Composable accepting parameters in Vue - Performance & Optimization

Performance: Composable accepting parameters

MEDIUM IMPACT

This affects how efficiently Vue manages reactive state and re-renders components when composables receive parameters.

Using a composable that accepts parameters to manage reactive state

Vue

import { ref } from 'vue';

export function useCounter(initial) {
  const count = ref(initial);
  // No watch on parameter, parameter used only once
  return { count };
}

Using the parameter only once avoids unnecessary reactive tracking and re-renders.

📈 Performance Gainsingle reactive setup, fewer re-renders

Using a composable that accepts parameters to manage reactive state

Vue

import { ref, watch } from 'vue';

export function useCounter(initial) {
  const count = ref(initial);
  watch(() => initial, (newVal) => {
    count.value = newVal;
  });
  return { count };
}

Watching the parameter directly causes the composable to react and update whenever the parameter changes, triggering extra reactivity and re-renders.

📉 Performance Costtriggers multiple reactive updates and re-renders on parameter change

Performance Comparison

Pattern	DOM Operations	Reflows	Paint Cost	Verdict
Watching parameter changes inside composable	Multiple reactive updates	Multiple reflows on parameter change	Higher paint cost due to frequent updates	[X] Bad
Using parameter only once as initial value	Single reactive update	Single reflow on state change	Lower paint cost with fewer updates	[OK] Good

Rendering Pipeline

When a composable accepts parameters, Vue tracks dependencies to update reactive state. If parameters are watched or reactive themselves, this can cause extra style recalculations and layout updates.

→Reactive Dependency Tracking

→Component Update

→Style Calculation

→Layout

⚠️ BottleneckReactive Dependency Tracking causing unnecessary component updates

Core Web Vital Affected

INP

This affects how efficiently Vue manages reactive state and re-renders components when composables receive parameters.

Optimization Tips

1Avoid watching composable parameters reactively unless necessary.

2Use parameters as initial values to prevent extra reactive tracking.

3Profile reactive updates to detect unnecessary re-renders caused by parameter changes.

Performance Quiz - 3 Questions

What is a performance risk when a Vue composable watches its input parameter reactively?

A It causes extra reactive updates and re-renders when the parameter changes.

B It reduces the initial load time of the component.

C It prevents the component from updating when needed.

D It decreases memory usage by sharing state.

DevTools: Performance

How to check: Record a performance profile while interacting with the component using the composable. Look for frequent reactive updates and component re-renders triggered by parameter changes.

What to look for: High number of reactive effect recalculations and component updates when parameters change indicates inefficient composable usage.

Practice

1.

What is the main purpose of passing parameters to a Vue composable function?

easy

A. To customize the composable's behavior for different uses

B. To make the composable function run only once

C. To prevent the composable from using Vue's reactivity

D. To automatically render a template inside the composable

Solution

1. Step 1: Understand composable functions

   Composable functions are reusable pieces of logic in Vue that can be shared across components.

2. Step 2: Role of parameters in composables

   Parameters allow passing different inputs to the composable so it can behave differently depending on the need.

3. Final Answer:

   To customize the composable's behavior for different uses -> Option A

4. Quick Check:

   Parameters customize composables = A [OK]

Hint: Parameters let composables adapt to different inputs [OK]

Common Mistakes:

• Thinking composables run only once
• Believing parameters disable reactivity
• Confusing composables with templates

2.

Which of the following is the correct way to define a composable that accepts a parameter initialCount?

function useCounter(???) {
  const count = ref(initialCount)
  return { count }
}

easy

A. useCounter((initialCount))

B. useCounter()

C. useCounter(initialCount)

D. useCounter(initialCount: number)

Solution

1. Step 1: Check JavaScript function parameter syntax

   In JavaScript, function parameters are named without types or parentheses inside the parentheses.

2. Step 2: Match parameter name with usage

   The parameter initialCount must be declared inside the parentheses as initialCount.

3. Final Answer:

   useCounter(initialCount) -> Option C

4. Quick Check:

   Function parameter syntax = D [OK]

Hint: Function parameters go inside parentheses without types [OK]

Common Mistakes:

• Adding types like in TypeScript without syntax
• Using parentheses inside parameter list
• Leaving parameters empty when needed

3.

Given this composable and usage, what will be logged?

import { ref } from 'vue'
function useGreeting(name) {
  const greeting = ref(`Hello, ${name}!`)
  return { greeting }
}

const { greeting } = useGreeting('Alice')
console.log(greeting.value)

medium

A. undefined

B. "Hello, Alice!"

C. "Hello, name!"

D. Error: greeting is not defined

Solution

1. Step 1: Understand parameter passing

   The composable receives 'Alice' as the name parameter, so the greeting string becomes "Hello, Alice!".

2. Step 2: Accessing the ref value

   The greeting is a ref, so to get the string, we use greeting.value, which logs "Hello, Alice!".

3. Final Answer:

   "Hello, Alice!" -> Option B

4. Quick Check:

   Ref .value logs "Hello, Alice!" = C [OK]

Hint: Use .value to get ref content in Vue composables [OK]

Common Mistakes:

• Logging the ref object instead of its value
• Confusing parameter name with string literal
• Forgetting to import ref from 'vue'

4.

Identify the error in this composable that accepts a parameter start:

import { ref } from 'vue'
function useTimer(start) {
  const time = ref(start)
  function increment() {
    time++
  }
  return { time, increment }
}

medium

A. Cannot increment a ref directly with ++ operator

B. Missing return statement for increment function

C. Parameter start is not reactive

D. Ref should be initialized with 0, not start

Solution

1. Step 1: Check how ref values are updated

   Refs hold their value inside the .value property, so you cannot increment the ref itself.

2. Step 2: Identify correct increment syntax

   The increment function should update time.value++ instead of time++.

3. Final Answer:

   Cannot increment a ref directly with ++ operator -> Option A

4. Quick Check:

   Update ref via .value, not ref itself = B [OK]

Hint: Use .value to update ref, not the ref variable itself [OK]

Common Mistakes:

• Trying to increment ref without .value
• Thinking parameters must be reactive
• Forgetting to return functions from composable

5.

You want a composable useFilteredList that accepts an array and a filter function as parameters. Which implementation correctly applies the filter reactively?

import { computed } from 'vue'

function useFilteredList(list, filterFn) {
  const filtered = computed(() => ??? )
  return { filtered }
}

What should replace ????

hard

A. list.filter(filterFn)

B. filterFn(list)

C. computed(() => list.filter(filterFn))

D. list.value.filter(filterFn)

Solution

1. Step 1: Understand composable parameters

   The list parameter is expected to be a ref(array) for proper reactivity in Vue composables.

2. Step 2: Reactive filtering with computed

   Inside the computed, access the array value with list.value.filter(filterFn) to apply the filter reactively whenever the list changes.

3. Final Answer:

   list.value.filter(filterFn) -> Option D

4. Quick Check:

   computed(() => list.value.filter(filterFn)) = C [OK]

Hint: Use computed(() => list.value.filter(fn)) for reactive filtering [OK]

Common Mistakes:

• Using list.filter(filterFn) without .value when list is ref
• Calling filterFn(list) instead of list.filter(filterFn)
• Unnecessarily nesting computed inside computed