Creating a Pinia store in Vue - Try It Yourself

Creating a Pinia Store

📖 Scenario: You are building a simple Vue 3 app that needs to share a counter value across components. To do this, you will create a Pinia store to hold and manage the counter state.

🎯 Goal: Build a Pinia store named useCounterStore that holds a count state starting at 0, a getter to get the double of count, and an action to increment count by 1.

📋 What You'll Learn

Create a Pinia store using defineStore.

Store must have a state with a count property initialized to 0.

Add a getter named doubleCount that returns count * 2.

Add an action named increment that increases count by 1.

💡 Why This Matters

🌍 Real World

Pinia stores help manage shared state in Vue apps, making it easy to keep data consistent across components.

💼 Career

Understanding Pinia is important for Vue developers to build scalable and maintainable applications.

1

Set up the Pinia store with initial state

Import defineStore from pinia and create a store named useCounterStore with a state function that returns an object with count set to 0.

Vue

import { defineStore } from 'pinia'

// Create the Pinia store with initial state
// Your code here

Hint

Remember to import defineStore from pinia and define the store with a state function returning an object.

2

Add a getter to compute double the count

Inside the useCounterStore definition, add a getters object with a getter named doubleCount that returns state.count * 2. Use the state parameter in the getter function.

Vue

import { defineStore } from 'pinia'

export const useCounterStore = defineStore('counter', {
  state: () => ({
    count: 0
  }),
  // Add getters here
  // Your code here
})

Hint

Getters are functions inside a getters object. Use state to access the store state.

3

Add an action to increment the count

Inside the useCounterStore definition, add an actions object with a method named increment that increases this.count by 1.

Vue

import { defineStore } from 'pinia'

export const useCounterStore = defineStore('counter', {
  state: () => ({
    count: 0
  }),
  getters: {
    doubleCount: (state) => state.count * 2
  },
  // Add actions here
  // Your code here
})

Hint

Actions are methods inside an actions object. Use this to access and modify state.

4

Export the complete Pinia store

Ensure the useCounterStore is exported as a named export using export const so it can be imported in Vue components.

Vue

import { defineStore } from 'pinia'

// Export the store
// Your code here

Hint

Make sure the store is exported with export const so other files can use it.

Practice

1. What is the main purpose of creating a Pinia store in a Vue application?

easy

A. To handle routing between pages

B. To style components with CSS

C. To hold and manage shared state across components

D. To write HTML templates

Solution

1. Step 1: Understand what Pinia stores do

   Pinia stores are designed to hold shared data (state) that multiple components can access and update.

2. Step 2: Identify the correct purpose

   Styling, routing, and HTML templates are handled by other parts of Vue or related libraries, not Pinia stores.

3. Final Answer:

   To hold and manage shared state across components -> Option C

4. Quick Check:

   Pinia stores = shared state management [OK]

Hint: Pinia stores manage shared data, not styles or routes [OK]

Common Mistakes:

• Confusing Pinia with CSS or routing libraries
• Thinking Pinia handles templates
• Assuming Pinia is for styling

2. Which of the following is the correct way to define a Pinia store named 'user' using defineStore?

easy

A. const userStore = createStore('user', { state: () => ({ name: '' }) })

B. const userStore = defineStore('user', { data: () => ({ name: '' }) })

C. const useUserStore = defineStore({ name: 'user', state: () => ({ name: '' }) })

D. const useUserStore = defineStore('user', { state: () => ({ name: '' }) })

Solution

1. Step 1: Recall the correct syntax for defineStore

   The correct syntax is defineStore('storeId', { state: () => ({ ... }) }) and assign it to a function named useStoreId.

2. Step 2: Check each option

   const useUserStore = defineStore('user', { state: () => ({ name: '' }) }) uses defineStore with correct parameters and naming. const userStore = createStore('user', { state: () => ({ name: '' }) }) uses createStore which is incorrect. const useUserStore = defineStore({ name: 'user', state: () => ({ name: '' }) }) passes an object without storeId string first. const userStore = defineStore('user', { data: () => ({ name: '' }) }) uses 'data' instead of 'state'.

3. Final Answer:

   const useUserStore = defineStore('user', { state: () => ({ name: '' }) }) -> Option D

4. Quick Check:

   defineStore('id', { state }) syntax = const useUserStore = defineStore('user', { state: () => ({ name: '' }) }) [OK]

Hint: Use defineStore('id', { state: () => ({}) }) with useId naming [OK]

Common Mistakes:

• Using createStore instead of defineStore
• Passing object without store id string
• Using 'data' instead of 'state' key

3. Given this Pinia store definition:

const useCounterStore = defineStore('counter', {
  state: () => ({ count: 0 }),
  actions: {
    increment() { this.count++ }
  }
})

What will be the value of count after calling increment() twice?

medium

A. 0

B. 2

C. undefined

D. 1

Solution

1. Step 1: Understand initial state and action

   The initial count is 0. The increment action increases count by 1 each time it is called.

2. Step 2: Calculate count after two increments

   Calling increment() twice adds 1 + 1 to count, so count becomes 2.

3. Final Answer:

   2 -> Option B

4. Quick Check:

   0 + 1 + 1 = 2 [OK]

Hint: Each increment adds 1; two calls add 2 total [OK]

Common Mistakes:

• Forgetting to call increment twice
• Assuming count resets each call
• Confusing initial value with updated value

4. Identify the error in this Pinia store code:

const useTodoStore = defineStore('todo', {
  state: () => ({ todos: [] }),
  actions: {
    addTodo(todo) {
      this.todos.push(todo)
    }
  }
})

// Usage
const todoStore = useTodoStore()
todoStore.addTodo('Learn Pinia')

medium

A. Missing import of defineStore from 'pinia'

B. State should be an object, not a function

C. Actions cannot modify state directly

D. Store id must be an object, not a string

Solution

1. Step 1: Check the store definition syntax

   The store is correctly defined with state as a function returning an object, and actions modifying state properly.

2. Step 2: Identify missing parts

   The code snippet does not show importing defineStore from 'pinia', which is required to use defineStore.

3. Final Answer:

   Missing import of defineStore from 'pinia' -> Option A

4. Quick Check:

   Always import defineStore before use [OK]

Hint: Always import defineStore from 'pinia' before defining stores [OK]

Common Mistakes:

• Forgetting to import defineStore
• Thinking state must be an object, not a function
• Believing actions cannot modify state

5. You want to create a Pinia store that keeps track of a user's profile with a name and age, and includes a getter to check if the user is an adult (age >= 18). Which of the following is the correct way to define this store?

hard

A. const useUserStore = defineStore('user', {
state: () => ({ name: '', age: 0 }),
getters: {
isAdult() { return this.age >= 18 }
}
})

B. const useUserStore = defineStore('user', {
state: { name: '', age: 0 },
getters: {
isAdult() { return this.age >= 18 }
}
})

C. const useUserStore = defineStore('user', {
state: { name: '', age: 0 },
getters: {
isAdult: (state) => state.age >= 18
}
})

D. const useUserStore = defineStore('user', {
state: () => ({ name: '', age: 0 }),
actions: {
isAdult() { return this.age >= 18 }
}
})

Solution

1. Step 1: Check state definition

   State must be a function returning an object. Options C and D do this correctly; A and B do not.

2. Step 2: Check getter syntax

   Getters can be defined as functions using 'this' to access state. const useUserStore = defineStore('user', { state: () => ({ name: '', age: 0 }), getters: { isAdult() { return this.age >= 18 } } }) uses this correctly. const useUserStore = defineStore('user', { state: { name: '', age: 0 }, getters: { isAdult: (state) => state.age >= 18 } }) has incorrect state. const useUserStore = defineStore('user', { state: { name: '', age: 0 }, getters: { isAdult() { return this.age >= 18 } } }) has incorrect state. const useUserStore = defineStore('user', { state: () => ({ name: '', age: 0 }), actions: { isAdult() { return this.age >= 18 } } }) puts isAdult in actions, which is wrong because isAdult is a computed property, not an action.

3. Final Answer:

   const useUserStore = defineStore('user', { state: () => ({ name: '', age: 0 }), getters: { isAdult() { return this.age >= 18 } } }) -> Option A

4. Quick Check:

   State as function + getter with this = const useUserStore = defineStore('user', { state: () => ({ name: '', age: 0 }), getters: { isAdult() { return this.age >= 18 } } }) [OK]

Hint: State is function; getters use this for reactive access [OK]

Common Mistakes:

• Defining state as object instead of function
• Putting computed logic inside actions
• Using arrow function getters with state parameter incorrectly