Getters for computed store values in Vue - Mini Project: Build & Apply

Getters for computed store values

📖 Scenario: You are building a simple Vue 3 app to manage a shopping cart. You want to keep track of the items in the cart and show the total price dynamically.

🎯 Goal: Create a Vue 3 store with a list of cart items, add a getter to compute the total price of all items, and use that getter in a component.

📋 What You'll Learn

Create a reactive store with a cartItems array containing objects with name and price.

Add a getter called totalPrice that computes the sum of all item prices.

Use the getter in a Vue component to display the total price.

Follow Vue 3 Composition API and store patterns.

💡 Why This Matters

🌍 Real World

Stores with getters are common in Vue apps to keep data organized and compute values like totals, averages, or filtered lists.

💼 Career

Understanding how to create computed getters in stores and use them in components is essential for building maintainable Vue applications.

1

Create the initial cart items array

Create a reactive store object called store with a property cartItems set to an array containing these exact objects: { name: 'Apple', price: 1.2 }, { name: 'Banana', price: 0.8 }, and { name: 'Cherry', price: 2.5 }.

Vue

const store = {
  cartItems: [
    // Your code here
  ]
}

Hint

Use an object named store with a property cartItems that is an array of objects.

2

Add a getter for total price

Add a getter property called totalPrice to the store object. This getter should return the sum of the price values of all items in cartItems using reduce.

Vue

const store = {
  cartItems: [
    { name: 'Apple', price: 1.2 },
    { name: 'Banana', price: 0.8 },
    { name: 'Cherry', price: 2.5 }
  ],
  // Your code here
}

Hint

Use a getter with get totalPrice() and inside use this.cartItems.reduce to sum prices.

3

Create a Vue component to display total price

Create a Vue 3 functional component called TotalPriceDisplay that imports store and uses the totalPrice getter to display the total price inside a <div> with the text: Total Price: $ followed by the value.

Vue

const store = {
  cartItems: [
    { name: 'Apple', price: 1.2 },
    { name: 'Banana', price: 0.8 },
    { name: 'Cherry', price: 2.5 }
  ],
  get totalPrice() {
    return this.cartItems.reduce((sum, item) => sum + item.price, 0);
  }
}

// Your code here

Hint

Use defineComponent and return a render function that shows Total Price: $ plus store.totalPrice.

4

Export the store and component

Add export statements to export both the store object and the TotalPriceDisplay component from the module.

Vue

import { defineComponent } from 'vue'

const store = {
  cartItems: [
    { name: 'Apple', price: 1.2 },
    { name: 'Banana', price: 0.8 },
    { name: 'Cherry', price: 2.5 }
  ],
  get totalPrice() {
    return this.cartItems.reduce((sum, item) => sum + item.price, 0);
  }
}

export const TotalPriceDisplay = defineComponent({
  name: 'TotalPriceDisplay',
  setup() {
    return () => (
      
        Total Price: ${store.totalPrice}
      
    )
  }
})

// Your code here

Hint

Use export const before store and TotalPriceDisplay.

Practice

1. What is the main purpose of getters in a Vue store?

easy

A. To compute and return derived state based on the store's data

B. To directly modify the store's state values

C. To register new components in the Vue app

D. To handle user input events in components

Solution

1. Step 1: Understand what getters do

   Getters compute values derived from the store's state without changing the state itself.

2. Step 2: Compare options with this role

   Only To compute and return derived state based on the store's data describes computing and returning derived state, which matches getters' purpose.

3. Final Answer:

   To compute and return derived state based on the store's data -> Option A

4. Quick Check:

   Getters compute derived state = B [OK]

Hint: Getters compute data, mutations change data [OK]

Common Mistakes:

• Confusing getters with mutations that modify state
• Thinking getters handle events or components
• Assuming getters register components

2. Which of the following is the correct syntax to define a getter named completedTasks in a Vue store?

easy

A. getters: { completedTasks: function() { return state.tasks.filter(t => t.done) } }

B. getters: { completedTasks: (state) => state.tasks.filter(t => t.done) }

C. getters: { completedTasks() { return this.tasks.filter(t => t.done) } }

D. getters: { completedTasks: (state) => { state.tasks.filter(t => t.done) } }

Solution

1. Step 1: Review correct getter syntax

   A getter is a function receiving state and returning a value. Arrow function with implicit return is common.

2. Step 2: Check each option

   getters: { completedTasks: (state) => state.tasks.filter(t => t.done) } correctly uses an arrow function with state parameter and returns filtered tasks. getters: { completedTasks() { return this.tasks.filter(t => t.done) } } incorrectly uses this which is not available. getters: { completedTasks: function() { return state.tasks.filter(t => t.done) } } lacks state parameter. getters: { completedTasks: (state) => { state.tasks.filter(t => t.done) } } misses return statement.

3. Final Answer:

   getters: { completedTasks: (state) => state.tasks.filter(t => t.done) } -> Option B

4. Quick Check:

   Getter syntax needs state param and return value = C [OK]

Hint: Getters get state as first param and return computed value [OK]

Common Mistakes:

• Using this inside getters instead of state
• Forgetting to return a value from the getter
• Not including state as a parameter

3. Given the Vue store state:

state: { items: [ { id: 1, done: true }, { id: 2, done: false } ] }

and getter:

getters: { doneCount: (state) => state.items.filter(i => i.done).length }

What will store.getters.doneCount return?

medium

A. 2

B. undefined

C. 1

D. 0

Solution

1. Step 1: Analyze the state array

   There are two items: one with done: true and one with done: false.

2. Step 2: Evaluate the getter logic

   The getter filters items where done is true, so only one item matches. Then it returns the length, which is 1.

3. Final Answer:

   1 -> Option C

4. Quick Check:

   Count of done items = 1 [OK]

Hint: Count items with done true using filter and length [OK]

Common Mistakes:

• Counting all items instead of filtered ones
• Returning the filtered array instead of its length
• Assuming undefined because of missing return

4. Identify the error in this Vue store getter definition:

getters: { activeUsers: (state) => { state.users.filter(u => u.active) } }

medium

A. Filter method cannot be used on arrays

B. Incorrect parameter name; should be 'context' instead of 'state'

C. Using arrow function is not allowed for getters

D. Missing return statement inside the getter function

Solution

1. Step 1: Check getter function body

   The getter uses curly braces but does not return the filtered array explicitly.

2. Step 2: Understand arrow function return rules

   When using curly braces in arrow functions, you must use return to output a value. Without it, the function returns undefined.

3. Final Answer:

   Missing return statement inside the getter function -> Option D

4. Quick Check:

   Arrow function with braces needs explicit return = A [OK]

Hint: Arrow functions with braces need explicit return [OK]

Common Mistakes:

• Assuming implicit return with curly braces
• Confusing parameter names for getters
• Thinking filter can't be used on arrays

5. You want a getter that returns the total price of items in a cart, but only for items with quantity > 0.
Given state:

state: { cart: [ { price: 10, quantity: 2 }, { price: 5 }, { price: 3, quantity: 4 } ] }

Which getter correctly computes this total?

hard

A. totalPrice: (state) => state.cart.filter(i => i.quantity > 0).reduce((sum, item) => sum + item.price * item.quantity, 0)

B. totalPrice: (state) => state.cart.reduce((sum, item) => sum + item.price * item.quantity, 0)

C. totalPrice: (state) => state.cart.map(i => i.price * i.quantity).reduce((a, b) => a + b)

D. totalPrice: (state) => state.cart.reduce((sum, item) => sum + item.price, 0)

Solution

1. Step 1: Understand the requirement

   Only items with quantity > 0 should be counted in total price calculation.

2. Step 2: Analyze each option

   totalPrice: (state) => state.cart.reduce((sum, item) => sum + item.price * item.quantity, 0) sums all items regardless of quantity. totalPrice: (state) => state.cart.filter(i => i.quantity > 0).reduce((sum, item) => sum + item.price * item.quantity, 0) filters items with quantity > 0 before summing, which is correct. totalPrice: (state) => state.cart.map(i => i.price * i.quantity).reduce((a, b) => a + b) sums all items including those with zero quantity. totalPrice: (state) => state.cart.reduce((sum, item) => sum + item.price, 0) sums only prices ignoring quantity.

3. Final Answer:

   totalPrice: (state) => state.cart.filter(i => i.quantity > 0).reduce((sum, item) => sum + item.price * item.quantity, 0) -> Option A

4. Quick Check:

   Filter then reduce for quantity > 0 total = A [OK]

Hint: Filter items with quantity > 0 before summing [OK]

Common Mistakes:

• Not filtering out items with zero quantity
• Summing prices without multiplying by quantity
• Using map without filtering first