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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>useReducer</code></h1><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><p>This makes the code more manageable and also helps with separating state management from rendering.</p><h2 id="syntax">Syntax</h2><pre><code class="jsx language-jsx">const [state, dispatch] = useReducer(reducer, initialState);</code></pre><ul><li><code>initialState</code><ul><li>The starting state, typically an object</li></ul></li><li><code>reducer</code><ul><li>A pure function that accepts two parameters:<ul><li>The current state</li><li>An action object</li></ul></li><li>The reducer function must update the current state (immutably) and return the new state</li><li>We can think of the reducer as working in the same manner as <code>state</code>/<code>setState</code> in the <code>useState</code> hook. The functional role is the same, it is just that the reducer offers more than one type of update.</li></ul></li></ul><h3 id="example-reducer">Example reducer</h3><pre><code class="js language-js">function reducer(state, action) {
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let newState;
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switch (action.type) {
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case "increase":
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newState = { counter: state.counter + 1 };
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break;
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case "descrease":
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newState = { counter: state.counter - 1 };
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break;
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default:
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throw new Error();
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}
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return newState;
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}</code></pre><p>In this example we are updating an object with the following shape:</p><pre><code class="js language-js">{
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counter: 0,
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}</code></pre><p>This would be the <code>initialState</code> that we pass to the <code>useReducer</code> hook along with a reference to <code>reducer</code> above.</p><p>To update the state we would invoke the <code>dispatch</code> function which applies one of the actions defined in the reducer. For example the following dispatch increments the counter by one:</p><pre><code class="js language-js">dispatch({ type: "increase" });</code></pre><p>To view the updated value:</p><pre><code class="js language-js">console.log(state.counter);</code></pre><h3 id="refining-the-syntax">Refining the syntax</h3><p>Because React doesn’t mutate state, the reducer doesn’t directly modify the current state in the <code>state</code> variable, it creates a new instance of the state object on each update.</p><p>In the reducer example above this is achieved by declaring a variable <code>newState</code> that is updated by each <code>action</code> type and then returned. There is a more elegant way of doing this using spread syntax:</p><pre><code class="js language-js">function reducer(state, action) {
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switch (action.type) {
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case "increase":
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return { ...state, counter: state.counter + 1 };
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break;
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case "decrease":
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return { ...state, counter: state.counter - 1 };
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break;
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default:
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throw new Error();
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}
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}</code></pre><h3 id="including-payloads">Including payloads</h3><p>In the examples so far, we have updated the the state directly via the action type however it is also possible to pass data along with the <code>action.type</code> as <code>action.payload</code>.</p><p>For example:</p><pre><code class="js language-js">dispatch(
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{
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type: 'increase_by_payload'
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payload: 3,
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});</code></pre><p>Then we would update our reducer to handle this case:</p><pre><code class="js language-js">function reducer(state, action) {
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switch (action.type) {
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...
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case 'increase_by_payload':
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return {...state, counter: state.counter + action.payload}
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default:
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throw new Error();
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}
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}</code></pre></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="Application_state_management_with_React_hooks.html">Application state management</a></span></span><ul class="context-list" style="zoom: 85%;"><li class="item"><div class="pandoc"><p>Although <span class="zettel-link-container cf"><span class="zettel-link" title="Zettel: useReducer"><a href="React_useReducer.html">React_useReducer</a></span></span> and <span class="zettel-link-container cf"><span class="zettel-link" title="Zettel: useContext"><a href="React_useContext.html">React_useContext</a></span></span> have many sound use cases by themselves, when they are combined they offer a way to acheive a system of global state management, without utilising third-party libraries like Redux.</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> |