Why composables matter in Vue - The Real Reasons

The Big Idea

Discover how composables can save you hours of debugging and rewriting code!

The Scenario

Imagine building a Vue app where you copy and paste the same logic in multiple components to handle things like fetching data or managing user input.

The Problem

Copying code everywhere makes your app hard to maintain, bugs spread easily, and updating one place means hunting down all copies manually.

The Solution

Composables let you write reusable logic once and share it across components cleanly, keeping your code DRY and easy to update.

Before vs After

✗ Before

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

✓ After

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

What It Enables

Composables unlock clean, reusable, and testable logic that scales effortlessly as your app grows.

Real Life Example

Imagine a login form and a profile page both needing to track user authentication state; composables let you share that logic without repeating code.

Key Takeaways

Manual code duplication leads to bugs and hard maintenance.

Composables package logic for easy reuse across components.

This approach keeps your Vue app clean, scalable, and easier to update.

Practice

1. Why are composables important in Vue applications?

easy

A. They compile Vue templates into JavaScript.

B. They automatically style components without CSS.

C. They replace the need for Vue Router.

D. They help reuse logic across multiple components easily.

Solution

1. Step 1: Understand composables' purpose

   Composables are functions that let you reuse logic in Vue components.

2. Step 2: Identify the correct benefit

   They help keep components clean by sharing logic, not styling or routing.

3. Final Answer:

   They help reuse logic across multiple components easily. -> Option D

4. Quick Check:

   Reusing logic = composables importance [OK]

Hint: Composables = reusable logic functions in Vue [OK]

Common Mistakes:

• Thinking composables handle styling
• Confusing composables with routing
• Believing composables compile templates

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

easy

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

B. const useCounter = new VueComponent() { count: 0 }

C. class useCounter { count = 0; }

D. const useCounter = () => { this.count = 0; }

Solution

1. Step 1: Recall composable syntax

   Composables are functions that return reactive state or methods.

2. Step 2: Check each option

   function useCounter() { const count = ref(0); return { count }; } defines a function returning a reactive ref, which is correct. Others misuse class, VueComponent, or this context.

3. Final Answer:

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

4. Quick Check:

   Composable = function returning reactive state [OK]

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

Common Mistakes:

• Using class syntax for composables
• Trying to use 'this' inside arrow functions
• Confusing composables with components

3. Given this composable:

function useToggle() {
  const on = ref(false);
  function toggle() { on.value = !on.value; }
  return { on, toggle };
}

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

medium

A. undefined

B. true

C. false

D. null

Solution

1. Step 1: Understand initial state

   The ref on starts as false.

2. Step 2: Apply toggle twice

   First toggle flips false to true. Second toggle flips true back to false.

3. Final Answer:

   false -> Option C

4. Quick Check:

   Toggle twice returns to initial false [OK]

Hint: Toggle flips boolean; two flips return to start [OK]

Common Mistakes:

• Assuming toggle only flips once
• Confusing ref.value with ref itself
• Expecting undefined or null values

4. What is wrong with this composable code?

function useData() {
  const data = ref(null);
  fetch('api/data').then(response => {
    data.value = response.json();
  });
  return { data };
}

medium

A. fetch returns a promise, so data.value is set to a promise, not actual data.

B. ref cannot hold null values initially.

C. The composable should not return data.

D. fetch cannot be used inside composables.

Solution

1. Step 1: Analyze fetch usage

   fetch returns a promise; response.json() also returns a promise.

2. Step 2: Identify mistake in assigning data.value

   Assigning data.value = response.json() sets it to a promise, not resolved data.

3. Final Answer:

   fetch returns a promise, so data.value is set to a promise, not actual data. -> Option A

4. Quick Check:

   response.json() returns promise, must await [OK]

Hint: Remember to await response.json() before assigning [OK]

Common Mistakes:

• Not awaiting response.json()
• Thinking ref can't start as null
• Believing fetch is disallowed in composables

5. You want to share a composable that tracks window width and updates reactively on resize. Which approach correctly uses Vue composables to achieve this?

hard

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

B. 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 };
}

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

D. function useWindowWidth() {
const width = ref(0);
setInterval(() => {
width.value = window.innerWidth;
}, 1000);
return { width };
}

Solution

1. Step 1: Understand reactive state and lifecycle

   We need reactive width and proper event listener cleanup to avoid leaks.

2. Step 2: Evaluate options for 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 }; } uses onMounted and onUnmounted to add and remove listeners correctly, ensuring cleanup.

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 B

4. Quick Check:

   Use lifecycle hooks to add/remove listeners [OK]

Hint: Add and remove listeners in lifecycle hooks for cleanup [OK]

Common Mistakes:

• Not removing event listeners causing memory leaks
• Using non-reactive variables for width
• Using setInterval instead of event listeners