Why the Composition API exists in Vue

Introduction

The Composition API was created to make Vue components easier to organize and reuse. It helps manage complex logic by grouping related code together.

When your component has many features and you want to keep code clean and organized.

When you want to reuse logic across multiple components easily.

When you need better TypeScript support in your Vue app.

When you want to improve readability by grouping related code together.

When you want more flexibility than the Options API offers.

Syntax

Vue

import { ref, computed } from 'vue';

export default {
  setup() {
    const count = ref(0);
    const double = computed(() => count.value * 2);

    function increment() {
      count.value++;
    }

    return { count, double, increment };
  }
}

The setup function is the main place to write logic in the Composition API.

Use ref to create reactive data, and computed for derived values.

Examples

Simple reactive data example using ref.

Vue

import { ref } from 'vue';

export default {
  setup() {
    const message = ref('Hello!');
    return { message };
  }
}

Using computed to create a value based on reactive data.

Vue

import { ref, computed } from 'vue';

export default {
  setup() {
    const count = ref(1);
    const squared = computed(() => count.value * count.value);
    return { count, squared };
  }
}

Adding a function to change reactive state inside setup.

Vue

import { ref } from 'vue';

export default {
  setup() {
    const isVisible = ref(true);
    function toggle() {
      isVisible.value = !isVisible.value;
    }
    return { isVisible, toggle };
  }
}

Sample Program

This Vue component uses the Composition API to manage a counter. The count is reactive, and double automatically updates when count changes. Clicking the button increases the count.

Vue

<template>
  <div>
    <p>Count: {{ count }}</p>
    <p>Double: {{ double }}</p>
    <button @click="increment">Add 1</button>
  </div>
</template>

<script setup>
import { ref, computed } from 'vue';

const count = ref(0);
const double = computed(() => count.value * 2);

function increment() {
  count.value++;
}
</script>

Important Notes

The Composition API helps keep related code close together, unlike the Options API which separates data, methods, and lifecycle hooks.

It works well with TypeScript because it uses plain JavaScript functions.

You can gradually adopt the Composition API in existing Vue projects.

Summary

The Composition API organizes component logic by grouping related code.

It makes code easier to read, reuse, and maintain.

It improves TypeScript support and flexibility in Vue apps.

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