Introduction
The release of Vue 3 brought 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 leverage Vue’s reactive features. This change enhanced the framework’s flexibility and code reusability. The inconsistent implementation of composables across projects often leads to convoluted and hard-to-maintain codebases.
This style guide harmonizes coding practices around composables, focusing on producing clean, maintainable, and testable code. While composables represent a new pattern, they remain functions at their core. The guide bases its recommendations on time-tested principles of good software design.
This guide serves as a comprehensive resource for both newcomers to Vue 3 and experienced developers aiming to standardize their team’s coding style.
Table of Contents
Open Table of Contents
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 well-structured composable improves understanding, usage, and maintenance. It consists of these components:
- Primary State: The main reactive state that the composable manages.
- State Metadata: States that hold values like API request status or errors.
- Methods: Functions that update the Primary State and State Metadata. These functions can call APIs, manage cookies, or integrate with other composables.
Following this structure makes your composables more intuitive and improves 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 follow the Single Responsibility Principle: one reason to change. This means each composable handles one specific task. Following this principle creates composables that are clear, maintainable, and testable.
// 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
Your team should establish and follow a consistent order for Composition API features throughout the codebase.
Here’s a recommended order:
- Initializing: Setup logic
- Refs: Reactive references
- Computed: Computed properties
- Methods: Functions for state manipulation
- Lifecycle Hooks: onMounted, onUnmounted, etc.
- Watch
Pick an order that works for your team and apply it consistently across all composables.
// 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
These guidelines provide best practices for writing clean, testable, and efficient Vue 3 composables. They combine established software design principles with practical experience, though they aren’t exhaustive.
Programming blends art and science. As you develop with Vue, you’ll discover patterns that match your needs. Focus on maintaining a consistent, scalable, and maintainable codebase. Adapt these guidelines to fit your project’s requirements.
Share your ideas, improvements, and real-world examples in the comments. Your input helps evolve these guidelines into a better resource for the Vue community.