How to filter lists in Vue using computed properties

The blog posts on my site have tags in their metadata. I display these tags on the left hand side of the main blog post page, so visitors can filter posts to specific topics that they're intretsed in.

The tags are an array within each post, e.g: tags: ['Vue', 'JavaScript', ...].

When the blog index page is being built by Nuxt, I carry out the following steps to remove any undefined and duplicated tags:

// use map to go through the blogPosts array and return the tags for each blogpost (post.tags) if they're not empty ('')
const allTags = => {
  // for each post I filter post.tags and return any tag that's not equal to undefined
  const filterdPostTags = (post.tags !== undefined ? post.tags : "").filter((postTag) => {
    // return the tag that isn't equal to undefined
    return postTag !== undefined;

  // return the filtered Post tags to allTags for each post within this map function on all blogposts
  return filterdPostTags;

// create a new const called 'tags' that's created from 'allTags' using new Set(). This automatically removes duplicate entries from the array passed in.

// .flat(1) flattens allTags to ensure there's no nested arrays from the filtering above
const tags = [ Set(allTags.flat(1))];

The tags const is stored in the blog index page state, I use this to render all the tags in the left hand column with a simple v-for directive that we've seen in previous posts:

  class="flex-none pb-4 mr-4 md:block md:pb-1 md:mr-0 hover:cursor-pointer"
  :class="selectedTag == tag ? 'font-bold' : ''"
  v-for="(tag, index) in tags"
  :key="tag + index"
  {{ tag }}

Each <li> here has a @click="setTag" event handler attached to it. This runs a method called setTag that takes the tag value and saves it to the local blog index state under selectedTag. I'm using Vue to bind dynamic data to the class so when the selectedTag value is the same as the tag value that Vue is rendering, it will apply the font-bold class to the <li>, and removes it when the values don't match. The class font-bold is appended/removed to the classes we apply with regular HTML:

<li class="flex-none pb-4 mr-4 md:block md:pb-1 md:mr-0 hover:cursor-pointer font-bold" data-tag="tag">tag</li>

Now that tags can be selected and stored, I can use this to create a computed property to calculate what posts should be shown in the main blog post' list.

Computed properties

Computed properties allow us to handle reactive data that requires complex logic. We could do all logic in the template but it's recommended that we keep any complex logic out of the template used a computed property instead. This keeps the template clean and much easier to read:

computed: {
  filteredBlogPosts () {
    const filteredPosts = this.blogPosts.filter((blogpost) => {
      // check if this.selectedtag has a value, if not just return the blogpost
      if (!this.selectedTag || this.selectedTag.length === 0) return blogpost

      // if this blog post's tag array includes the selectedTag then we return this blogpost in to the filteredPosts array
      if (blogpost.tags.includes(this.selectedTag)) {
        return blogpost

    // once we've filtered all blogposts, we return this to the template and Vue will render the posts
    return filteredPosts

The template can now render each post within filteredBlogPosts using my <ListItem> component:

<ListItem v-for="post in filteredBlogPosts" :key="post.slug" :post-data="post" :route="path" />

Computed properties look very similar to methods, but there's a big advantage of using a computed property over a method. Computed properties are cached, depending on their reactive dependencies. They're only re-calculated when Vue has detected a change in the depedencies, where as methods will run whever a re-render happens.