Composables for reusable logic in Vue - Step-by-Step Execution

Concept Flow - Composables for reusable logic

Create composable function

↓

Define reactive state inside composable

↓

Return state and functions

↓

Import composable in component

↓

Use returned state and functions

↓

Component reacts to composable state changes

↓

Reusable logic shared across components

This flow shows how you create a composable function with reactive state and logic, then use it inside components to share behavior easily.

Execution Sample

Vue

import { ref } from 'vue'

export function useCounter() {
  const count = ref(0)
  function increment() { count.value++ }
  return { count, increment }
}

Defines a composable that provides a reactive count and an increment function.

Execution Table

Step	Action	State Before	State After	Output/Effect
1	Call useCounter()	No count	count = 0	Composable returns count and increment
2	Component uses count.value	count = 0	count = 0	Component shows count as 0
3	Call increment()	count = 0	count = 1	count.value increments to 1
4	Component reacts to count change	count = 1	count = 1	UI updates to show count as 1
5	Call increment() again	count = 1	count = 2	count.value increments to 2
6	Component reacts to count change	count = 2	count = 2	UI updates to show count as 2
7	No more actions	count = 2	count = 2	Execution stops

💡 No more actions to perform, composable state stable

Variable Tracker

Variable	Start	After Step 1	After Step 3	After Step 5	Final
count.value	undefined	0	1	2	2

Key Moments - 2 Insights

Why does count.value update inside the component when increment() is called?

Because count is a reactive ref returned by the composable, when increment() changes count.value, Vue tracks this and updates the component automatically (see execution_table steps 3 and 4).

Can multiple components use the same composable and keep their own count?

Yes, each call to useCounter() creates a new reactive count ref, so each component has its own independent state (not shown in table but implied by composable pattern).

Visual Quiz - 3 Questions

Look at the execution_table at step 3, what is the value of count after increment() is called?

A 0

B undefined

C 1

D 2

💡 Hint

Check the 'State After' column at step 3 in execution_table

At which step does the component update its UI to show the new count value?

A Step 2

B Step 4

C Step 5

D Step 1

💡 Hint

Look for 'UI updates' in the Output/Effect column in execution_table

If we call increment() twice, what will count.value be after the second call?

A 2

B 0

C 1

D undefined

💡 Hint

See variable_tracker for count.value changes after steps 3 and 5

Concept Snapshot

Composable functions in Vue are reusable logic blocks.
They use reactive state like ref() inside.
Return state and functions to components.
Each component calling composable gets independent state.
Components react automatically to reactive changes.
Use composables to share logic cleanly.

Full Transcript

This visual trace shows how Vue composables work. First, you create a composable function that defines reactive state using ref(). It returns this state and any functions that change it. When a component calls this composable, it gets its own reactive state. Calling functions like increment() updates the reactive state. Vue detects these changes and updates the component's UI automatically. Each component using the composable has its own independent state. This pattern helps share logic easily and keeps components clean.

Practice

1. What is the main purpose of a composable in Vue?

easy

A. To reuse reactive logic across multiple components

B. To define global CSS styles

C. To create new Vue directives

D. To register Vue components globally

Solution

1. Step 1: Understand composables role

   Composables are functions that encapsulate reactive logic to be reused.

2. Step 2: Identify main purpose

   They help share logic like state or effects across components without duplication.

3. Final Answer:

   To reuse reactive logic across multiple components -> Option A

4. Quick Check:

   Composable = reusable logic [OK]

Hint: Composables share logic, not styles or components [OK]

Common Mistakes:

• Confusing composables with CSS or templates
• Thinking composables register components
• Assuming composables create directives

2. Which of the following is the correct way to define a composable in Vue 3?

easy

A. function useCounter() { return reactive({ count: 0 }) }

B. const useCounter = new Vue({ data() { return { count: 0 } } })

C. const useCounter = reactive({ count: 0 })

D. function useCounter() { const count = ref(0); return { count }; }

Solution

1. Step 1: Check composable function syntax

   A composable is a function that returns reactive state or methods.

2. Step 2: Validate options

   function useCounter() { const count = ref(0); return { count }; } defines a function returning a ref wrapped count, which is correct. const useCounter = new Vue({ data() { return { count: 0 } } }) wrongly uses Vue instance syntax. const useCounter = reactive({ count: 0 }) returns reactive object directly without function. function useCounter() { return reactive({ count: 0 }) } returns reactive but misses ref usage and best practice.

3. Final Answer:

   function useCounter() { const count = ref(0); return { count }; } -> Option D

4. Quick Check:

   Composable = function returning reactive state [OK]

Hint: Composable must be a function returning reactive data [OK]

Common Mistakes:

• Defining composables as Vue instances
• Returning reactive objects without a function
• Not using ref or reactive inside composables

3. Given this composable:

import { ref } from 'vue';
export function useToggle() {
  const isOn = ref(false);
  function toggle() { isOn.value = !isOn.value; }
  return { isOn, toggle };
}

What will be the value of isOn.value after calling toggle() twice starting from initial state?

medium

A. true

B. undefined

C. false

D. null

Solution

1. Step 1: Analyze initial state and toggle function

   Initially, isOn.value is false. Each toggle flips it to the opposite.

2. Step 2: Apply two toggles

   First toggle: false -> true. Second toggle: true -> false.

3. Final Answer:

   false -> Option C

4. Quick Check:

   Two toggles flip false twice back to false [OK]

Hint: Toggle flips boolean; even toggles return original [OK]

Common Mistakes:

• Assuming toggle sets true always
• Forgetting to access .value of ref
• Confusing initial state with toggled state

4. What is wrong with this composable code?

import { ref } from 'vue';
export function useCount() {
  const count = ref(0);
  function increment() {
    count++;
  }
  return { count, increment };
}

medium

A. The count variable should be declared with reactive()

B. You cannot increment a ref directly with ++ operator

C. ref should be imported from 'vue/reactive' instead

D. The function should be named useCounter, not useCount

Solution

1. Step 1: Understand ref usage

   Ref values are accessed and mutated via the .value property.

2. Step 2: Identify error in increment function

   The code uses count++ which tries to increment the ref object itself, not its value. It should be count.value++.

3. Final Answer:

   You cannot increment a ref directly with ++ operator -> Option B

4. Quick Check:

   Use count.value++ to update ref [OK]

Hint: Always use .value to read or write ref values [OK]

Common Mistakes:

• Incrementing ref without .value
• Confusing reactive and ref imports
• Thinking function name affects behavior

5. You want to create a composable that tracks window width and updates reactively on resize. Which approach correctly implements this?

hard

A. 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 };
}

B. function useWindowWidth() {
const width = ref(window.innerWidth);
window.addEventListener('resize', () => {
width.value = window.innerWidth;
});
return { width };
}

C. function useWindowWidth() {
const width = reactive({ value: window.innerWidth });
window.onresize = () => {
width.value = window.innerWidth;
};
return width;
}

D. function useWindowWidth() {
const width = ref(0);
window.addEventListener('resize', () => {
width = window.innerWidth;
});
return { width };
}

Solution

1. Step 1: Understand reactive window width tracking

   We need a ref to hold width and update it on resize events.

2. Step 2: Check event listener 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 }; } correctly adds listener on mount and removes it on unmount to avoid leaks. function useWindowWidth() { const width = ref(window.innerWidth); window.addEventListener('resize', () => { width.value = window.innerWidth; }); return { width }; } adds listener immediately without cleanup. function useWindowWidth() { const width = reactive({ value: window.innerWidth }); window.onresize = () => { width.value = window.innerWidth; }; return width; } uses reactive incorrectly and overwrites window.onresize. function useWindowWidth() { const width = ref(0); window.addEventListener('resize', () => { width = window.innerWidth; }); return { width }; } tries to assign to ref variable directly, which is invalid.

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 A

4. Quick Check:

   Manage listeners with lifecycle hooks [OK]

Hint: Add and remove listeners inside lifecycle hooks [OK]

Common Mistakes:

• Not removing event listeners causing memory leaks
• Assigning directly to ref variable instead of .value
• Overwriting window.onresize instead of adding listener