Ref and reactive in Composition API in Vue - Performance & Optimization

Performance: Ref and reactive in Composition API

MEDIUM IMPACT

This affects how Vue tracks changes and updates the DOM efficiently during user interactions.

Managing reactive state for a simple counter and a complex object

Vue

import { ref, reactive } from 'vue';
const count = ref(0);
const user = reactive({ name: 'Alice', age: 30 });

Using ref for primitives keeps reactivity simple and efficient, while reactive is used for objects to track nested changes properly.

📈 Performance GainReduces reactivity overhead and avoids unnecessary re-renders, improving input responsiveness.

Managing reactive state for a simple counter and a complex object

Vue

import { reactive } from 'vue';
const count = reactive({ value: 0 });
const user = reactive({ name: 'Alice', age: 30 });

Using reactive for a single primitive value wraps it in an object, causing more complex reactivity tracking and unnecessary component updates.

📉 Performance CostTriggers more reactivity overhead and can cause extra component re-renders.

Performance Comparison

Pattern	DOM Operations	Reflows	Paint Cost	Verdict
Using reactive for primitives	More proxy layers	Multiple re-renders	Higher paint cost	[X] Bad
Using ref for primitives	Minimal proxy layers	Single re-render	Lower paint cost	[OK] Good
Replacing nested reactive objects	Triggers full reactivity recalculation	Multiple reflows	Higher paint cost	[X] Bad
Mutating nested reactive properties	Fine-grained updates	Single reflow	Lower paint cost	[OK] Good

Rendering Pipeline

Ref and reactive create reactive proxies that Vue uses to detect changes. When a reactive value changes, Vue schedules component updates, affecting style calculation, layout, and paint.

→Reactivity Tracking

→Component Update

→Layout

→Paint

⚠️ BottleneckComponent Update stage is most expensive due to re-rendering triggered by reactive changes.

Core Web Vital Affected

INP

This affects how Vue tracks changes and updates the DOM efficiently during user interactions.

Optimization Tips

1Use ref for primitive values to keep reactivity simple and efficient.

2Use reactive for objects to track nested property changes precisely.

3Mutate nested reactive properties directly instead of replacing objects to minimize re-renders.

Performance Quiz - 3 Questions

Which Vue Composition API feature is best for tracking a single number to minimize reactivity overhead?

A computed

B ref

C reactive

D watch

DevTools: Performance

How to check: Record a performance profile while interacting with the component; look for frequent component re-renders and long scripting times.

What to look for: High scripting time and many component updates indicate inefficient reactive usage; fewer updates mean better performance.

Practice

1. In Vue's Composition API, what is the main difference between ref and reactive?

easy

A. ref is used to create components, reactive is used to create directives.

B. ref is only for arrays, reactive is only for strings.

C. ref is for simple values accessed with .value, reactive is for objects and arrays accessed directly.

D. ref and reactive are exactly the same and can be used interchangeably.

Solution

1. Step 1: Understand ref usage

   ref holds simple reactive values like numbers or strings and requires accessing the value with .value.

2. Step 2: Understand reactive usage

   reactive wraps objects or arrays to make their properties reactive and accessed directly without .value.

3. Final Answer:

   ref is for simple values accessed with .value, reactive is for objects and arrays accessed directly. -> Option C

4. Quick Check:

   ref vs reactive difference = D [OK]

Hint: Remember: ref = simple value with .value, reactive = object direct access [OK]

Common Mistakes:

• Thinking ref works directly on objects without .value
• Confusing reactive as only for primitive values
• Assuming ref and reactive are interchangeable

2. Which of the following is the correct way to declare a reactive object in Vue 3 Composition API?

easy

A. const state = ref({ count: 0 })

B. const state = reactive({ count: 0 })

C. const state = reactive(0)

D. const state = ref(0)

Solution

1. Step 1: Identify correct reactive syntax for objects

   reactive is used to wrap objects to make their properties reactive.

2. Step 2: Check each option

   const state = ref({ count: 0 }) uses ref with an object, which is allowed but requires .value to access. const state = reactive({ count: 0 }) correctly uses reactive with an object. const state = reactive(0) uses reactive with a number, which is invalid. const state = ref(0) uses ref with a number, which is valid but not for objects.

3. Final Answer:

   const state = reactive({ count: 0 }) -> Option B

4. Quick Check:

   reactive object syntax = C [OK]

Hint: Use reactive() for objects, ref() for primitives [OK]

Common Mistakes:

• Using reactive() on primitive values
• Using ref() for objects without .value access
• Confusing reactive and ref usage

3. Given the code:

import { ref, reactive } from 'vue';
const count = ref(0);
const state = reactive({ clicks: 0 });

count.value++;
state.clicks++;

console.log(count.value, state.clicks);

What will be logged to the console?

medium

A. 0 1

B. 0 0

C. 1 0

D. 1 1

Solution

1. Step 1: Understand ref and reactive increments

   count is a ref holding 0, incremented by count.value++ to 1. state is reactive object with clicks 0, incremented by state.clicks++ to 1.

2. Step 2: Check console.log output

   Logging count.value and state.clicks will print 1 1.

3. Final Answer:

   1 1 -> Option D

4. Quick Check:

   Increment ref and reactive values = A [OK]

Hint: Increment ref with .value, reactive directly [OK]

Common Mistakes:

• Forgetting to use .value with ref
• Assuming reactive needs .value
• Mixing up initial values

4. What is wrong with this Vue Composition API code?

import { ref } from 'vue';
const user = ref({ name: 'Alice' });
user.name = 'Bob';

medium

A. You must use user.value.name to update the name property.

B. ref cannot hold objects, use reactive instead.

C. You should use user.value() to access the object.

D. There is no error; the code works fine.

Solution

1. Step 1: Understand ref with objects

   ref wraps the object, so to access or update properties inside, you must use .value.

2. Step 2: Identify the error in property update

   The code tries to update user.name directly, which is incorrect. It should be user.value.name.

3. Final Answer:

   You must use user.value.name to update the name property. -> Option A

4. Quick Check:

   Access ref object properties with .value = A [OK]

Hint: Use .value to access or update ref object properties [OK]

Common Mistakes:

• Accessing ref object properties without .value
• Confusing ref and reactive usage
• Thinking ref cannot hold objects

5. You want to create a reactive state for a form with fields name (string) and tags (array). You also want to track a simple counter for submissions. Which setup correctly uses ref and reactive together?

hard

A. const form = reactive({ name: '', tags: [] }); const submitCount = ref(0);

B. const form = ref({ name: '', tags: [] }); const submitCount = reactive(0);

C. const form = reactive(''); const submitCount = ref([]);

D. const form = ref(''); const submitCount = reactive({ count: 0 });

Solution

1. Step 1: Identify correct usage for form object

   The form has multiple fields, so reactive is best to wrap the object with name and tags properties.

2. Step 2: Identify correct usage for simple counter

   The submission count is a simple number, so ref is appropriate to track it reactively.

3. Final Answer:

   const form = reactive({ name: '', tags: [] }); const submitCount = ref(0); -> Option A

4. Quick Check:

   Use reactive for objects, ref for simple values = B [OK]

Hint: Use reactive for objects/arrays, ref for simple values [OK]

Common Mistakes:

• Using ref for complex objects without .value
• Using reactive on primitive values
• Mixing up which data needs ref or reactive