Why composables matter in Vue - Performance Evidence

Performance: Why composables matter

MEDIUM IMPACT

This affects how efficiently Vue components share and reuse logic, impacting rendering speed and memory usage.

Sharing reactive logic across multiple components

Vue

import { ref } from 'vue';

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

// Components import and call useCounter() to share logic

Logic is centralized and reused, reducing duplicated reactive setups and memory overhead.

📈 Performance GainSaves memory and CPU by sharing composable logic, improving interaction responsiveness

Sharing reactive logic across multiple components

Vue

export default {
  data() {
    return { count: 0 };
  },
  methods: {
    increment() {
      this.count++;
    }
  }
};

// Repeated in many components separately

Duplicating reactive state and methods in many components causes larger bundle size and repeated reactive setups.

📉 Performance CostAdds extra reactive watchers per component, increasing memory and CPU usage on interaction

Performance Comparison

Pattern	DOM Operations	Reflows	Paint Cost	Verdict
Duplicated reactive logic in each component	Multiple reactive watchers per component	Multiple reflows on state change	Higher paint cost due to redundant updates	[X] Bad
Shared reactive logic via composables	Single reactive watcher per logic unit reused	Minimal reflows triggered	Lower paint cost with optimized updates	[OK] Good

Rendering Pipeline

Composables organize reactive state and logic outside components, so Vue can optimize reactivity and reduce redundant watchers during rendering.

→Reactive Setup

→Component Render

→Update

⚠️ BottleneckReactive Setup when duplicated logic creates many watchers

Core Web Vital Affected

INP

This affects how efficiently Vue components share and reuse logic, impacting rendering speed and memory usage.

Optimization Tips

1Use composables to share reactive logic and avoid duplication.

2Duplicated reactive state increases watchers and slows interaction.

3Centralizing logic reduces memory use and improves responsiveness.

Performance Quiz - 3 Questions

How do composables improve Vue app performance?

A By increasing the number of reactive watchers for safety

B By reusing reactive logic and reducing duplicated watchers

C By forcing components to reload on every state change

D By disabling reactivity to speed up rendering

DevTools: Performance

How to check: Record an interaction in the Performance panel, then inspect the number of reactive updates and component renders triggered.

What to look for: Look for fewer reactive watcher setups and less frequent component re-renders indicating efficient composable use.

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