Why the Composition API exists in Vue - Test Your Understanding

Practice - 5 Tasks

1fill in blank

easy

Complete the code to import the Composition API function for reactive state.

Vue

import { [1] } from 'vue';

const count = [1](0);

Drag options to blanks, or click blank then click option'

Awatch

Breactive

Ccomputed

Dref

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'reactive' instead of 'ref' for a primitive value.

Trying to use 'computed' to create state.

Importing 'watch' which is for side effects, not state.

The ref function creates a reactive reference to a value, which is the basic way to hold reactive state in the Composition API.

2fill in blank

medium

Complete the code to define a reactive object using the Composition API.

Vue

import { [1] } from 'vue';

const state = [1]({ count: 0 });

Drag options to blanks, or click blank then click option'

Acomputed

Bref

Creactive

Dwatch

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'ref' for an object instead of 'reactive'.

Trying to use 'computed' to create reactive state.

Confusing 'watch' with state creation.

The reactive function creates a reactive object, which is useful for grouping multiple reactive properties together.

3fill in blank

hard

Fix the error in the code by choosing the correct way to access a reactive ref's value.

Vue

import { ref } from 'vue';

const count = ref(0);
console.log(count[1]);

Drag options to blanks, or click blank then click option'

A.value

B.val

C[]

D()

Attempts:

3 left

💡 Hint

Common Mistakes

Trying to access the ref like a normal variable without '.value'.

Using '.val' which is incorrect.

Using parentheses or brackets which cause errors.

To get the actual value inside a ref, you must use the .value property.

4fill in blank

hard

Fill both blanks to create a computed property that doubles a reactive count.

Vue

import { ref, [1] } from 'vue';

const count = ref(1);
const double = [2](() => count.value * 2);

Drag options to blanks, or click blank then click option'

Acomputed

Breactive

Cwatch

Dref

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'ref' instead of 'computed' for derived values.

Trying to use 'reactive' or 'watch' here.

The computed function creates a reactive value that updates automatically based on other reactive values.

5fill in blank

hard

Fill all three blanks to create a reactive object, a computed property, and a watcher that logs changes.

Vue

import { [1], [2], [3] } from 'vue';

const state = [1]({ count: 0 });
const double = [2](() => state.count * 2);
[3](() => console.log('Count changed:', state.count));

Drag options to blanks, or click blank then click option'

Areactive

Bcomputed

Cwatch

Dref

Attempts:

3 left

💡 Hint

Common Mistakes

Mixing up 'ref' and 'reactive' for objects.

Using 'ref' instead of 'computed' for derived values.

Confusing 'watch' with 'computed'.

reactive creates a reactive object, computed creates a derived reactive value, and watch runs code when reactive data changes.

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