7.4: React State

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7.4: React State
Introduction
State is the concept in React that represents temporary data stored in the app, and data that potentially triggers DOM manipulation when changed. When we run the ReactDOM render function React manipulates the DOM once when the page loads. Over the life of the React app in the browser, React may manipulate the DOM multiple times as the user makes actions that modify the app state.
Events
Before we learn how to use state, let's learn how to specify event listener callbacks in React. The following example calls incrementCount when the button is clicked. See the full list of React listener attributes here: https://reactjs.org/docs/events.html#supported-events.
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<button onClick={incrementCount}>Click me</button>
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Note in the following example we define the callback function incrementCount inside the component function Counter. We would not typically define a function inside a function in JS, but this is standard convention in React functional components.
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import React from 'react';
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​
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export default function Counter() {
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  // Define count variable shared across the component
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  let count = 0;
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  // Define incrementCount function to increment count on an event
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  const incrementCount = (event) => {
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    count += 1;
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    console.log(`current value of count is: ${count}`);
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  };
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​
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  // Return the JSX to be rendered in the browser
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  return (
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    <div>
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      <p>You clicked {count} times</p>
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      <button onClick={incrementCount}>Click me</button>
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    </div>
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  );
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}
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This code may not work as you expect. We display the count variable value on line 16. Clicking the button triggers incrementCount, and the value of count changes, but its value on screen never does.
In order to get React to manipulate the DOM on state variable changes, we have to use specific built-in features of React.
Hooks and useState
In React class components (see 7.2.3: Class Components), there is a special class property this.state that is an object that stores the local state for that component. When properties in this.state change, the JSX that depends on those properties gets re-rendered.
In React functional components, this.state is replaced by what is known as "hooks". Hooks are the more modern React system for DOM manipulation, replacing both this.state and what were known as "component lifecycle methods" that we will learn about in Module 8: Advanced React. The React team launched hooks in Feb 2019, thus many companies that used React before 2019 still rely on class components.
The hooks function useState is a React function that returns an array with 2 elements.
1.A variable representing the state value, e.g. X
2.A function that updates this value, whose name is typically setX, where X is the variable name from #1.
useState takes 1 input, the initial state value. In the example below count begins as 0.
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import React, { useState } from 'react';
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​
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export default function Counter() {
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  // Initialise count to 0
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  const [count, setCount] = useState(0);
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​
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  // Callback function to update state
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  const incrementCount = (event) => {
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    console.log(`current value of count is: ${count}`);
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    // Call the setCount hook function to manipulate the DOM
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    setCount(count + 1);
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  };
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  console.log('Executing Counter component function');
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​
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  // Return JSX to render
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  return (
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    <div>
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      <p>You clicked {count} times</p>
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      <button onClick={incrementCount}>Click me</button>
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    </div>
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  );
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}
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Note the following when we run this code.
1.count gets updated when we press the button!
2.The console.log on line 14 runs every time the counter gets updated
When we call the state "hook function" setCount, React takes the value and calls the component function again, where count in the re-rendered component is the new value, but count in the original component is still the old value. The variable count and hook function setCount are related from when we first created them with useState. The subsequently called component function renders JSX with the new value and React puts it on screen.
Differences Between Manual DOM Manipulation and React State
In the past we might have manipulated the DOM manually when increment count button were clicked by running code like document.getElementById('count') to get the element that displays the count, and setting that element's text value to the new count.
With React, this process of updating the display element's value happens automatically when the count state changes. This makes our lives easier as developers because we no longer need to manually manage state; React manages it automatically for us.
In summary, we can describe this difference as that between an "imperative" paradigm (manual DOM manipulation) where we need to command the DOM to change every time, and a "declarative" paradigm (React), where we declare how the UI should look like and what state it depends on in components, and React automatically updates the UI when the state changes.
Full Example
Clicking "Click me" updates the count state and re-renders Counter component with new value of count.
App.jsx
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import React, { useState } from 'react';
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​
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function Counter() {
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  // Initialise count to 0
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  const [count, setCount] = useState(0);
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​
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  // Callback function to update state
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  const incrementCount = (event) => {
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    console.log(`current value of count is: ${count}`);
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    // Call the setCount hook function to manipulate the DOM
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    setCount(count + 1);
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  };
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  console.log('Executing Counter component function');
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​
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  // Return JSX to render
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  return (
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    <div>
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      <p>You clicked {count} times</p>
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      <button onClick={incrementCount}>Click me</button>
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    </div>
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  );
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}
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​
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export default function App() {
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  console.log('Executing App component function');
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  return (
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    <div>
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      This is App.jsx
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      <Counter/>
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    </div>
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  );
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}
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Optimising Nested Components
When a component is rendered, React also renders all its child components. Note how the Fahrenheit console.log happens when count changes. To avoid this for performance reasons, we might move BigWord and Fahrenheit components up inside the App component so that they are not re-rendered every time count changes in Counter.
App.jsx
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import React, { useState } from 'react';
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​
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function BigWord() {
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  console.log('Executing BigWord component function');
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  return <h1>HELLO</h1>;
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}
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​
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function Fahrenheit({ celsiusTemp }) {
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  console.log('Executing Fahrenheit component function');
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  const fahrenheitTemp = (celsiusTemp * 9) / 5 + 32;
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  return (
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    <p>
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      {celsiusTemp} in Fahrenheit is: {fahrenheitTemp}
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    </p>
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  );
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}
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​
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function Counter() {
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  const [count, setCount] = useState(0);
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​
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  const incrementCount = (event) => {
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    console.log(`Current value of count is: ${count}`);
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    setCount(count + 1);
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  };
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​
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  console.log('Executing Counter component function');
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​
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  // In this implementation, BigWord and Fahrenheit components get
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  // re-rendered every time the "count" state changes. This is
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  // inefficient and undesirable.
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  return (
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    <div>
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      <BigWord />
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      <Fahrenheit celsiusTemp={count} />
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      <p>You clicked {count} times</p>
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      <button onClick={incrementCount}>Click me</button>
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    </div>
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  );
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}
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​
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export default function App() {
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  console.log('Executing App component function');
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  return (
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    <div>
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      This is App.jsx
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      <Counter/>
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    </div>
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  );
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}
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Set State Functions Do Not Instantly Update Variables
A major feature of using the React library to do DOM manipulation is that calling a set state hook function triggers the library to update the DOM. Basically this happens whenever the current context of the function ends, i.e., a click event function runs (on line 9) and a set state function is called within. React waits until the function ends to update the DOM.
Notice the debugger statement on line 12. This will pause execution of the JavaScript at that line- this makes it clear that after the setCount function call, the browser screen has not been updated yet.
React Updating State Values
Another thing to pay attention to is the value of count. It has also not changed. Calling the set state function does not set the count state variable. This value changes after the function ends.
This means that you cannot call a set state hook function and rely on an updated state value further on in the function.
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import React, { useState } from 'react';
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​
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export default function Counter() {
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  // Initialise count to 0
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  const [count, setCount] = useState(0);
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​
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  // Callback function to update state
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  const incrementCount = (event) => {
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    console.log(`current value of count is: ${count}`);
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    // Call the setCount hook function to manipulate the DOM
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    setCount(count + 1);
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    debugger;
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    console.log('THIS WILL NOT BE THE UPDATED COUNT VALUE')
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    console.log(count);
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  };
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  console.log('Executing Counter component function');
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​
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  // Return JSX to render
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  return (
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    <div>
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      <p>You clicked {count} times</p>
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      <button onClick={incrementCount}>Click me</button>
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    </div>
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  );
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}
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Further Reading
Past students have found this video helpful in explaining React Hooks.
Web Dev Simplified has a series that does deep dives into React hooks. This video is specifically for useState.
7.3: Using React with Express and Webpack
7.5: Controlled Forms
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Contents
Introduction
Events
Hooks and useState
Differences Between Manual DOM Manipulation and React State
Full Example
Optimising Nested Components
Set State Functions Do Not Instantly Update Variables