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<!DOCTYPE html><html><head><meta content="text/html; charset=utf-8" http-equiv="Content-Type" /><meta content="width=device-width, initial-scale=1" name="viewport" /><!--replace-start-0--><!--replace-start-5--><!--replace-start-8--><title>useEffect - My Zettelkasten</title><!--replace-end-8--><!--replace-end-5--><!--replace-end-0--><link href="https://cdn.jsdelivr.net/npm/fomantic-ui@2.8.7/dist/semantic.min.css" rel="stylesheet" /><link href="https://fonts.googleapis.com/css?family=Merriweather|Libre+Franklin|Roboto+Mono&display=swap" rel="stylesheet" /><!--replace-start-1--><!--replace-start-4--><!--replace-start-7--><link href="https://raw.githubusercontent.com/srid/neuron/master/assets/neuron.svg" rel="icon" /><meta content="useEffect should be used for any background process that does not arise as a result of a user interaction or standard state change. 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<!--replace-end-7--><!--replace-end-4--><!--replace-end-1--></head><body><div class="ui fluid container universe"><!--replace-start-2--><!--replace-start-3--><!--replace-start-6--><div class="ui text container" id="zettel-container" style="position: relative"><div class="zettel-view"><article class="ui raised attached segment zettel-content"><div class="pandoc"><h1 id="title-h1"><code>useEffect</code></h1><p><code>useEffect</code> should be used for any background process that does not arise as a result of a user interaction or standard state change. The canonical example would be dispatching a request to an API to retrieve data to display in a component’s initial render.</p><p><code>useEffect</code> enshrines but also simplifies the <span class="zettel-link-container cf"><span class="zettel-link" title="Zettel: Lifecycle methods"><a href="Lifecycle_methods.html">lifecyle methods</a></span></span> that are used with class-based React components.</p><h2 id="demonstration">Demonstration</h2><pre><code class="jsx language-jsx">const [workflowTitle, setWorkflowTitle] = useState("");
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const [loading, setLoading] = useState(false);
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useEffect(() => {
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const fetchWorkflowTitle = () => {
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axios
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.post(`${process.env.REACT_APP_ENDPOINT}/load-workflows`, {
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workflow_id: project_id,
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user_id: isAuth?.userId,
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})
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.then((res) => {
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setWorkflowTitle(res?.data?.meta.name);
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})
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.finally(() => setLoading(false))
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.catch((err) => {
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console.error(err);
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});
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};
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setLoading(true);
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fetchWorkflowTitle();
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}, [project_id, isAuth]);</code></pre><p>In this example we fetch some data from a REST API and store the response value as state, which can then be accessed within our JSX template.</p><h2 id="function-placement">Function placement</h2><p>In the example, the function is declared inside of the <code>useEffect</code> hook and then called. We do this to avoid adding the function to the dependency array, which we couldn’t do if we defined it outside of the hook, somewhere in the component. If we did this, the function would run on every render by default and would be recalled again because of its placement in the dependency array, which leads to the ‘too many rerenders’ error.</p><p>An alternative would be to define it outside of the component itself but this is difficult since it requires state from the component to run.</p><h2 id="dependency-array">Dependency array</h2><p>Note the array that is the second argument to <code>useEffect</code>. This is the <strong>dependency array</strong> and is how we manage the lifecycle of the side-effect.</p><ul><li><p>After the initial call on render, the function within the <code>useEffect</code> block will run any time one of the dependencies update. The dependency array typically refers to variables that are defined within the hook but can also contain references to variables outside of it.</p></li><li><p>If you do not include the array, the effect runs on every render</p></li><li><p>If you pass an empty array, the effect will only run once on the initial render</p></li></ul><h2 id="cleanup-functions">Cleanup functions</h2><p>The syntax of the <code>useEffect</code> hook also allows you to handle cleanup: something you want to do when the component unmounts (another example of how this hook recasts the traditional <span class="zettel-link-container cf"><span class="zettel-link" title="Zettel: Lifecycle methods"><a href="Lifecycle_methods.html">lifecycle methods</a></span></span>) . In addition to running on unmount, the cleanup function will run before the effect runs again (i.e. when it runs in response to a change in one of the elements of the dependency araray). This is chiefly used to prevent <span class="zettel-link-container cf"><span class="zettel-link" title="Zettel: Memory leaks"><a href="Memory_leaks.html">memory leaks</a></span></span> and the <a href="React_errors.md#state-update-on-unmounted-component">‘update on unmounted component error’</a>.</p><p>You do this by having a <code>return</code> clause after the main effect. Schematically:</p><pre><code class="js language-js">useEffect(() => {
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// Run your effect;
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return () => {
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// Put your cleanup function here
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};
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}, [input]);</code></pre><p>Here is an example of using a cleanup to cancel a subscription</p><pre><code class="js language-js">useEffect(() => {
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let isApiSubscribed = true;
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axios.get(API).then((response) => {
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if (isApiSubscribed) {
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// handle success
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}
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});
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return () => {
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isApiSubscribed = false;
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};
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}, []);</code></pre><h2 id="resources">Resources</h2><p><a>Understanding the React useEffect cleanup function</a></p></div></article><nav class="ui attached segment deemphasized backlinksPane" id="neuron-backlinks-pane"><h3 class="ui header">Backlinks</h3><ul class="backlinks"><li><span class="zettel-link-container cf"><span class="zettel-link"><a href="Memoization_in_React.html">Memoization with useCallback and useMemo</a></span></span><ul class="context-list" style="zoom: 85%;"><li class="item"><div class="pandoc"><p>Note that the syntax is similar to <span class="zettel-link-container cf"><span class="zettel-link" title="Zettel: useEffect"><a href="React_useEffect.html">React_useEffect</a></span></span>: there is a dependency array. The effect is the same: the function contained within <code>useCallback</code> will only re-rendered if one of these dependencies changes. However (see next section) the function will run in its memoized form on every click.</p></div></li></ul></li></ul></nav><nav class="ui attached segment deemphasized bottomPane" id="neuron-tags-pane"><div><span class="ui basic label zettel-tag" title="Tag">javascript</span><span class="ui basic label zettel-tag" title="Tag">react</span></div></nav><nav class="ui bottom attached icon compact inverted menu blue" id="neuron-nav-bar"><!--replace-start-9--><!--replace-end-9--><a class="right item" href="impulse.html" title="Open Impulse"><i class="wave square icon"></i></a></nav></div></div><!--replace-end-6--><!--replace-end-3--><!--replace-end-2--><div class="ui center aligned container footer-version"><div class="ui tiny image"><a href="https://neuron.zettel.page"><img alt="logo" src="https://raw.githubusercontent.com/srid/neuron/master/assets/neuron.svg" title="Generated by Neuron 1.9.35.3" /></a></div></div></div></body></html> |