Type-safe stores with Pinia in Vue - Mini Project: Build & Apply

Type-safe stores with Pinia

📖 Scenario: You are building a simple Vue app to manage a list of tasks. You want to keep the tasks in a central store using Pinia. To avoid mistakes, you will make the store type-safe using TypeScript.

🎯 Goal: Create a Pinia store with TypeScript that holds a list of tasks. Each task has an id (number) and a title (string). Add a getter to count tasks and an action to add a new task.

📋 What You'll Learn

Create a Pinia store named useTaskStore using defineStore

Define a TypeScript interface Task with id: number and title: string

Store state as an array of Task objects named tasks

Add a getter taskCount returning the number of tasks

Add an action addTask that takes a Task and adds it to tasks

💡 Why This Matters

🌍 Real World

Type-safe stores help prevent bugs by ensuring data shapes are consistent. This is useful in any Vue app managing shared state.

💼 Career

Many Vue jobs require managing state with Pinia and TypeScript. Knowing how to create type-safe stores is a valuable skill.

1

Set up the Task interface and initial state

Create a TypeScript interface called Task with id as a number and title as a string. Then create a Pinia store named useTaskStore using defineStore with an empty array of tasks as the state.

Vue

import { defineStore } from 'pinia'

// Define Task interface and create useTaskStore with empty tasks array
// Your code here

Hint

Use interface Task { id: number; title: string } and state: () => ({ tasks: [] as Task[] }).

2

Add a getter to count tasks

Add a getter named taskCount inside useTaskStore that returns the length of the tasks array.

Vue

import { defineStore } from 'pinia'

interface Task {
  id: number
  title: string
}

export const useTaskStore = defineStore('task', {
  state: () => ({
    tasks: [] as Task[]
  }),
  // Add getter taskCount here
  // Your code here
})

Hint

Use getters: { taskCount: (state) => state.tasks.length }.

3

Add an action to add a new task

Add an action named addTask inside useTaskStore that takes a parameter task of type Task and pushes it into the tasks array.

Vue

import { defineStore } from 'pinia'

interface Task {
  id: number
  title: string
}

export const useTaskStore = defineStore('task', {
  state: () => ({
    tasks: [] as Task[]
  }),
  getters: {
    taskCount: (state) => state.tasks.length
  },
  // Add action addTask here
  // Your code here
})

Hint

Use actions: { addTask(task: Task) { this.tasks.push(task) } }.

4

Export the store and use it in a Vue component

Ensure the store useTaskStore is exported. Then, in a Vue component, import useTaskStore and create a store instance by calling it inside setup().

Vue

import { defineStore } from 'pinia'
import { defineComponent } from 'vue'

interface Task {
  id: number
  title: string
}

export const useTaskStore = defineStore('task', {
  state: () => ({
    tasks: [] as Task[]
  }),
  getters: {
    taskCount: (state) => state.tasks.length
  },
  actions: {
    addTask(task: Task) {
      this.tasks.push(task)
    }
  }
})

// In a Vue component, import and create the store instance inside setup()
// Your code here

Hint

Import useTaskStore and call it inside setup() to create taskStore.

Practice

1. What is the main benefit of using type-safe stores with Pinia in a Vue application?

easy

A. It speeds up the app by caching data on the server.

B. It automatically styles components based on state.

C. It replaces Vue Router for navigation.

D. It helps catch errors by checking data types during development.

Solution

1. Step 1: Understand type safety in programming

   Type safety means the system checks that data matches expected types, reducing bugs.

2. Step 2: Connect type safety to Pinia stores

   Pinia stores with types ensure state and actions use correct data types, catching mistakes early.

3. Final Answer:

   It helps catch errors by checking data types during development. -> Option D

4. Quick Check:

   Type safety = catches type errors [OK]

Hint: Type-safe means checking data types to avoid bugs [OK]

Common Mistakes:

• Confusing type safety with styling or UI features
• Thinking Pinia replaces routing
• Assuming type safety improves runtime speed

2. Which of the following is the correct way to define a Pinia store with TypeScript for a state property count of type number?

easy

A. const useStore = defineStore('main', { state: () => ({ count: '0' }) })

B. const useStore = defineStore('main', { state: () => { count: 0 as number } })

C. const useStore = defineStore('main', { state: () => ({ count: 0 }) })

D. const useStore = defineStore('main', { state: () => ({ count: Number }) })

Solution

1. Step 1: Check state initialization syntax

   State is a function returning an object with initial values; count should be a number, not string or type.

2. Step 2: Identify correct type-safe initialization

   Using count: 0 correctly sets count as number; as number is unnecessary here.

3. Final Answer:

   const useStore = defineStore('main', { state: () => ({ count: 0 }) }) -> Option C

4. Quick Check:

   State count initialized as number = const useStore = defineStore('main', { state: () => ({ count: 0 }) }) [OK]

Hint: Initialize state with correct value type directly [OK]

Common Mistakes:

• Using string '0' instead of number 0
• Using type casts unnecessarily in state
• Assigning type constructors like Number instead of values

3. Given this Pinia store using TypeScript:

const useCounterStore = defineStore('counter', {
  state: () => ({ count: 5 }),
  actions: {
    increment() { this.count++ }
  }
})

const store = useCounterStore()
store.increment()
console.log(store.count)

What will be logged to the console?

medium

A. 5

B. 6

C. undefined

D. Error: count is readonly

Solution

1. Step 1: Understand initial state and action effect

   The state count starts at 5. The increment action increases it by 1.

2. Step 2: Trace the code execution

   Calling store.increment() changes count from 5 to 6, then logging outputs 6.

3. Final Answer:

   6 -> Option B

4. Quick Check:

   5 + 1 = 6 [OK]

Hint: Increment adds 1 to count, so output is 6 [OK]

Common Mistakes:

• Thinking state is immutable and causes error
• Forgetting increment changes count
• Assuming initial value remains unchanged

4. Identify the error in this Pinia store definition using TypeScript:

const useStore = defineStore('main', {
  state: () => ({ count: 0 }),
  actions: {
    increment() {
      this.count = this.count + 1
    },
    reset() {
      this.count = 'zero'
    }
  }
})

medium

A. Using string 'zero' to assign to number state property count causes a type error.

B. The increment method should use this.count++ instead of addition.

C. State must be an object, not a function returning an object.

D. The store name 'main' is invalid and must be uppercase.

Solution

1. Step 1: Check state type and assignments

   The state count is a number initialized to 0.

2. Step 2: Identify type mismatch in actions

   In reset(), assigning string 'zero' to count breaks type safety, causing error.

3. Final Answer:

   Using string 'zero' to assign to number state property count causes a type error. -> Option A

4. Quick Check:

   Assigning wrong type to state = Using string 'zero' to assign to number state property count causes a type error. [OK]

Hint: State types must match assigned values exactly [OK]

Common Mistakes:

• Ignoring type mismatch errors
• Thinking addition syntax is wrong
• Believing state must be plain object, not function
• Assuming store names must be uppercase

5. You want to create a type-safe Pinia store for a user profile with state properties name (string), age (number), and an action updateProfile that accepts a partial object to update state. Which approach correctly ensures type safety?

hard

A. Define an interface for state, use it in store, and type updateProfile parameter as Partial<State>.

B. Use any type for state and action parameters to allow flexibility.

C. Define state without types and cast inside updateProfile to avoid errors.

D. Use separate stores for each property to avoid typing complexity.

Solution

1. Step 1: Define a TypeScript interface for the state

   Creating an interface with name: string and age: number sets clear expected types.

2. Step 2: Use Partial<State> in updateProfile parameter

   This allows updating any subset of properties while keeping type safety.

3. Step 3: Implement store using interface and typed action

   Using the interface in defineStore and typing updateProfile ensures safe updates.

4. Final Answer:

   Define an interface for state, use it in store, and type updateProfile parameter as Partial<State>. -> Option A

5. Quick Check:

   Interface + Partial parameter = Define an interface for state, use it in store, and type updateProfile parameter as Partial<State>. [OK]

Hint: Use interfaces and Partial type for safe partial updates [OK]

Common Mistakes:

• Using any type loses type safety
• Casting types inside actions hides errors
• Splitting state unnecessarily complicates design