composables" — modular functions that utilize Vue's reactive features. This change has injected greater flexibility and code reusability into the framework. However, it has also birthed challenges, notably the inconsistent implementation of composables across projects, which often results in convoluted and hard-to-maintain codebases.
This style guide aims to harmonize coding practices around composables, with a focus on producing clean, maintainable, and testable code. Though composables may appear to be a new beast, they are fundamentally just functions. Hence, this guide grounds its recommendations in time-tested principles of good software design.
Whether you're just stepping into Vue 3 or are an experienced developer aiming to standardize your team's coding style, this guide serves as a comprehensive resource.
Table of Contents
- File Naming
- Composable Naming
- Folder Structure
- Argument Passing
- Error Handling
- Avoid Mixing UI and Business Logic
- Anatomy of a Composable
- Functional Core, Imperative Shell
- Single Responsibility Principle
- File Structure of a Composable
File Naming
Rule 1.1: Prefix with use and Follow PascalCase
// Good
useCounter.ts
useApiRequest.ts
// Bad
counter.ts
APIrequest.ts
Composable Naming
Rule 2.1: Use Descriptive Names
// Good
export function useUserData() {}
// Bad
export function useData() {}
Folder Structure
Rule 3.1: Place in composables Directory
src/
└── composables/
├── useCounter.ts
└── useUserData.ts
Argument Passing
Rule 4.1: Use Object Arguments for Four or More Parameters
// Good: For Multiple Parameters
useUserData({ id: 1, fetchOnMount: true, token: 'abc', locale: 'en' });
// Also Good: For Fewer Parameters
useCounter(1, true, 'session');
// Bad
useUserData(1, true, 'abc', 'en');
Error Handling
Rule 5.1: Expose Error State
// Good
const error = ref(null);
try {
// Do something
} catch (err) {
error.value = err;
}
return { error };
// Bad
try {
// Do something
} catch (err) {
console.error("An error occurred:", err);
}
return {};
Avoid Mixing UI and Business Logic
Rule 6.2: Decouple UI from Business Logic in Composables
Composables should focus on managing state and business logic, avoiding UI-specific behavior like toasts or alerts. Keeping UI logic separate from business logic will ensure that your composable is reusable and testable.
// Good
export function useUserData(userId) {
const user = ref(null);
const error = ref(null);
const fetchUser = async () => {
try {
const response = await axios.get(`/api/users/${userId}`);
user.value = response.data;
} catch (e) {
error.value = e;
}
};
return { user, error, fetchUser };
}
// In component
setup() {
const { user, error, fetchUser } = useUserData(userId);
watch(error, (newValue) => {
if (newValue) {
showToast("An error occurred."); // UI logic in component
}
});
return { user, fetchUser };
}
// Bad
export function useUserData(userId) {
const user = ref(null);
const fetchUser = async () => {
try {
const response = await axios.get(`/api/users/${userId}`);
user.value = response.data;
} catch (e) {
showToast("An error occurred."); // UI logic inside composable
}
};
return { user, fetchUser };
}
Anatomy of a Composable
Rule 7.2: Structure Your Composables Well
A composable that adheres to a well-defined structure is easier to understand, use, and maintain. Ideally, it should consist of the following components:
- Primary State: The main read-only state that the composable manages.
- Supportive State: Additional read-only states that hold values like the status of API requests or errors.
- Methods: Functions responsible for updating the Primary and Supportive states. These can call APIs, manage cookies, or even call other composables.
By ensuring your composables follow this anatomical structure, you make it easier for developers to consume them, which can improve code quality across your project.
// Good Example: Anatomy of a Composable
// Well-structured according to Anatomy of a Composable
export function useUserData(userId) {
// Primary State
const user = ref(null);
// Supportive State
const status = ref('idle');
const error = ref(null);
// Methods
const fetchUser = async () => {
status.value = 'loading';
try {
const response = await axios.get(`/api/users/${userId}`);
user.value = response.data;
status.value = 'success';
} catch (e) {
status.value = 'error';
error.value = e;
}
};
return { user, status, error, fetchUser };
}
// Bad Example: Anatomy of a Composable
// Lacks well-defined structure and mixes concerns
export function useUserDataAndMore(userId) {
// Muddled State: Not clear what's Primary or Supportive
const user = ref(null);
const count = ref(0);
const message = ref('Initializing...');
// Methods: Multiple responsibilities and side-effects
const fetchUserAndIncrement = async () => {
message.value = 'Fetching user and incrementing count...';
try {
const response = await axios.get(`/api/users/${userId}`);
user.value = response.data;
} catch (e) {
message.value = 'Failed to fetch user.';
}
count.value++; // Incrementing count, unrelated to user fetching
};
// More Methods: Different kind of task entirely
const setMessage = (newMessage) => {
message.value = newMessage;
};
return { user, count, message, fetchUserAndIncrement, setMessage };
}
Functional Core, Imperative Shell
Rule 8.2: (optional) use functional core imperative shell pattern
Structure your composable such that the core logic is functional and devoid of side effects, while the imperative shell handles the Vue-specific or side-effecting operations. Following this principle makes your composable easier to test, debug, and maintain.
Example: Functional Core, Imperative Shell
// good
// Functional Core
const calculate = (a, b) => a + b;
// Imperative Shell
export function useCalculatorGood() {
const result = ref(0);
const add = (a, b) => {
result.value = calculate(a, b); // Using the functional core
};
// Other side-effecting code can go here, e.g., logging, API calls
return { result, add };
}
// wrong
// Mixing core logic and side effects
export function useCalculatorBad() {
const result = ref(0);
const add = (a, b) => {
// Side-effect within core logic
console.log("Adding:", a, b);
result.value = a + b;
};
return { result, add };
}
Single Responsibility Principle
Rule 9.1: Use SRP for composables
A composable should adhere to the Single Responsibility Principle, meaning it should have only one reason to change. In other words, a composable should have only one job or responsibility. A violation of this principle can result in composables that are difficult to understand, maintain, and test.
// Good
export function useCounter() {
const count = ref(0);
const increment = () => {
count.value++;
};
const decrement = () => {
count.value--;
};
return { count, increment, decrement };
}
// Bad
export function useUserAndCounter(userId) {
const user = ref(null);
const count = ref(0);
const fetchUser = async () => {
try {
const response = await axios.get(`/api/users/${userId}`);
user.value = response.data;
} catch (error) {
console.error("An error occurred while fetching user data:", error);
}
};
const increment = () => {
count.value++;
};
const decrement = () => {
count.value--;
};
return { user, fetchUser, count, increment, decrement };
}
File Structure of a Composable
Rule 10.1: Rule: Consistent Ordering of Composition API Features
While the precise order can be adapted to meet the needs of your project or team, it is crucial that the chosen order is maintained consistently throughout your codebase.
Here's a suggestion for a file structure:
- Initializing: Code for setting up initialization logic.
- Refs: Code for reactive references.
- Computed: Code for computed properties.
- Methods: Functions and methods that will be used.
- Lifecycle Hooks: Lifecycle hooks like onMounted, onUnmounted, etc.
- Watch
this is just one example of a possible order, its just important that you have a order and ideally in your project the order is always the same
// Example in useCounter.ts
import { ref, computed, onMounted } from "vue";
export default function useCounter() {
// Initializing
// Initialize variables, make API calls, or any setup logic
// For example, using a router
// ...
// Refs
const count = ref(0);
// Computed
const isEven = computed(() => count.value % 2 === 0);
// Methods
const increment = () => {
count.value++;
};
const decrement = () => {
count.value--;
};
// Lifecycle
onMounted(() => {
console.log("Counter is mounted");
});
return {
count,
isEven,
increment,
decrement,
};
}
Conclusion
The guidelines presented in this article aim to offer best practices for writing clean, testable, and efficient Vue 3 composables. While these recommendations stem from a mix of established software design principles and practical experience, they are by no means exhaustive or universally applicable.
Programming is often more of an art than a science. As you grow in your Vue journey, you may find different strategies and patterns that work better for your specific use-cases. The key is to maintain a consistent, scalable, and maintainable codebase. Therefore, feel free to adapt these guidelines according to the needs of your project.
I'm open to further ideas, improvements, and real-world examples. If you have any suggestions or different approaches that work well for you, please don't hesitate to share in the comments below. Together, we can evolve these guidelines to be an even more useful resource for the Vue community.
Happy coding!
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Top comments (10)
I like where this article is going, and I have some notes/critiques.
Likes
I like Rule 4.1 as a guideline (object instead of many params); better scalability. I also strongly agree with 9.1 (SRP). It helps scope the work done by the composable (easier maintenance/debugging). 6.2 is another strong agree. 👍
Critiques
Rule 2.1's example is essentially restating 1.1. The only diff was the prefixing "use*", but that doesn't improve comprehension of the composable. Maybe a better example would be (bad)
useDatavs (good)useUserData. The ladder is more descriptive, which is 2.1's point.8.2 is a good guideline (using functional programming core funcs), but the example is too contrived to see the value of the practice.
10.1's example where ref, computed, and functions are grouped can lead to the same problems that Options API did, but maybe the point is to keep composables small/simple enough where that concern is moot.
Comments
Rule 6.2 has no preceding 6.1 (nor does 7 & 8). That's a weird/confusing outline practice and looks accidental.
Also worth noting is 7.2's point on primary & supportive state being read-only. If that's a hard-and-fast goal, computed should be returned instead of ref, which the composable consumer can still change. I'd like to trust devs to follow good convention when they can do what's easier, but experience teaches me to be more pragmatic.
Overall, a good and thoughtful collection of composable conventions!
Thank you for your comments. I did update some points regarding your feedback.
Regarding 8.2, I think the idea of functional programming is worth its own blog post to fully reap the benefits, so I agree with you.
Concerning 10.1, I did refactor a large composable, and grouping the elements helped. Of course, ideally, you shouldn't have a composable that is so large that you need to do this in the first place.
I agree with 7.2. You could use a computed property, but ultimately, the composable itself has to change a primary state somehow. You could have an internal user object and then export the same user as a computed property from the composable. However, I'm not sure if this approach is over-engineered, as I haven't seen this technique used elsewhere. But it's interesting to have immutable states for a composable.
In the end, with this blog post, I aimed to establish some guidelines that could be helpful, as I've noticed that developers often have differing views on composables. I believe there's a need for some kind of guide to unify these perspectives.
Great article!
I think 8.2 could be explained more deeply as functional/imperative is not a basic concept.
I also would love to see some examples that feel a bit more 'real life' and bring out the finer lines between good and bad.. i think the examples chosen are a bit on the obvious side (like the unrelated responsibility is too unrelated.. maybe make it logging in vs getting user data as those are two different things part of the same flow)
A follow up that talks about taking in reactive vs non reactive data and returning reactive data and a best practice structure of how to do that in almost every scenario would also be awesome.
Thank you for your insightful comment!
I agree, section 8.2 could be elaborated further, and more real-world examples would enrich the discussion.
The idea behind this post was to summarise some best practices, but you've highlighted important areas that deserve their own posts. I'm intrigued by your suggestion on a follow-up post regarding reactive versus non-reactive data handling.
Stay tuned for more in-depth explorations in upcoming posts, and thanks again for your valuable feedback!
Art! 👍 I like your blog.
Also 6.2, you watch here for an error to come from composable.
What if there is more BE calls and more erros may appear in 1 composable? Then if you use the composable in multiple components, it may display the error many times... Any smart solution here?
good question
I wrote a detailed blog post only about Error Handling
you can Check IT Out Here
dev.to/alexanderop/best-practices-...
Why the file should be named with prefix "use"? Vue 3 docs itself is not using this for filenames.
naming is not a hard rule, but your Project should have a better structure
I dont Like how the vue Docs IS doing IT
they have a Mouse.js File which Exports useMouse why not directly useMouse?
when you use vscode and you Type use you directly can than see all your composables
I love how you lay out not just examples but specifically good vs bad examples. Very helpful!