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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>useState</code></h1><p><code>useState</code> is the primary way in which React updates the application state based on user interaction.</p><ul><li><code>useState</code> <strong>receives</strong>: the initial state as an argument</li><li><code>useState</code> <strong>returns</strong>: the current state and a function that updates it</li></ul><p>The two items that are returned are returned as an array. We could refer to them with array syntax (i.e with <code>[0]</code> and <code>[1]</code> ) but it is more concise to use the destructuring syntax. But all that’s happening is that a method belonging to the <code>React</code> library (i.e. <code>useState</code> ) is taking an argument (the initial state) and returning two values as an array of two elements. The first of these values is a variable, the second is a function.</p><pre><code class="jsx language-jsx">(1) const [count, setCount] = useState(0);
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(2) const handleClick = () => { setCount(count + 1) };
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(3) <span>{count}</span>
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(4) <button onClick={handleClick)>Add one</button></code></pre><ul><li>At line 1 we initialise the <code>useState</code> hook.<ul><li>We use destructuring syntax to declare two variables</li><li>The first variable stores our initial state. The value of the initial state is an argument passed to <code>useState</code> , i.e. <code>0</code>. Do not confuse the variable with the value it refers to.</li><li>The second variable is a method that mutates the state referred to by the first variable into some new value, i.e. from <code>0</code> to a different integer</li></ul></li><li>At line 3, which in reality, will be within your JSX and part of the <code>render()</code> block of the function, we render the initial state in the React DOM. This will change and be re-rendered in accordance with the state logic</li><li>At line 4 we have a user interface that executes the function on line 2. This function applies the state change represented by the second variable in the destructuring assignment at line 1 (<code>setCount</code> ): it mutates the initial state (<code>0</code>) by the value of `1’.</li></ul><blockquote><p>You can utilise as many <code>useState</code> declarations as your component requires.</p></blockquote><h2 id="passing-variables-as-initial-state">Passing variables as initial state</h2><p>We don’t always have to declare the data types within the destructuring assignment. We can store them in variables and pass the variables to the destructuring assignment on <code>useState</code> . Below we do this for a function that mutates a string value:</p><pre><code class="jsx language-jsx">const hi = "hello";
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const bye = "goodbye";
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const [initText, changeText] = useState(hi);
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<p>{initText}</p>
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<button onClick={()=>changeText(bye)}>ChangeText</button></code></pre><h2 id="lazily-setting-initial-state">Lazily setting initial state</h2><p>Although you will typically use a literal value or variable as your initial state, React provides a way for you to set this value through a function. This will only run once at the first render so as not to interfere with the second update parameter of <code>useState</code>. This is called <strong>lazy initialisation</strong></p><pre><code class="jsx language-jsx">const getInitialState = () => Number(window.localStorage.getItem("count"));
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const [count, setCount] = React.useState(getInitialState);</code></pre><h2 id="accessing-previous-state">Accessing previous state</h2><p>It is also possible to access the previous state via the <code>useState</code> hook. We can access this as a callback on the updater function.</p><p>Let’s say we have a simple counter set up like so:</p><pre><code class="js language-js">const [count, setCount] = useState(0);</code></pre><p>We can access the prior value of <code>count</code> with:</p><pre><code class="js language-js">const getPrevValue = () => setCount((prev) => console.log(prev));</code></pre><h3 id="naming-best-practise">Naming best practise</h3><p>When destructuring the two variables from <code>useState</code>, React follows uses the following semantics: <code>[something, setSomething]</code> . This isn’t required but it helps to keep everything clear, particularly the fact that the first thing destructured is a variable and the second is a function.</p><blockquote><p><a href="https://stackoverflow.com/questions/54807454/what-is-prevstate-in-reactjs">https://stackoverflow.com/questions/54807454/what-is-prevstate-in-reactjs</a></p></blockquote></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="React_useReducer.html">useReducer</a></span></span><ul class="context-list" style="zoom: 85%;"><li class="item"><div class="pandoc"><p>The <code>useReducer</code> hook is best used in scenarios where you are manipulating state in a way that is too complex for the trivial <span class="zettel-link-container cf"><span class="zettel-link" title="Zettel: useState"><a href="React_useState.html">React_useState</a></span></span> use case. <code>useState</code> is best employed when you are updating a single value or toggling a boolean. If you are updating the state of an object or more complex data structure, it is often more efficient to employ <code>useReducer</code>.</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>
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