Creating a Pinia store in Vue

Introduction

A Pinia store helps you keep and manage data in one place for your Vue app. It makes sharing data easy and organized.

You want to share data between different Vue components.

You need to keep track of user login status across pages.

You want to manage a shopping cart in an online store app.

You want to update data in one place and see changes everywhere.

You want a simple way to organize app state without confusion.

Syntax

Vue

import { defineStore } from 'pinia'

export const useStore = defineStore('storeId', {
  state: () => ({
    // your data here
  }),
  getters: {
    // computed properties here
  },
  actions: {
    // functions to change state here
  }
})

defineStore creates a store with a unique ID.

state is a function returning an object holding your data.

Examples

A simple counter store with a number and a function to add one.

Vue

import { defineStore } from 'pinia'

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

A user store with data, a message getter, and a login action.

Vue

import { defineStore } from 'pinia'

export const useUserStore = defineStore('user', {
  state: () => ({ name: '', loggedIn: false }),
  getters: {
    welcomeMessage: (state) => `Hello, ${state.name}`
  },
  actions: {
    login(name) {
      this.name = name
      this.loggedIn = true
    }
  }
})

Sample Program

This example shows a simple todo list using a Pinia store. You can add todos and toggle their done status. The store keeps the list and updates it for the app.

Vue

import { createApp, ref } from 'vue'
import { createPinia, defineStore } from 'pinia'

// Create a store
export const useTodoStore = defineStore('todo', {
  state: () => ({
    todos: []
  }),
  actions: {
    addTodo(text) {
      this.todos.push({ text, done: false })
    },
    toggleTodo(index) {
      this.todos[index].done = !this.todos[index].done
    }
  }
})

// Vue app component
const App = {
  template: `
    <div>
      <h1>Todo List</h1>
      <input v-model="newTodo" @keyup.enter="add" placeholder="Add todo" aria-label="Add todo" />
      <ul>
        <li v-for="(todo, i) in todoStore.todos" :key="i">
          <label>
            <input type="checkbox" v-model="todo.done" />
            <span :style="{ textDecoration: todo.done ? 'line-through' : 'none' }">{{ todo.text }}</span>
          </label>
        </li>
      </ul>
    </div>
  `,
  setup() {
    const todoStore = useTodoStore()
    const newTodo = ref('')

    function add() {
      if (newTodo.value.trim()) {
        todoStore.addTodo(newTodo.value.trim())
        newTodo.value = ''
      }
    }

    return { todoStore, newTodo, add }
  }
}

// Create app and use Pinia
const app = createApp(App)
const pinia = createPinia()
app.use(pinia)
app.mount('#app')

Important Notes

Always give your store a unique ID string in defineStore.

Use actions to change state safely and keep code organized.

Pinia works well with Vue 3 and Composition API for clean code.

Summary

Pinia stores hold shared data for Vue apps.

Define state, getters, and actions inside defineStore.

Use stores to keep your app data organized and easy to update.

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