Why the Composition API exists in Vue - Performance Evidence

Performance: Why the Composition API exists

MEDIUM IMPACT

This affects how Vue components load and update, impacting rendering speed and responsiveness.

Organizing reactive state and logic in Vue components

Vue

import { ref, watch } from 'vue';
export default {
  setup() {
    const count = ref(0);
    function increment() {
      count.value++;
    }
    watch(count, (newVal) => {
      console.log('Count changed:', newVal);
    });
    return { count, increment };
  }
}

Composition API groups related reactive state and logic, enabling finer control over reactivity and reducing unnecessary updates.

📈 Performance GainReduces redundant reactivity triggers and improves update granularity, lowering INP.

Organizing reactive state and logic in Vue components

Vue

export default {
  data() {
    return { count: 0 };
  },
  methods: {
    increment() {
      this.count++;
    }
  },
  watch: {
    count(newVal) {
      console.log('Count changed:', newVal);
    }
  }
}

Options API scatters related logic across data, methods, and watch, causing less predictable reactivity and more component re-renders.

📉 Performance CostTriggers multiple reactive dependencies and re-renders due to less granular control.

Performance Comparison

Pattern	DOM Operations	Reflows	Paint Cost	Verdict
Options API scattered logic	More reactive dependencies	Multiple re-renders	Higher paint cost due to redundant updates	[X] Bad
Composition API grouped logic	Fewer reactive dependencies	Single targeted re-render	Lower paint cost with precise updates	[OK] Good

Rendering Pipeline

The Composition API organizes reactive dependencies more explicitly, allowing Vue's reactivity system to track changes precisely and update only affected parts of the DOM.

→Reactive Dependency Tracking

→Component Update

→DOM Patch

⚠️ BottleneckComponent Update stage due to unnecessary re-renders in less organized code

Core Web Vital Affected

INP

This affects how Vue components load and update, impacting rendering speed and responsiveness.

Optimization Tips

1Group related reactive state and logic with the Composition API for better update control.

2Avoid scattering reactive code across multiple Options API properties to reduce redundant reactivity.

3Use Vue DevTools Performance panel to monitor component update frequency and duration.

Performance Quiz - 3 Questions

How does the Composition API improve Vue component performance compared to the Options API?

A By removing reactivity completely

B By increasing the number of reactive dependencies

C By grouping reactive logic to reduce unnecessary re-renders

D By forcing full component reloads on every change

DevTools: Performance

How to check: Record a performance profile while interacting with the component, then inspect component update times and number of re-renders.

What to look for: Look for fewer component updates and shorter update durations indicating efficient reactive tracking.

Practice

1. Why was the Composition API introduced in Vue?

easy

A. To remove support for reactive data

B. To replace templates with plain JavaScript

C. To organize component logic by grouping related code

D. To make Vue incompatible with TypeScript

Solution

1. Step 1: Understand Vue component logic challenges

   Before Composition API, logic was scattered in options like data, methods, computed, making reuse and reading harder.

2. Step 2: Identify Composition API purpose

   The Composition API groups related logic together, improving readability and reuse.

3. Final Answer:

   To organize component logic by grouping related code -> Option C

4. Quick Check:

   Composition API = Organize logic [OK]

Hint: Composition API groups related code for clarity [OK]

Common Mistakes:

• Thinking it removes templates
• Believing it disables reactivity
• Assuming it breaks TypeScript support

2. Which of the following is the correct way to import the ref function from Vue's Composition API?

easy

A. import ref from 'vue';

B. import { ref } from 'vue';

C. const ref = require('vue');

D. import { Ref } from 'vue';

Solution

1. Step 1: Recall ES module import syntax in Vue 3+

   Vue exports Composition API functions as named exports, so use curly braces for import.

2. Step 2: Check each option

   import { ref } from 'vue'; uses named import correctly. import ref from 'vue'; tries default import which is wrong. const ref = require('vue'); uses CommonJS require which is outdated. import { Ref } from 'vue'; imports 'Ref' (capitalized) which is a type, not a function.

3. Final Answer:

   import { ref } from 'vue'; -> Option B

4. Quick Check:

   Named import with curly braces = import { ref } from 'vue'; [OK]

Hint: Use curly braces for named imports from Vue [OK]

Common Mistakes:

• Using default import instead of named import
• Using require() instead of import
• Confusing types with functions

3. What will be the output of this Vue Composition API code snippet inside a component's setup()?

import { ref } from 'vue';
export default {
  setup() {
    const count = ref(0);
    count.value++;
    return { count };
  }
}

What is the value of count when rendered?

medium

A. 0

B. An error occurs

C. undefined

D. 1

Solution

1. Step 1: Understand ref and its value property

   The ref(0) creates a reactive object with initial value 0 accessible via count.value.

2. Step 2: Analyze the increment operation

   count.value++ increases the value from 0 to 1 before returning it.

3. Final Answer:

   1 -> Option D

4. Quick Check:

   ref value incremented once = 1 [OK]

Hint: Remember to access .value with ref to get or set data [OK]

Common Mistakes:

• Expecting count to be 0 without increment
• Trying to use count directly without .value
• Thinking ref returns a primitive, not an object

4. Identify the error in this Composition API code snippet:

import { reactive } from 'vue';
export default {
  setup() {
    const state = reactive({ count: 0 });
    function increment() {
      state.count += 1
    }
    return { state, increment };
  }
}

medium

A. No error; code works correctly

B. Reactive object properties should not be mutated directly

C. Reactive must be imported as ref

D. Missing semicolon after increment function

Solution

1. Step 1: Review reactive usage

   The reactive function creates a reactive object whose properties can be mutated directly.

2. Step 2: Check the increment function

   The function increments state.count correctly without syntax errors.

3. Final Answer:

   No error; code works correctly -> Option A

4. Quick Check:

   Reactive objects allow direct property mutation [OK]

Hint: Reactive objects allow direct property changes [OK]

Common Mistakes:

• Thinking reactive properties are immutable
• Confusing reactive with ref usage
• Assuming missing semicolon causes error

5. You want to reuse a piece of logic that tracks a user's online status across multiple components. How does the Composition API help achieve this compared to the Options API?

hard

A. By allowing you to write a reusable function that returns reactive state and methods

B. By forcing you to duplicate the same data and methods in each component

C. By requiring you to use global variables for shared state

D. By removing support for reactive data in reusable code

Solution

1. Step 1: Understand logic reuse challenges in Options API

   Options API scatters logic in component options, making reuse hard without mixins or plugins.

2. Step 2: Recognize Composition API's reusable functions

   Composition API lets you write functions (composables) that return reactive state and methods to share logic easily.

3. Final Answer:

   By allowing you to write a reusable function that returns reactive state and methods -> Option A

4. Quick Check:

   Composition API enables reusable logic functions [OK]

Hint: Use composable functions to share reactive logic [OK]

Common Mistakes:

• Thinking duplication is necessary
• Believing global variables are required
• Assuming reactive data can't be reused