Composables for reusable logic in Vue - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is a composable in Vue 3?

A composable is a function that uses Vue's Composition API to encapsulate and reuse reactive logic across components.

beginner

How do composables help in Vue development?

Composables help by organizing code into reusable pieces, making components cleaner and logic easier to share and test.

intermediate

Which Vue Composition API functions are commonly used inside composables?

Functions like ref, reactive, computed, and watch are commonly used to create reactive state and side effects inside composables.

beginner

How do you use a composable inside a Vue component?

You import the composable function and call it inside the <code>setup()</code> function of your component to access its reactive state and methods.

intermediate

Why should composables avoid using component-specific code like template refs or DOM manipulation?

Because composables are meant to be reusable logic units, they should stay independent of component-specific details to keep them flexible and testable.

What is the main purpose of a composable in Vue?

A To reuse reactive logic across components

B To style components with CSS

C To define HTML templates

D To manage Vuex state

Composables are functions that encapsulate reactive logic for reuse, not for styling or templates.

Where do you call a composable function inside a Vue component?

A Inside the <code>template</code> section

B Inside the <code>methods</code> option

C Inside the <code>data()</code> option

D Inside the <code>setup()</code> function

Composable functions are called inside the setup() function to access reactive state.

Which of these is NOT typically part of a composable?

A Reactive state with <code>ref</code>

B Computed properties

C Direct DOM manipulation

D Watchers for side effects

Composables should avoid direct DOM manipulation to stay reusable and independent.

What Vue API is essential for creating reactive variables in composables?

A <code>template</code>

B <code>ref</code>

C <code>emit</code>

D <code>props</code>

ref creates reactive variables used inside composables.

Why is it good to use composables in Vue projects?

A They make code reusable and easier to maintain

B They automatically generate CSS styles

C They replace the need for components

D They handle routing between pages

Composables help organize and reuse logic, improving maintainability.

Explain what a composable is in Vue and how it helps with code reuse.

Think about how you can share logic between components without repeating code.

Describe best practices when writing composables to keep them reusable and clean.

Consider what makes a piece of code easy to reuse in different places.

Practice

1. What is the main purpose of a composable in Vue?

easy

A. To reuse reactive logic across multiple components

B. To define global CSS styles

C. To create new Vue directives

D. To register Vue components globally

Solution

1. Step 1: Understand composables role

   Composables are functions that encapsulate reactive logic to be reused.

2. Step 2: Identify main purpose

   They help share logic like state or effects across components without duplication.

3. Final Answer:

   To reuse reactive logic across multiple components -> Option A

4. Quick Check:

   Composable = reusable logic [OK]

Hint: Composables share logic, not styles or components [OK]

Common Mistakes:

• Confusing composables with CSS or templates
• Thinking composables register components
• Assuming composables create directives

2. Which of the following is the correct way to define a composable in Vue 3?

easy

A. function useCounter() { return reactive({ count: 0 }) }

B. const useCounter = new Vue({ data() { return { count: 0 } } })

C. const useCounter = reactive({ count: 0 })

D. function useCounter() { const count = ref(0); return { count }; }

Solution

1. Step 1: Check composable function syntax

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

2. Step 2: Validate options

   function useCounter() { const count = ref(0); return { count }; } defines a function returning a ref wrapped count, which is correct. const useCounter = new Vue({ data() { return { count: 0 } } }) wrongly uses Vue instance syntax. const useCounter = reactive({ count: 0 }) returns reactive object directly without function. function useCounter() { return reactive({ count: 0 }) } returns reactive but misses ref usage and best practice.

3. Final Answer:

   function useCounter() { const count = ref(0); return { count }; } -> Option D

4. Quick Check:

   Composable = function returning reactive state [OK]

Hint: Composable must be a function returning reactive data [OK]

Common Mistakes:

• Defining composables as Vue instances
• Returning reactive objects without a function
• Not using ref or reactive inside composables

3. Given this composable:

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

What will be the value of isOn.value after calling toggle() twice starting from initial state?

medium

A. true

B. undefined

C. false

D. null

Solution

1. Step 1: Analyze initial state and toggle function

   Initially, isOn.value is false. Each toggle flips it to the opposite.

2. Step 2: Apply two toggles

   First toggle: false -> true. Second toggle: true -> false.

3. Final Answer:

   false -> Option C

4. Quick Check:

   Two toggles flip false twice back to false [OK]

Hint: Toggle flips boolean; even toggles return original [OK]

Common Mistakes:

• Assuming toggle sets true always
• Forgetting to access .value of ref
• Confusing initial state with toggled state

4. What is wrong with this composable code?

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

medium

A. The count variable should be declared with reactive()

B. You cannot increment a ref directly with ++ operator

C. ref should be imported from 'vue/reactive' instead

D. The function should be named useCounter, not useCount

Solution

1. Step 1: Understand ref usage

   Ref values are accessed and mutated via the .value property.

2. Step 2: Identify error in increment function

   The code uses count++ which tries to increment the ref object itself, not its value. It should be count.value++.

3. Final Answer:

   You cannot increment a ref directly with ++ operator -> Option B

4. Quick Check:

   Use count.value++ to update ref [OK]

Hint: Always use .value to read or write ref values [OK]

Common Mistakes:

• Incrementing ref without .value
• Confusing reactive and ref imports
• Thinking function name affects behavior

5. You want to create a composable that tracks window width and updates reactively on resize. Which approach correctly implements this?

hard

A. function useWindowWidth() {
const width = ref(window.innerWidth);
onMounted(() => {
window.addEventListener('resize', () => {
width.value = window.innerWidth;
});
});
onUnmounted(() => {
window.removeEventListener('resize', () => {
width.value = window.innerWidth;
});
});
return { width };
}

B. function useWindowWidth() {
const width = ref(window.innerWidth);
window.addEventListener('resize', () => {
width.value = window.innerWidth;
});
return { width };
}

C. function useWindowWidth() {
const width = reactive({ value: window.innerWidth });
window.onresize = () => {
width.value = window.innerWidth;
};
return width;
}

D. function useWindowWidth() {
const width = ref(0);
window.addEventListener('resize', () => {
width = window.innerWidth;
});
return { width };
}

Solution

1. Step 1: Understand reactive window width tracking

   We need a ref to hold width and update it on resize events.

2. Step 2: Check event listener lifecycle management

   function useWindowWidth() { const width = ref(window.innerWidth); onMounted(() => { window.addEventListener('resize', () => { width.value = window.innerWidth; }); }); onUnmounted(() => { window.removeEventListener('resize', () => { width.value = window.innerWidth; }); }); return { width }; } correctly adds listener on mount and removes it on unmount to avoid leaks. function useWindowWidth() { const width = ref(window.innerWidth); window.addEventListener('resize', () => { width.value = window.innerWidth; }); return { width }; } adds listener immediately without cleanup. function useWindowWidth() { const width = reactive({ value: window.innerWidth }); window.onresize = () => { width.value = window.innerWidth; }; return width; } uses reactive incorrectly and overwrites window.onresize. function useWindowWidth() { const width = ref(0); window.addEventListener('resize', () => { width = window.innerWidth; }); return { width }; } tries to assign to ref variable directly, which is invalid.

3. Final Answer:

   function useWindowWidth() { const width = ref(window.innerWidth); onMounted(() => { window.addEventListener('resize', () => { width.value = window.innerWidth; }); }); onUnmounted(() => { window.removeEventListener('resize', () => { width.value = window.innerWidth; }); }); return { width }; } -> Option A

4. Quick Check:

   Manage listeners with lifecycle hooks [OK]

Hint: Add and remove listeners inside lifecycle hooks [OK]

Common Mistakes:

• Not removing event listeners causing memory leaks
• Assigning directly to ref variable instead of .value
• Overwriting window.onresize instead of adding listener