Why composables matter in Vue - Test Your Understanding

Practice - 5 Tasks

1fill in blank

easy

Complete the code to import the Vue Composition API function used to create reactive state.

Vue

import { [1] } from 'vue';

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

Acomputed

Breactive

Cwatch

Dref

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'reactive' instead of 'ref' when a simple reactive value is needed.

Confusing 'computed' with state creation.

Importing from the wrong package.

The ref function creates a reactive reference to a value, which is a basic building block in Vue composables.

2fill in blank

medium

Complete the code to define a composable function that returns a reactive count.

Vue

export function useCounter() {
  const count = [1](0);
  return { count };
}

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

Aref

Breactive

Ccomputed

Dwatch

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'reactive' which is for objects, not primitives.

Trying to use 'computed' without a getter function.

The ref function is used to create a reactive primitive value like a number inside a composable.

3fill in blank

hard

Fix the error in the composable by completing the code to update the count value.

Vue

export function useCounter() {
  const count = ref(0);
  function increment() {
    count[1]++;
  }
  return { count, increment };
}

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

A()

B.value

C[0]

D[]

Attempts:

3 left

💡 Hint

Common Mistakes

Trying to increment the ref directly without '.value'.

Using parentheses or brackets which cause errors.

In Vue's ref, the actual value is accessed and updated via the .value property.

4fill in blank

hard

Fill both blanks to create a composable that returns a reactive object with a method to reset the count.

Vue

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

export function useCounter() {
  const state = [1]({ count: 0 });
  function reset() {
    state.count = 0;
  }
  return { state, reset };
}

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

Areactive

Bref

Ccomputed

Dwatch

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'ref' to create an object instead of 'reactive'.

Not importing both functions as shown.

reactive creates a reactive object, and ref is also imported but not used here. The blank for import includes both to show common imports.

5fill in blank

hard

Fill all three blanks to create a composable that uses a computed property to double the count.

Vue

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

export function useCounter() {
  const count = [1](0);
  const double = [3](() => count.value * 2);
  return { count, double };
}

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

Aref

Breactive

Ccomputed

Dwatch

Attempts:

3 left

💡 Hint

Common Mistakes

Mixing up 'ref' and 'reactive' for primitive values.

Not using 'computed' for derived reactive values.

ref creates the reactive count, computed creates the double value, and reactive is imported but unused here to show common imports.

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