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<link rel="search" type="application/opensearchdescription+xml" title="foomo project docs" href="/opensearch.xml"><title data-react-helmet="true">Performance | foomo project docs</title><meta data-react-helmet="true" name="twitter:card" content="summary_large_image"><meta data-react-helmet="true" property="og:url" content="https://www.foomo.org/docs/frontend/performance"><meta data-react-helmet="true" name="docsearch:language" content="en"><meta data-react-helmet="true" name="docsearch:version" content="current"><meta data-react-helmet="true" name="docsearch:docusaurus_tag" content="docs-default-current"><meta data-react-helmet="true" property="og:title" content="Performance | foomo project docs"><meta data-react-helmet="true" name="description" content="JS"><meta data-react-helmet="true" property="og:description" content="JS"><link data-react-helmet="true" rel="icon" href="/img/favicon.ico"><link data-react-helmet="true" rel="canonical" href="https://www.foomo.org/docs/frontend/performance"><link data-react-helmet="true" rel="alternate" href="https://www.foomo.org/docs/frontend/performance" hreflang="en"><link data-react-helmet="true" rel="alternate" href="https://www.foomo.org/docs/frontend/performance" hreflang="x-default"><link data-react-helmet="true" rel="preconnect" href="https://SUATUVZDDM-dsn.algolia.net" crossorigin="anonymous"><link rel="stylesheet" href="/assets/css/styles.47738ace.css">
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id="js">JS<a class="hash-link" href="#js" title="Direct link to heading">​</a></h2><p>JS is nowadays extremely fast and yet we have many performance issues. Here you can find few common mistakes that occur in JS that can decrease performance.</p><h3 class="anchor anchorWithStickyNavbar_y2LR" id="extensive-use-of-map-filter">Extensive use of .map, .filter<a class="hash-link" href="#extensive-use-of-map-filter" title="Direct link to heading">​</a></h3><p>Let&#x27;s say you have a large list of objects and you would like to filter them and transform them in some form. Usually we do this:</p><div class="codeBlockContainer_J+bg language-js theme-code-block"><div class="codeBlockContent_csEI js"><pre tabindex="0" class="prism-code language-js codeBlock_rtdJ thin-scrollbar" style="color:#bfc7d5;background-color:#292d3e"><code class="codeBlockLines_1zSZ"><span class="token-line" style="color:#bfc7d5"><span class="token keyword" style="font-style:italic">const</span><span class="token plain"> largeArray </span><span class="token operator" style="color:rgb(137, 221, 255)">=</span><span class="token plain"> </span><span class="token punctuation" style="color:rgb(199, 146, 234)">[</span><span class="token plain"> </span><span class="token spread operator" style="color:rgb(137, 221, 255)">...</span><span class="token punctuation" style="color:rgb(199, 146, 234)">.</span><span class="token plain"> </span><span class="token punctuation" style="color:rgb(199, 146, 234)">]</span><span class="token plain"></span><br></span><span class="token-line" style="color:#bfc7d5"><span class="token plain">largeArray</span><span class="token punctuation" style="color:rgb(199, 146, 234)">.</span><span class="token method function property-access" style="color:rgb(130, 170, 255)">map</span><span class="token punctuation" style="color:rgb(199, 146, 234)">(</span><span class="token parameter">obj</span><span class="token plain"> </span><span class="token arrow operator" style="color:rgb(137, 221, 255)">=&gt;</span><span class="token plain"> </span><span class="token function" style="color:rgb(130, 170, 255)">transformObj</span><span class="token punctuation" style="color:rgb(199, 146, 234)">(</span><span class="token plain">obj</span><span class="token punctuation" style="color:rgb(199, 146, 234)">)</span><span class="token punctuation" style="color:rgb(199, 146, 234)">)</span><span class="token punctuation" style="color:rgb(199, 146, 234)">.</span><span class="token method function property-access" style="color:rgb(130, 170, 255)">filter</span><span class="token punctuation" style="color:rgb(199, 146, 234)">(</span><span class="token plain">omitBadObject</span><span class="token punctuation" style="color:rgb(199, 146, 234)">)</span><br></span></code></pre><button type="button" aria-label="Copy code to clipboard" class="copyButton_M3SB clean-btn">Copy</button></div></div><p>In the above case we first loop through whole set, transform it and then filter things out. Not only does this goes through all the items twice, but it also first time goes through all the items and then filters them.
One optimization would be to first filter them and then transform them, but ideally we should just use a normal for loop or forEach where you go through items only once.</p><div class="codeBlockContainer_J+bg language-js theme-code-block"><div class="codeBlockContent_csEI js"><pre tabindex="0" class="prism-code language-js codeBlock_rtdJ thin-scrollbar" style="color:#bfc7d5;background-color:#292d3e"><code class="codeBlockLines_1zSZ"><span class="token-line" style="color:#bfc7d5"><span class="token keyword" style="font-style:italic">const</span><span class="token plain"> finalArray </span><span class="token operator" style="color:rgb(137, 221, 255)">=</span><span class="token plain"> </span><span class="token punctuation" style="color:rgb(199, 146, 234)">[</span><span class="token punctuation" style="color:rgb(199, 146, 234)">]</span><span class="token plain"></span><br></span><span class="token-line" style="color:#bfc7d5"><span class="token plain">largeArray</span><span class="token punctuation" style="color:rgb(199, 146, 234)">.</span><span class="token method function property-access" style="color:rgb(130, 170, 255)">forEach</span><span class="token punctuation" style="color:rgb(199, 146, 234)">(</span><span class="token parameter">obj</span><span class="token plain"> </span><span class="token arrow operator" style="color:rgb(137, 221, 255)">=&gt;</span><span class="token plain"> </span><span class="token punctuation" style="color:rgb(199, 146, 234)">{</span><span class="token plain"></span><br></span><span class="token-line" style="color:#bfc7d5"><span class="token plain"> </span><span class="token keyword control-flow" style="font-style:italic">if</span><span class="token plain"> </span><span class="token punctuation" style="color:rgb(199, 146, 234)">(</span><span class="token function" style="color:rgb(130, 170, 255)">omitBadObject</span><span class="token punctuation" style="color:rgb(199, 146, 234)">(</span><span class="token plain">obj</span><span class="token punctuation" style="color:rgb(199, 146, 234)">)</span><span class="token punctuation" style="color:rgb(199, 146, 234)">)</span><span class="token plain"> </span><span class="token punctuation" style="color:rgb(199, 146, 234)">{</span><span class="token plain"></span><br></span><span class="token-line" style="color:#bfc7d5"><span class="token plain"> finalArray</span><span class="token punctuation" style="color:rgb(199, 146, 234)">.</span><span class="token method function property-access" style="color:rgb(130, 170, 255)">push</span><span class="token punctuation" style="color:rgb(199, 146, 234)">(</span><span class="token function" style="color:rgb(130, 170, 255)">transformObj</span><span class="token punctuation" style="color:rgb(199, 146, 234)">(</span><span class="token plain">obj</span><span class="token punctuation" style="color:rgb(199, 146, 234)">)</span><span class="token punctuation" style="color:rgb(199, 146, 234)">)</span><span class="token plain"></span><br></span><span class="token-line" style="color:#bfc7d5"><span class="token plain"> </span><span class="token punctuation" style="color:rgb(199, 146, 234)">}</span><span class="token plain"></span><br></span><span class="token-line" style="color:#bfc7d5"><span class="token plain"></span><span class="token punctuation" style="color:rgb(199, 146, 234)">}</span><span class="token punctuation" style="color:rgb(199, 146, 234)">)</span><br></span></code></pre><button type="button" aria-label="Copy code to clipboard" class="copyButton_M3SB clean-btn">Copy</button></div></div><p>This code will skip another loop of items.</p></div><footer class="theme-doc-footer docusaurus-mt-lg"><div class="theme-doc-footer-edit-meta-row row"><div class="col"><a href="https://github.com/foomo/foomo-docs/tree/main/foomo/docs/frontend/performance.md" target="_blank" rel="noreferrer noopener" class="theme-edit-this-page"><svg fill="currentColor" height="20" width="20" viewBox="0 0 40 40" 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