Vue.js • Chapter 22 • Foundations to Advanced
Composables
Each topic includes substantial explanation, ten focused examples, its own Vue code example, step-by-step reasoning, expected behavior, and practice.
22.1 What a Composable Is
A composable packages reusable stateful logic with the Composition API. In Chapter 22, apply this definition specifically to What a Composable Is and trace how it changes the rendered interface. In Chapter 22, trace the value or control flow from its source to the rendered interface so you can explain why Vue changes the screen.
For What a Composable Is in Chapter 22, inspect reactive inputs, component ownership, rendered output, edge cases, and what causes another update. Keep writable state ownership explicit, derive values when possible, and separate display calculations from network, DOM, storage, timer, or other external work.
This lesson connects What a Composable Is to composables. Start with one working case, verify the expected output, then add one edge case and explain what Vue tracks, reuses, creates, removes, or updates.
Concept in plain language
A composable packages reusable stateful logic with the Composition API. In Chapter 22, apply this definition specifically to What a Composable Is and trace how it changes the rendered interface.
10 teaching examples
Example 1: Smallest useful case
Build the smallest photo gallery that demonstrates What a Composable Is. Keep one input and one visible result, then explain reactive inputs, component ownership, rendered output, edge cases, and what causes another update. A composable packages reusable stateful logic with the Composition API. In Chapter 22, apply this definition specifically to What a Composable Is and trace how it changes the rendered interface. This is example 1 for Chapter 22, topic 1.
Example 2: Change one reactive value
Change one value involved in What a Composable Is inside the notification center. Predict what Vue will update before running it, then compare the prediction with the rendered result.
Example 3: Parent-child comparison
Use What a Composable Is across two components in the task board. Compare which component owns the writable data and which component only receives or presents it.
Example 4: Edge case
Add an empty, missing, invalid, delayed, or rapidly changing value to the language selector. Handle the What a Composable Is edge case explicitly instead of leaving stale output.
Example 5: Accessibility review
Use What a Composable Is in the quiz screen while checking semantic HTML, labels, focus order, keyboard access, and understandable status feedback. A composable packages reusable stateful logic with the Composition API. In Chapter 22, apply this definition specifically to What a Composable Is and trace how it changes the rendered interface. This is example 5 for Chapter 22, topic 1.
Example 6: State ownership review
Remove duplicated state from the analytics view. For What a Composable Is, derive values when possible and keep the writable source with the component or store that owns it.
Example 7: Slow-network scenario
Assume the support form has a slow request while using What a Composable Is. Decide what remains interactive, what shows pending feedback, and how stale responses are prevented.
Example 8: Refactoring exercise
Extract the What a Composable Is responsibility from a crowded course dashboard into a focused component or composable with a narrow API.
Example 9: Performance experiment
Measure the profile editor before optimizing What a Composable Is. Check reactive work, repeated calculations, list size, component updates, and user-visible delay.
Example 10: Production review
Review What a Composable Is in the search panel for errors, security, localization, RTL, narrow screens, accessibility, and what monitoring should report. A composable packages reusable stateful logic with the Composition API. In Chapter 22, apply this definition specifically to What a Composable Is and trace how it changes the rendered interface. This is example 10 for Chapter 22, topic 1.
Vue code example
<script setup>
import { ref } from 'vue'
const topic = "What a Composable Is"
const active = ref(false)
</script>
<template>
<section><h2>{{ topic }}</h2><button :aria-pressed="active" @click="active=!active">Toggle state</button><p>{{ active ? 'Active' : 'Inactive' }}</p></section>
</template>Step-by-step code explanation
- Identify the Vue responsibility demonstrated by What a Composable Is.
- Read the reactive state, props, route/store input, or injected value before the template.
- Trace the event, dependency, watcher, lifecycle hook, or navigation action that changes the display.
- Test one normal path and one edge case, including cleanup when external work is involved.
- Confirm accessibility, mobile width, and RTL behavior for user-facing controls and translated text.
Expected behavior: A small Vue interface demonstrates What a Composable Is and updates according to the interaction or data in the example.
Practice exercise
Create a small Vue feature focused on What a Composable Is from Chapter 22. Predict the result before running it, test one edge case, and explain which reactive value, prop, event, route, store, or lifecycle step caused the update. Then check accessibility, narrow-screen width, and RTL behavior where relevant.
22.2 Extracting Reusable Logic
Extracting Reusable Logic is a focused part of Vue application design in Chapter 22. Study its inputs, reactive dependencies, component ownership, rendered result, edge cases, and cleanup requirements. In Chapter 22, trace the value or control flow from its source to the rendered interface so you can explain why Vue changes the screen.
For Extracting Reusable Logic in Chapter 22, inspect reactive inputs, component ownership, rendered output, edge cases, and what causes another update. Keep writable state ownership explicit, derive values when possible, and separate display calculations from network, DOM, storage, timer, or other external work.
This lesson connects Extracting Reusable Logic to composables. Start with one working case, verify the expected output, then add one edge case and explain what Vue tracks, reuses, creates, removes, or updates.
Concept in plain language
Extracting Reusable Logic is a focused part of Vue application design in Chapter 22. Study its inputs, reactive dependencies, component ownership, rendered result, edge cases, and cleanup requirements.
10 teaching examples
Example 1: Smallest useful case
Build the smallest language selector that demonstrates Extracting Reusable Logic. Keep one input and one visible result, then explain reactive inputs, component ownership, rendered output, edge cases, and what causes another update. Extracting Reusable Logic is a focused part of Vue application design in Chapter 22. Study its inputs, reactive dependencies, component ownership, rendered result, edge cases, and cleanup requirements. This is example 1 for Chapter 22, topic 2.
Example 2: Change one reactive value
Change one value involved in Extracting Reusable Logic inside the quiz screen. Predict what Vue will update before running it, then compare the prediction with the rendered result.
Example 3: Parent-child comparison
Use Extracting Reusable Logic across two components in the analytics view. Compare which component owns the writable data and which component only receives or presents it.
Example 4: Edge case
Add an empty, missing, invalid, delayed, or rapidly changing value to the support form. Handle the Extracting Reusable Logic edge case explicitly instead of leaving stale output.
Example 5: Accessibility review
Use Extracting Reusable Logic in the course dashboard while checking semantic HTML, labels, focus order, keyboard access, and understandable status feedback. Extracting Reusable Logic is a focused part of Vue application design in Chapter 22. Study its inputs, reactive dependencies, component ownership, rendered result, edge cases, and cleanup requirements. This is example 5 for Chapter 22, topic 2.
Example 6: State ownership review
Remove duplicated state from the profile editor. For Extracting Reusable Logic, derive values when possible and keep the writable source with the component or store that owns it.
Example 7: Slow-network scenario
Assume the search panel has a slow request while using Extracting Reusable Logic. Decide what remains interactive, what shows pending feedback, and how stale responses are prevented.
Example 8: Refactoring exercise
Extract the Extracting Reusable Logic responsibility from a crowded shopping cart into a focused component or composable with a narrow API.
Example 9: Performance experiment
Measure the lesson tracker before optimizing Extracting Reusable Logic. Check reactive work, repeated calculations, list size, component updates, and user-visible delay.
Example 10: Production review
Review Extracting Reusable Logic in the booking form for errors, security, localization, RTL, narrow screens, accessibility, and what monitoring should report. Extracting Reusable Logic is a focused part of Vue application design in Chapter 22. Study its inputs, reactive dependencies, component ownership, rendered result, edge cases, and cleanup requirements. This is example 10 for Chapter 22, topic 2.
Vue code example
<script setup>
import { ref } from 'vue'
const topic = "Extracting Reusable Logic"
const active = ref(false)
</script>
<template>
<section><h2>{{ topic }}</h2><button :aria-pressed="active" @click="active=!active">Toggle state</button><p>{{ active ? 'Active' : 'Inactive' }}</p></section>
</template>Step-by-step code explanation
- Identify the Vue responsibility demonstrated by Extracting Reusable Logic.
- Read the reactive state, props, route/store input, or injected value before the template.
- Trace the event, dependency, watcher, lifecycle hook, or navigation action that changes the display.
- Test one normal path and one edge case, including cleanup when external work is involved.
- Confirm accessibility, mobile width, and RTL behavior for user-facing controls and translated text.
Expected behavior: A small Vue interface demonstrates Extracting Reusable Logic and updates according to the interaction or data in the example.
Practice exercise
Create a small Vue feature focused on Extracting Reusable Logic from Chapter 22. Predict the result before running it, test one edge case, and explain which reactive value, prop, event, route, store, or lifecycle step caused the update. Then check accessibility, narrow-screen width, and RTL behavior where relevant.
22.3 Composables with Refs
A ref stores a reactive value; JavaScript uses .value while templates usually unwrap it automatically. In Chapter 22, apply this definition specifically to Composables with Refs and trace how it changes the rendered interface. In Chapter 22, trace the value or control flow from its source to the rendered interface so you can explain why Vue changes the screen.
For Composables with Refs in Chapter 22, inspect reactive dependencies, update timing, derived values, side effects, and cleanup. Keep writable state ownership explicit, derive values when possible, and separate display calculations from network, DOM, storage, timer, or other external work.
This lesson connects Composables with Refs to composables. Start with one working case, verify the expected output, then add one edge case and explain what Vue tracks, reuses, creates, removes, or updates.
Concept in plain language
A ref stores a reactive value; JavaScript uses .value while templates usually unwrap it automatically. In Chapter 22, apply this definition specifically to Composables with Refs and trace how it changes the rendered interface.
10 teaching examples
Example 1: Smallest useful case
Build the smallest support form that demonstrates Composables with Refs. Keep one input and one visible result, then explain reactive dependencies, update timing, derived values, side effects, and cleanup. A ref stores a reactive value; JavaScript uses .value while templates usually unwrap it automatically. In Chapter 22, apply this definition specifically to Composables with Refs and trace how it changes the rendered interface. This is example 1 for Chapter 22, topic 3.
Example 2: Change one reactive value
Change one value involved in Composables with Refs inside the course dashboard. Predict what Vue will update before running it, then compare the prediction with the rendered result.
Example 3: Parent-child comparison
Use Composables with Refs across two components in the profile editor. Compare which component owns the writable data and which component only receives or presents it.
Example 4: Edge case
Add an empty, missing, invalid, delayed, or rapidly changing value to the search panel. Handle the Composables with Refs edge case explicitly instead of leaving stale output.
Example 5: Accessibility review
Use Composables with Refs in the shopping cart while checking semantic HTML, labels, focus order, keyboard access, and understandable status feedback. A ref stores a reactive value; JavaScript uses .value while templates usually unwrap it automatically. In Chapter 22, apply this definition specifically to Composables with Refs and trace how it changes the rendered interface. This is example 5 for Chapter 22, topic 3.
Example 6: State ownership review
Remove duplicated state from the lesson tracker. For Composables with Refs, derive values when possible and keep the writable source with the component or store that owns it.
Example 7: Slow-network scenario
Assume the booking form has a slow request while using Composables with Refs. Decide what remains interactive, what shows pending feedback, and how stale responses are prevented.
Example 8: Refactoring exercise
Extract the Composables with Refs responsibility from a crowded message panel into a focused component or composable with a narrow API.
Example 9: Performance experiment
Measure the admin table before optimizing Composables with Refs. Check reactive work, repeated calculations, list size, component updates, and user-visible delay.
Example 10: Production review
Review Composables with Refs in the photo gallery for errors, security, localization, RTL, narrow screens, accessibility, and what monitoring should report. A ref stores a reactive value; JavaScript uses .value while templates usually unwrap it automatically. In Chapter 22, apply this definition specifically to Composables with Refs and trace how it changes the rendered interface. This is example 10 for Chapter 22, topic 3.
Vue code example
<script setup>
import { ref } from 'vue'
const topic = "Composables with Refs"
const active = ref(false)
</script>
<template>
<section><h2>{{ topic }}</h2><button :aria-pressed="active" @click="active=!active">Toggle state</button><p>{{ active ? 'Active' : 'Inactive' }}</p></section>
</template>Step-by-step code explanation
- Identify the Vue responsibility demonstrated by Composables with Refs.
- Read the reactive state, props, route/store input, or injected value before the template.
- Trace the event, dependency, watcher, lifecycle hook, or navigation action that changes the display.
- Test one normal path and one edge case, including cleanup when external work is involved.
- Confirm accessibility, mobile width, and RTL behavior for user-facing controls and translated text.
Expected behavior: A small Vue interface demonstrates Composables with Refs and updates according to the interaction or data in the example.
Practice exercise
Create a small Vue feature focused on Composables with Refs from Chapter 22. Predict the result before running it, test one edge case, and explain which reactive value, prop, event, route, store, or lifecycle step caused the update. Then check accessibility, narrow-screen width, and RTL behavior where relevant.
22.4 Composables with Side Effects
A composable packages reusable stateful logic with the Composition API. In Chapter 22, apply this definition specifically to Composables with Side Effects and trace how it changes the rendered interface. In Chapter 22, trace the value or control flow from its source to the rendered interface so you can explain why Vue changes the screen.
For Composables with Side Effects in Chapter 22, inspect reactive inputs, component ownership, rendered output, edge cases, and what causes another update. Keep writable state ownership explicit, derive values when possible, and separate display calculations from network, DOM, storage, timer, or other external work.
This lesson connects Composables with Side Effects to composables. Start with one working case, verify the expected output, then add one edge case and explain what Vue tracks, reuses, creates, removes, or updates.
Concept in plain language
A composable packages reusable stateful logic with the Composition API. In Chapter 22, apply this definition specifically to Composables with Side Effects and trace how it changes the rendered interface.
10 teaching examples
Example 1: Smallest useful case
Build the smallest search panel that demonstrates Composables with Side Effects. Keep one input and one visible result, then explain reactive inputs, component ownership, rendered output, edge cases, and what causes another update. A composable packages reusable stateful logic with the Composition API. In Chapter 22, apply this definition specifically to Composables with Side Effects and trace how it changes the rendered interface. This is example 1 for Chapter 22, topic 4.
Example 2: Change one reactive value
Change one value involved in Composables with Side Effects inside the shopping cart. Predict what Vue will update before running it, then compare the prediction with the rendered result.
Example 3: Parent-child comparison
Use Composables with Side Effects across two components in the lesson tracker. Compare which component owns the writable data and which component only receives or presents it.
Example 4: Edge case
Add an empty, missing, invalid, delayed, or rapidly changing value to the booking form. Handle the Composables with Side Effects edge case explicitly instead of leaving stale output.
Example 5: Accessibility review
Use Composables with Side Effects in the message panel while checking semantic HTML, labels, focus order, keyboard access, and understandable status feedback. A composable packages reusable stateful logic with the Composition API. In Chapter 22, apply this definition specifically to Composables with Side Effects and trace how it changes the rendered interface. This is example 5 for Chapter 22, topic 4.
Example 6: State ownership review
Remove duplicated state from the admin table. For Composables with Side Effects, derive values when possible and keep the writable source with the component or store that owns it.
Example 7: Slow-network scenario
Assume the photo gallery has a slow request while using Composables with Side Effects. Decide what remains interactive, what shows pending feedback, and how stale responses are prevented.
Example 8: Refactoring exercise
Extract the Composables with Side Effects responsibility from a crowded notification center into a focused component or composable with a narrow API.
Example 9: Performance experiment
Measure the task board before optimizing Composables with Side Effects. Check reactive work, repeated calculations, list size, component updates, and user-visible delay.
Example 10: Production review
Review Composables with Side Effects in the language selector for errors, security, localization, RTL, narrow screens, accessibility, and what monitoring should report. A composable packages reusable stateful logic with the Composition API. In Chapter 22, apply this definition specifically to Composables with Side Effects and trace how it changes the rendered interface. This is example 10 for Chapter 22, topic 4.
Vue code example
<script setup>
import { ref } from 'vue'
const topic = "Composables with Side Effects"
const active = ref(false)
</script>
<template>
<section><h2>{{ topic }}</h2><button :aria-pressed="active" @click="active=!active">Toggle state</button><p>{{ active ? 'Active' : 'Inactive' }}</p></section>
</template>Step-by-step code explanation
- Identify the Vue responsibility demonstrated by Composables with Side Effects.
- Read the reactive state, props, route/store input, or injected value before the template.
- Trace the event, dependency, watcher, lifecycle hook, or navigation action that changes the display.
- Test one normal path and one edge case, including cleanup when external work is involved.
- Confirm accessibility, mobile width, and RTL behavior for user-facing controls and translated text.
Expected behavior: A small Vue interface demonstrates Composables with Side Effects and updates according to the interaction or data in the example.
Practice exercise
Create a small Vue feature focused on Composables with Side Effects from Chapter 22. Predict the result before running it, test one edge case, and explain which reactive value, prop, event, route, store, or lifecycle step caused the update. Then check accessibility, narrow-screen width, and RTL behavior where relevant.
22.5 Designing a Clean Composable API
A composable packages reusable stateful logic with the Composition API. In Chapter 22, apply this definition specifically to Designing a Clean Composable API and trace how it changes the rendered interface. In Chapter 22, trace the value or control flow from its source to the rendered interface so you can explain why Vue changes the screen.
For Designing a Clean Composable API in Chapter 22, inspect reactive inputs, component ownership, rendered output, edge cases, and what causes another update. Keep writable state ownership explicit, derive values when possible, and separate display calculations from network, DOM, storage, timer, or other external work.
This lesson connects Designing a Clean Composable API to composables. Start with one working case, verify the expected output, then add one edge case and explain what Vue tracks, reuses, creates, removes, or updates.
Concept in plain language
A composable packages reusable stateful logic with the Composition API. In Chapter 22, apply this definition specifically to Designing a Clean Composable API and trace how it changes the rendered interface.
10 teaching examples
Example 1: Smallest useful case
Build the smallest booking form that demonstrates Designing a Clean Composable API. Keep one input and one visible result, then explain reactive inputs, component ownership, rendered output, edge cases, and what causes another update. A composable packages reusable stateful logic with the Composition API. In Chapter 22, apply this definition specifically to Designing a Clean Composable API and trace how it changes the rendered interface. This is example 1 for Chapter 22, topic 5.
Example 2: Change one reactive value
Change one value involved in Designing a Clean Composable API inside the message panel. Predict what Vue will update before running it, then compare the prediction with the rendered result.
Example 3: Parent-child comparison
Use Designing a Clean Composable API across two components in the admin table. Compare which component owns the writable data and which component only receives or presents it.
Example 4: Edge case
Add an empty, missing, invalid, delayed, or rapidly changing value to the photo gallery. Handle the Designing a Clean Composable API edge case explicitly instead of leaving stale output.
Example 5: Accessibility review
Use Designing a Clean Composable API in the notification center while checking semantic HTML, labels, focus order, keyboard access, and understandable status feedback. A composable packages reusable stateful logic with the Composition API. In Chapter 22, apply this definition specifically to Designing a Clean Composable API and trace how it changes the rendered interface. This is example 5 for Chapter 22, topic 5.
Example 6: State ownership review
Remove duplicated state from the task board. For Designing a Clean Composable API, derive values when possible and keep the writable source with the component or store that owns it.
Example 7: Slow-network scenario
Assume the language selector has a slow request while using Designing a Clean Composable API. Decide what remains interactive, what shows pending feedback, and how stale responses are prevented.
Example 8: Refactoring exercise
Extract the Designing a Clean Composable API responsibility from a crowded quiz screen into a focused component or composable with a narrow API.
Example 9: Performance experiment
Measure the analytics view before optimizing Designing a Clean Composable API. Check reactive work, repeated calculations, list size, component updates, and user-visible delay.
Example 10: Production review
Review Designing a Clean Composable API in the support form for errors, security, localization, RTL, narrow screens, accessibility, and what monitoring should report. A composable packages reusable stateful logic with the Composition API. In Chapter 22, apply this definition specifically to Designing a Clean Composable API and trace how it changes the rendered interface. This is example 10 for Chapter 22, topic 5.
Vue code example
<script setup>
import { ref } from 'vue'
const topic = "Designing a Clean Composable API"
const active = ref(false)
</script>
<template>
<section><h2>{{ topic }}</h2><button :aria-pressed="active" @click="active=!active">Toggle state</button><p>{{ active ? 'Active' : 'Inactive' }}</p></section>
</template>Step-by-step code explanation
- Identify the Vue responsibility demonstrated by Designing a Clean Composable API.
- Read the reactive state, props, route/store input, or injected value before the template.
- Trace the event, dependency, watcher, lifecycle hook, or navigation action that changes the display.
- Test one normal path and one edge case, including cleanup when external work is involved.
- Confirm accessibility, mobile width, and RTL behavior for user-facing controls and translated text.
Expected behavior: A small Vue interface demonstrates Designing a Clean Composable API and updates according to the interaction or data in the example.
Practice exercise
Create a small Vue feature focused on Designing a Clean Composable API from Chapter 22. Predict the result before running it, test one edge case, and explain which reactive value, prop, event, route, store, or lifecycle step caused the update. Then check accessibility, narrow-screen width, and RTL behavior where relevant.
Chapter 22 review — 10 questions and answers
1. What is the purpose of What a Composable Is?
Answer: A composable packages reusable stateful logic with the Composition API. In Chapter 22, apply this definition specifically to What a Composable Is and trace how it changes the rendered interface.
2. What should you inspect when What a Composable Is behaves unexpectedly?
Answer: Inspect reactive inputs, component ownership, rendered output, edge cases, and what causes another update. Reduce the feature to a small component and trace reactive input through the rendered result.
3. What is the purpose of Extracting Reusable Logic?
Answer: Extracting Reusable Logic is a focused part of Vue application design in Chapter 22. Study its inputs, reactive dependencies, component ownership, rendered result, edge cases, and cleanup requirements.
4. What should you inspect when Extracting Reusable Logic behaves unexpectedly?
Answer: Inspect reactive inputs, component ownership, rendered output, edge cases, and what causes another update. Reduce the feature to a small component and trace reactive input through the rendered result.
5. What is the purpose of Composables with Refs?
Answer: A ref stores a reactive value; JavaScript uses .value while templates usually unwrap it automatically. In Chapter 22, apply this definition specifically to Composables with Refs and trace how it changes the rendered interface.
6. What should you inspect when Composables with Refs behaves unexpectedly?
Answer: Inspect reactive dependencies, update timing, derived values, side effects, and cleanup. Reduce the feature to a small component and trace reactive input through the rendered result.
7. What is the purpose of Composables with Side Effects?
Answer: A composable packages reusable stateful logic with the Composition API. In Chapter 22, apply this definition specifically to Composables with Side Effects and trace how it changes the rendered interface.
8. What should you inspect when Composables with Side Effects behaves unexpectedly?
Answer: Inspect reactive inputs, component ownership, rendered output, edge cases, and what causes another update. Reduce the feature to a small component and trace reactive input through the rendered result.
9. What is the purpose of Designing a Clean Composable API?
Answer: A composable packages reusable stateful logic with the Composition API. In Chapter 22, apply this definition specifically to Designing a Clean Composable API and trace how it changes the rendered interface.
10. What should you inspect when Designing a Clean Composable API behaves unexpectedly?
Answer: Inspect reactive inputs, component ownership, rendered output, edge cases, and what causes another update. Reduce the feature to a small component and trace reactive input through the rendered result.