A beginners guide to Vuex

A beginners guide to Vuex

In this article, I’ll offer you a high-level overview of Vuex and demonstrate how to implement it into a simple app.

In single-page applications, the concept of state relates to any piece of data that can change. An example of state could be the details of a logged-in user, or data fetched from an API.

Handling state in single-page apps can be a tricky process. As an application gets larger and more complex, you start to encounter situations where a given piece of state needs to be used in multiple components, or you find yourself passing state through components that don’t need it, just to get it to where it needs to be. This is also known as “prop drilling”, and can lead to some unwieldy code.

Vuex is the official state management solution for Vue. It works by having a central store for shared state, and providing methods to allow any component in your application to access that state. In essence, Vuex ensures your views remain consistent with your application data, regardless of which function triggers a change to your application data.

A Shopping Cart Example

Let’s consider a real-world example to demonstrate the problem that Vuex solves.

When you go to a shopping site, you’ll usually have a list of products. Each product has an Add to Cart button and sometimes an Items Remaining label indicating the current stock or the maximum number of items you can order for the specified product. Each time a product is purchased, the current stock of that product is reduced. When this happens, the Items Remaining label should update with the correct figure. When the product’s stock level reaches 0, the label should read Out of Stock. In addition, the Add to Cart button should be disabled or hidden to ensure customers can’t order products that are currently not in inventory.

Now ask yourself how you’d implement this logic. It may be trickier than you think. And let me throw in a curve ball. You’ll need another function for updating stock records when new stock comes in. When the depleted product’s stock is updated, both the Items Remaining label and the Add to Cart button should be updated instantly to reflect the new state of the stock.

Depending on your programming prowess, your solution may start to look a bit like spaghetti. Now, let’s imagine your boss tells you to develop an API that allows third-party sites to sell the products directly from the warehouse. The API needs to ensure that the main shopping website remains in sync with the products’ stock levels. At this point you feel like pulling your hair out and demanding why you weren’t told to implement this earlier. You feel like all your hard work has gone to waste, as you’ll need to completely rework your code to cope with this new requirement.

This is where a state management pattern library can save you from such headaches. It will help you organize the code that handles your front-end data in a way that makes adding new requirements a breeze.

Prerequisites

Before we start, I’ll assume that you:

  • have a basic knowledge of Vue.js
  • are familiar with ES6 and ES7 language features

You’ll also need to have a recent version of Node.js that’s not older than version 6.0. At the time of writing, Node.js v10.13.0 (LTS) and npm version 6.4.1 are the most recent.

Finally, you should have the most recent version of the Vue CLI installed:

npm install -g @vue/cli 

Build a Counter Using Local State

In this section, we’re going to build a simple counter that keeps track of its state locally. Once we’re done, I’ll go over the fundamental concepts of Vuex, before looking at how to rewrite the counter app to use Vue’s official state management solution.

Getting Set Up

Let’s generate a new project using the CLI:

vue create vuex-counter 

A wizard will open up to guide you through the project creation. Select Manually select features and ensure that you choose to install Vuex.

Next, change into the new directory and in the src/components folder, rename HelloWorld.vue to Counter.vue:

cd vuex-counter
mv src/components/HelloWorld.vue src/components/Counter.vue

Finally, open up src/App.vue and replace the existing code with the following:



    # Vuex Counter






You can leave the styles as they are.

Creating the Counter

Let’s start off by initializing a count and outputting it to the page. We’ll also inform the user whether the count is currently even or odd. Open up src/components/Counter.vue and replace the code with the following:




Clicked {{ count }} times! Count is {{ parity }}.





As you can see, we have one state variable called count and a computed function called parity which returns the string even or odd depending on the whether count is an odd or even number.

To see what we’ve got so far, start the app from within the root folder by running npm run serve and navigate to http://localhost:8080.

Feel free to change the value of the counter to show that the correct output for both counter and parity is displayed. When you’re satisfied, make sure to reset it back to 0 before we proceed to the next step.

Incrementing and Decrementing

Right after the computed property in the `


Since Vuex stores are reactive, whenever the value of `$store.state.count` changes, the view will change as well. All this happens behind the scenes, making your code look simple and cleaner.

#### The `mapState` Helper

Now, suppose you have multiple states you want to display in your views. Declaring a long list of computed properties can get verbose, so Vuex provides a [mapState](https://vuex.vuejs.org/api/#mapstate) helper. This can be used to generate multiple computed properties easily. Here’s an example:

Welcome, {{ loggedInUser.name }}.

Count is {{ count }}.


Here’s an even simpler alternative where we can pass an array of strings to the `mapState` helper function:

export default { computed: mapState([ 'count', 'loggedInUser' ]) }


This version of the code and the one above it do exactly the same thing. You should note that `mapState` returns an object. If you want to use it with other computed properties, you can use the [spread operator](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Operators/Spread_syntax). Here’s how:

computed: { ...mapState([ 'count', 'loggedInUser' ]), parity: function() { return this.count % 2 === 0 ? 'even' : 'odd' } }


### **Getters**

In a Vuex store, getters are the equivalent to Vue’s computed properties. They allow you to create _derived state_ that can be shared between different components. Here’s a quick example:

getters: { depletedProducts: state => { return state.products.filter(product => product.stock <= 0) } }


Results of `getter` handlers (when accessed as properties) are cached and can be called as many times as you wish. They’re also reactive to state changes. In other words, if the state it depends upon changes, the `getter` function is automatically executed and the new result is cached. Any component that has accessed a `getter` handler will get updated instantly. This is how you can access a `getter` handler from a component:

computed: { depletedProducts() { return this.$store.getters.depletedProducts; } }


#### The `mapGetters` Helper

You can simplify the `getters` code by using the `mapGetters` helper:

import { mapGetters } from 'vuex'

export default { //.. computed: { ...mapGetters([ 'depletedProducts', 'anotherGetter' ]) } }


There’s an option for passing arguments to a `getter` handler by returning a function. This is useful if you want to perform a query within the `getter`:

getters: { getProductById: state => id => { return state.products.find(product => product.id === id); } }

store.getters.getProductById(5)


Do note that each time a `getter` handler is accessed via a method, it will always run and the result won’t be cached.

Compare:

// property notation, result cached store.getters.depletedProducts

// method notation, result not cached store.getters.getProductById(5)


### **Changing State with Mutations**

An important aspect of the Vuex architecture is that components never alter the state directly. Doing so can lead to odd bugs and inconsistencies in the app’s state.

Instead, the way to change state in a Vuex store is by committing a _mutation_. For those of you familiar with Redux, these are similar to _reducers_.

Here is an example of a mutation that increases a `count` variable stored in `state`:

export default new Vuex.Store({ state:{ count: 1 }, mutations: { increment(state) { state.count++ } } })


You can’t call a mutation handler directly. Instead, you trigger one by “committing a mutation” like this:

methods: { updateCount() { this.$store.commit('increment'); } }


You can also pass parameters to a mutation:

// store.js mutations: { incrementBy(state, n) { state.count += n; } }

// component updateCount() { this.$store.commit('incrementBy', 25); }


In the above example, we’re passing the mutation an integer by which it should increase the count. You can also pass an object as a parameter. This way, you can include multiple fields easily without overloading your mutation handler:

// store.js mutations: { incrementBy(state, payload) { state.count += payload.amount; } }

// component updateCount() { this.$store.commit('incrementBy', { amount: 25 }); }


You can also perform an _object-style commit_ that looks like this:

store.commit({ type: 'incrementBy', amount: 25 })


The mutation handler will remain the same.

#### The `mapMutations` Helper

Similar to `mapState` and `mapGetters`, you can also use the `mapMutations` helper to reduce the boilerplate for your mutation handlers:

import { mapMutations } from 'vuex'

export default{ methods: { ...mapMutations([ 'increment', // maps to this.increment() 'incrementBy' // maps to this.incrementBy(amount) ]) } }


On a final note, mutation handlers must be synchronous. You can attempt to write an asynchronous mutation function, but you’ll come to find out later down the road that it causes unnecessary complications. Let’s move on to actions.

### **Actions**

Actions are functions that don’t change the state themselves. Instead, they commit mutations after performing some logic (which is often asynchronous). Here’s a simple example of an action:

//.. actions: { increment(context) { context.commit('increment'); } }


Action handlers receive a `context` object as their first argument, which gives us access to store properties and methods. For example:

*   `context.commit`: commit a mutation
*   `context.state`: access state
*   `context.getters`: access getters

You can also use _argument destructing_ to extract the store attributes you need for your code. For example:

actions: { increment({ commit }) { commit('increment'); } }


As mentioned above, actions can be asynchronous. Here’s an example:

actions: { incrementAsync: async({ commit }) => { return await setTimeout(() => { commit('increment') }, 1000); } }


In this example, the mutation is committed after 1,000 milliseconds.

Like mutations, action handlers aren’t called directly, but rather via a dedicated `dispatch` method on the store, like so:

actions: { incrementAsync: async({ commit }) => { return await setTimeout(() => { commit('increment') }, 1000); } }


You can dispatch an action in a component like this:

this.$store.dispatch('increment')



#### The `mapActions` Helper

Alternatively, you can use the `mapActions` helper to assign action handlers to local methods:

actions: { incrementAsync: async({ commit }) => { return await setTimeout(() => { commit('increment') }, 1000); } }


## Re-build Counter App Using Vuex

Now that we’ve had a look at the core concepts of Vuex, it’s time to implement what we’ve learned and rewrite our counter to make use of Vue’s official state management solution.

If you fancy a challenge, you might like to have a go at doing this yourself before reading on …

When we generated our project using `Vue CLI`, we selected `Vuex` as one of the features. A couple of things happened:

1.  `Vuex` was installed as a package dependency. Check your `package.json` to confirm this.
2.  A `store.js` file was created and injected into your Vue.js application via `main.js`.

To convert our “local state” counter app to a Vuex application, open `src/store.js` and update the code as follows:

import Vue from 'vue' import Vuex from 'vuex'

Vue.use(Vuex)

export default new Vuex.Store({ state: { count: 0 }, getters: { parity: state => state.count % 2 === 0 ? 'even' : 'odd' }, mutations: { increment(state) { state.count++; }, decrement(state) { state.count--; } }, actions: { increment: ({ commit }) => commit('increment'), decrement: ({ commit }) => commit('decrement'), incrementIfOdd: ({ commit, getters }) => getters.parity === 'odd' ? commit('increment') : false, incrementAsync: ({ commit }) => { setTimeout(() => { commit('increment') }, 1000); } } });


Here we can see how a complete Vuex store is structured in practice. Please go back over the theory part of this article if anything here is unclear to you.

Next, update the `src/components/Counter.vue` component by replacing the existing code within the `` block. We’ll switch the local state and functions to the newly created ones in the Vuex store:

import { mapState mapGetters, mapActions } from 'vuex'

export default { name: 'Counter', computed: { ...mapState([ 'count' ]), ...mapGetters([ 'parity' ]) }, methods: mapActions([ 'increment', 'decrement', 'incrementIfOdd', 'incrementAsync' ]) }


The template code should remain the same, as we’re sticking to the previous variable and function names. See how much cleaner the code now is.

If you don’t want to use the state and getter map helpers, you can access the store data directly from your template like this:

Clicked {{ $store.state.count }} times! Count is {{ $store.getters.parity }}.


After you’ve saved your changes, make sure to test your application. From an end-user perspective, the counter application should function exactly the same as before. The only difference is that the counter is now operating from a Vuex store.

[https://codepen.io](https://codepen.io/SitePoint/pen/ZmaMKO)

## Conclusion

In this article, we’ve looked at what Vuex is, what problem it solves, how to install it, as well as its core concepts. We then applied these concepts to refactor our counter app to work with Vuex. Hopefully this introduction will serve you well in implementing Vuex in your own projects.

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