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                                <h1 id="getters">Getters</h1>
<p>Sometimes we may need to compute derived state based on store state, for example filtering through a list of items and counting them:</p>
<pre class="language-"><code class="lang-js">computed<span class="token punctuation">:</span> <span class="token punctuation">{</span>
  doneTodosCount <span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    <span class="token keyword">return</span> <span class="token keyword">this</span><span class="token punctuation">.</span>$store<span class="token punctuation">.</span>state<span class="token punctuation">.</span>todos<span class="token punctuation">.</span><span class="token function">filter</span><span class="token punctuation">(</span>todo <span class="token operator">=</span><span class="token operator">&gt;</span> todo<span class="token punctuation">.</span>done<span class="token punctuation">)</span><span class="token punctuation">.</span>length
  <span class="token punctuation">}</span>
<span class="token punctuation">}</span>
</code></pre>
<p>If more than one component needs to make use of this, we have to either duplicate the function, or extract it into a shared helper and import it in multiple places - both are less than ideal.</p>
<p>Vuex allows us to define &quot;getters&quot; in the store. You can think of them as computed properties for stores. Like computed properties, a getter&apos;s result is cached based on its dependencies, and will only re-evaluate when some of its dependencies have changed.</p>
<p>Getters will receive the state as their 1st argument:</p>
<pre class="language-"><code class="lang-js"><span class="token keyword">const</span> store <span class="token operator">=</span> <span class="token keyword">new</span> <span class="token class-name">Vuex<span class="token punctuation">.</span>Store</span><span class="token punctuation">(</span><span class="token punctuation">{</span>
  state<span class="token punctuation">:</span> <span class="token punctuation">{</span>
    todos<span class="token punctuation">:</span> <span class="token punctuation">[</span>
      <span class="token punctuation">{</span> id<span class="token punctuation">:</span> <span class="token number">1</span><span class="token punctuation">,</span> text<span class="token punctuation">:</span> <span class="token string">&apos;...&apos;</span><span class="token punctuation">,</span> done<span class="token punctuation">:</span> <span class="token boolean">true</span> <span class="token punctuation">}</span><span class="token punctuation">,</span>
      <span class="token punctuation">{</span> id<span class="token punctuation">:</span> <span class="token number">2</span><span class="token punctuation">,</span> text<span class="token punctuation">:</span> <span class="token string">&apos;...&apos;</span><span class="token punctuation">,</span> done<span class="token punctuation">:</span> <span class="token boolean">false</span> <span class="token punctuation">}</span>
    <span class="token punctuation">]</span>
  <span class="token punctuation">}</span><span class="token punctuation">,</span>
  getters<span class="token punctuation">:</span> <span class="token punctuation">{</span>
    doneTodos<span class="token punctuation">:</span> state <span class="token operator">=</span><span class="token operator">&gt;</span> <span class="token punctuation">{</span>
      <span class="token keyword">return</span> state<span class="token punctuation">.</span>todos<span class="token punctuation">.</span><span class="token function">filter</span><span class="token punctuation">(</span>todo <span class="token operator">=</span><span class="token operator">&gt;</span> todo<span class="token punctuation">.</span>done<span class="token punctuation">)</span>
    <span class="token punctuation">}</span>
  <span class="token punctuation">}</span>
<span class="token punctuation">}</span><span class="token punctuation">)</span>
</code></pre>
<h3 id="property-style-access">Property-Style Access</h3>
<p>The getters will be exposed on the <code>store.getters</code> object, and you access values as properties:</p>
<pre class="language-"><code class="lang-js">store<span class="token punctuation">.</span>getters<span class="token punctuation">.</span>doneTodos <span class="token comment" spellcheck="true">// -&gt; [{ id: 1, text: &apos;...&apos;, done: true }]</span>
</code></pre>
<p>Getters will also receive other getters as the 2nd argument:</p>
<pre class="language-"><code class="lang-js">getters<span class="token punctuation">:</span> <span class="token punctuation">{</span>
  <span class="token comment" spellcheck="true">// ...</span>
  doneTodosCount<span class="token punctuation">:</span> <span class="token punctuation">(</span>state<span class="token punctuation">,</span> getters<span class="token punctuation">)</span> <span class="token operator">=</span><span class="token operator">&gt;</span> <span class="token punctuation">{</span>
    <span class="token keyword">return</span> getters<span class="token punctuation">.</span>doneTodos<span class="token punctuation">.</span>length
  <span class="token punctuation">}</span>
<span class="token punctuation">}</span>
</code></pre>
<pre class="language-"><code class="lang-js">store<span class="token punctuation">.</span>getters<span class="token punctuation">.</span>doneTodosCount <span class="token comment" spellcheck="true">// -&gt; 1</span>
</code></pre>
<p>We can now easily make use of it inside any component:</p>
<pre class="language-"><code class="lang-js">computed<span class="token punctuation">:</span> <span class="token punctuation">{</span>
  doneTodosCount <span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    <span class="token keyword">return</span> <span class="token keyword">this</span><span class="token punctuation">.</span>$store<span class="token punctuation">.</span>getters<span class="token punctuation">.</span>doneTodosCount
  <span class="token punctuation">}</span>
<span class="token punctuation">}</span>
</code></pre>
<p>Note that getters accessed as properties are cached as part of Vue&apos;s reactivity system.</p>
<h3 id="method-style-access">Method-Style Access</h3>
<p>You can also pass arguments to getters by returning a function. This is particularly useful when you want to query an array in the store:</p>
<pre class="language-"><code class="lang-js">getters<span class="token punctuation">:</span> <span class="token punctuation">{</span>
  <span class="token comment" spellcheck="true">// ...</span>
  getTodoById<span class="token punctuation">:</span> <span class="token punctuation">(</span>state<span class="token punctuation">)</span> <span class="token operator">=</span><span class="token operator">&gt;</span> <span class="token punctuation">(</span>id<span class="token punctuation">)</span> <span class="token operator">=</span><span class="token operator">&gt;</span> <span class="token punctuation">{</span>
    <span class="token keyword">return</span> state<span class="token punctuation">.</span>todos<span class="token punctuation">.</span><span class="token function">find</span><span class="token punctuation">(</span>todo <span class="token operator">=</span><span class="token operator">&gt;</span> todo<span class="token punctuation">.</span>id <span class="token operator">===</span> id<span class="token punctuation">)</span>
  <span class="token punctuation">}</span>
<span class="token punctuation">}</span>
</code></pre>
<pre class="language-"><code class="lang-js">store<span class="token punctuation">.</span>getters<span class="token punctuation">.</span><span class="token function">getTodoById</span><span class="token punctuation">(</span><span class="token number">2</span><span class="token punctuation">)</span> <span class="token comment" spellcheck="true">// -&gt; { id: 2, text: &apos;...&apos;, done: false }</span>
</code></pre>
<p>Note that getters accessed via methods will run each time you call them, and the result is not cached.</p>
<h3 id="the-mapgetters-helper">The <code>mapGetters</code> Helper</h3>
<p>The <code>mapGetters</code> helper simply maps store getters to local computed properties:</p>
<pre class="language-"><code class="lang-js"><span class="token keyword">import</span> <span class="token punctuation">{</span> mapGetters <span class="token punctuation">}</span> <span class="token keyword">from</span> <span class="token string">&apos;vuex&apos;</span>

<span class="token keyword">export</span> <span class="token keyword">default</span> <span class="token punctuation">{</span>
  <span class="token comment" spellcheck="true">// ...</span>
  computed<span class="token punctuation">:</span> <span class="token punctuation">{</span>
    <span class="token comment" spellcheck="true">// mix the getters into computed with object spread operator</span>
    <span class="token operator">...</span><span class="token function">mapGetters</span><span class="token punctuation">(</span><span class="token punctuation">[</span>
      <span class="token string">&apos;doneTodosCount&apos;</span><span class="token punctuation">,</span>
      <span class="token string">&apos;anotherGetter&apos;</span><span class="token punctuation">,</span>
      <span class="token comment" spellcheck="true">// ...</span>
    <span class="token punctuation">]</span><span class="token punctuation">)</span>
  <span class="token punctuation">}</span>
<span class="token punctuation">}</span>
</code></pre>
<p>If you want to map a getter to a different name, use an object:</p>
<pre class="language-"><code class="lang-js"><span class="token operator">...</span><span class="token function">mapGetters</span><span class="token punctuation">(</span><span class="token punctuation">{</span>
  <span class="token comment" spellcheck="true">// map `this.doneCount` to `this.$store.getters.doneTodosCount`</span>
  doneCount<span class="token punctuation">:</span> <span class="token string">&apos;doneTodosCount&apos;</span>
<span class="token punctuation">}</span><span class="token punctuation">)</span>
</code></pre>

                                
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