Composable accepting parameters in Vue - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is a composable in Vue 3?

A composable is a reusable function that uses Vue's Composition API features like ref and reactive to share logic between components.

beginner

How do you pass parameters to a composable in Vue?

You pass parameters by defining them as function arguments when creating the composable. This allows the composable to use dynamic values.

intermediate

Why use parameters in composables?

Parameters make composables flexible and reusable by allowing different inputs to customize their behavior for each use case.

beginner

Example: What does this composable do?
<pre>import { ref } from 'vue';
function useCounter(start = 0) {
  const count = ref(start);
  const increment = () => count.value++;
  return { count, increment };
}</pre>

This composable creates a counter starting at the given start value (default 0). It returns a reactive count and an increment function to increase it.

beginner

How does Vue's ref help in composables?

ref creates a reactive value that Vue tracks for changes. When used in composables, it allows components to reactively update when the composable's state changes.

How do you make a composable accept a starting value in Vue?

A By hardcoding the value inside the composable

B By using a global variable

C By adding a parameter to the composable function

D By passing props to the composable

Composable functions accept parameters like any normal function, allowing dynamic input.

What Vue feature makes the composable's state reactive?

A ref

B props

C emit

D watch

ref creates reactive data that Vue tracks for changes.

Which of these is a benefit of passing parameters to composables?

A Prevents composables from being reactive

B Makes composables reusable with different data

C Forces composables to be global

D Disables component updates

Parameters allow composables to work with different inputs, increasing reuse.

What is the default value of the parameter in this composable?

function useCounter(start = 0) { ... }

A 1

B undefined

C null

D 0

The parameter start has a default value of 0.

How do you access the reactive value inside a ref in Vue?

A Using the <code>.value</code> property

B Directly like a normal variable

C Using <code>.reactive</code>

D Using <code>.data</code>

Reactive values inside ref are accessed with .value.

Explain how to create a composable in Vue that accepts parameters and why this is useful.

Think about how functions normally take inputs and how that applies to composables.

Describe how Vue's ref works inside a composable and how you use it with parameters.

Consider how reactive state and parameters combine to make composables powerful.

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