Sharing composables across components in Vue

Introduction

Sharing composables lets you reuse logic easily in many parts of your app. It keeps your code clean and simple.

You want to use the same reactive data or functions in multiple components.

You need to keep your code DRY (Don't Repeat Yourself) by sharing logic.

You want to separate concerns by moving logic out of components.

You want to test logic separately from UI components.

Syntax

Vue

import { ref } from 'vue';

// useCounter.js
export function useCounter() {
  const count = ref(0);
  function increment() {
    count.value++;
  }
  return { count, increment };
}

// In a component
import { useCounter } from './useCounter';

const { count, increment } = useCounter();

Composable functions start with use by convention.

They return reactive state and functions to use in components.

Examples

A simple toggle composable to share boolean state and toggle function.

Vue

// useToggle.js
import { ref } from 'vue';
export function useToggle(initial = false) {
  const state = ref(initial);
  function toggle() {
    state.value = !state.value;
  }
  return { state, toggle };
}

Using the toggle composable inside a component to get shared logic.

Vue

// In a component
import { useToggle } from './useToggle';

const { state, toggle } = useToggle(true);

Sample Program

This example shows two components using the same composable useCounter. Each has its own count state and increment function. They work independently but share the same logic.

Vue

<script setup>
import { ref } from 'vue';

// useCounter.js
function useCounter() {
  const count = ref(0);
  function increment() {
    count.value++;
  }
  return { count, increment };
}

// Component A
const { count, increment } = useCounter();

// Component B
const { count: countB, increment: incrementB } = useCounter();
</script>

<template>
  <section>
    <h2>Component A</h2>
    <p>Count: {{ count }}</p>
    <button @click="increment">Increment A</button>
  </section>
  <section>
    <h2>Component B</h2>
    <p>Count: {{ countB }}</p>
    <button @click="incrementB">Increment B</button>
  </section>
</template>

Important Notes

Each call to a composable creates a new independent state.

To share state across components, use a global store or provide/inject.

Summary

Composables let you reuse logic by exporting functions with reactive state.

Import and call composables inside components to get fresh state and methods.

They keep your code clean and easy to maintain.

Practice

1. What is the main purpose of a composable in Vue 3?
function useCounter() { const count = ref(0); return { count }; }

easy

A. To register global Vue directives

B. To define a new component template

C. To style components with CSS

D. To share reusable reactive logic across multiple components

Solution

1. Step 1: Understand what a composable function does

   A composable is a function that returns reactive state or methods to be reused.

2. Step 2: Identify the purpose of the example composable

   The example returns a reactive count variable to share logic across components.

3. Final Answer:

   To share reusable reactive logic across multiple components -> Option D

4. Quick Check:

   Composable = reusable logic [OK]

Hint: Composable functions return reactive state for reuse [OK]

Common Mistakes:

• Thinking composables define templates
• Confusing composables with CSS styling
• Assuming composables register directives

2. Which is the correct way to import and use a composable named useTimer inside a Vue component's setup() function?

easy

A. import { useTimer } from './composables'; const { time } = useTimer();

B. import useTimer from './composables'; const time = useTimer;

C. const useTimer = require('./composables'); const { time } = useTimer();

D. import useTimer from './composables'; setup() { return useTimer; }

Solution

1. Step 1: Import the composable correctly

   Use ES module import syntax with curly braces if composable is a named export.

2. Step 2: Call the composable function inside setup()

   Call useTimer() to get reactive state, then destructure needed properties.

3. Final Answer:

   import { useTimer } from './composables'; const { time } = useTimer(); -> Option A

4. Quick Check:

   Import named + call function = correct usage [OK]

Hint: Import named composable and call it inside setup() [OK]

Common Mistakes:

• Using require() instead of import
• Not calling the composable function
• Importing default when composable is named export

3. Given this composable and component code, what will be logged?

// composable.js
import { ref } from 'vue';
export function useCount() {
  const count = ref(0);
  function increment() { count.value++; }
  return { count, increment };
}

// Component.vue
import { useCount } from './composable';
export default {
  setup() {
    const { count, increment } = useCount();
    increment();
    increment();
    console.log(count.value);
    return {};
  }
}

medium

A. 2

B. 0

C. undefined

D. Error: count is not defined

Solution

1. Step 1: Analyze composable logic

   Count starts at 0 and increments by 1 each time increment() is called.

2. Step 2: Trace component setup execution

   Increment is called twice, so count.value becomes 2, then logged.

3. Final Answer:

   2 -> Option A

4. Quick Check:

   Two increments = count 2 [OK]

Hint: Count increments twice, so value is 2 [OK]

Common Mistakes:

• Forgetting to call increment()
• Confusing ref with plain variable
• Expecting count to be 0 without increments

4. Identify the error in this composable usage:

import { ref } from 'vue';
export function useData() {
  const data = ref(null);
  fetch('/api').then(res => res.json()).then(json => data = json);
  return { data };
}

medium

A. Using ref instead of reactive

B. Missing await keyword before fetch

C. Reassigning the ref variable instead of its value

D. Not returning the data variable

Solution

1. Step 1: Check how ref is updated

   Ref variables must update their .value, not be reassigned.

2. Step 2: Identify the mistake in the fetch callback

   The code assigns data = json instead of data.value = json.

3. Final Answer:

   Reassigning the ref variable instead of its value -> Option C

4. Quick Check:

   Update ref.value, not ref itself [OK]

Hint: Update ref.value, not the ref variable [OK]

Common Mistakes:

• Assigning directly to ref variable
• Forgetting to handle async properly
• Confusing reactive with ref usage

5. You want to share a composable that tracks a user's online status across multiple components. Which approach ensures each component has its own independent reactive state?

hard

A. Export a single reactive object and import it directly in all components

B. Export a function that returns new refs each call, then import and call it in each component

C. Define reactive state inside a global variable outside the composable

D. Use a Vuex store instead of composables for reactive state

Solution

1. Step 1: Understand independent reactive state per component

   Each component needs its own fresh reactive refs to avoid shared state.

2. Step 2: Identify correct composable pattern

   Exporting a function that returns new refs each call creates isolated state per component.

3. Final Answer:

   Export a function that returns new refs each call, then import and call it in each component -> Option B

4. Quick Check:

   Function returns fresh state per call [OK]

Hint: Return new refs inside function for isolated state [OK]

Common Mistakes:

• Sharing a single reactive object causing shared state
• Using global variables outside composables
• Confusing Vuex with composables for local state