122 lines
5.1 KiB
Markdown
122 lines
5.1 KiB
Markdown
---
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id: 5a24c314108439a4d4036173
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title: Set State with this.setState
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challengeType: 6
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isRequired: false
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forumTopicId: 301412
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---
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## Description
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<section id='description'>
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The previous challenges covered component <code>state</code> and how to initialize state in the <code>constructor</code>. There is also a way to change the component's <code>state</code>. React provides a method for updating component <code>state</code> called <code>setState</code>. You call the <code>setState</code> method within your component class like so: <code>this.setState()</code>, passing in an object with key-value pairs. The keys are your state properties and the values are the updated state data. For instance, if we were storing a <code>username</code> in state and wanted to update it, it would look like this:
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```jsx
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this.setState({
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username: 'Lewis'
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});
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```
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React expects you to never modify <code>state</code> directly, instead always use <code>this.setState()</code> when state changes occur. Also, you should note that React may batch multiple state updates in order to improve performance. What this means is that state updates through the <code>setState</code> method can be asynchronous. There is an alternative syntax for the <code>setState</code> method which provides a way around this problem. This is rarely needed but it's good to keep it in mind! Please consult the <a target="_blank" href="https://facebook.github.io/react/docs/state-and-lifecycle.html">React documentation</a> for further details.
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</section>
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## Instructions
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<section id='instructions'>
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There is a <code>button</code> element in the code editor which has an <code>onClick()</code> handler. This handler is triggered when the <code>button</code> receives a click event in the browser, and runs the <code>handleClick</code> method defined on <code>MyComponent</code>. Within the <code>handleClick</code> method, update the component <code>state</code> using <code>this.setState()</code>. Set the <code>name</code> property in <code>state</code> to equal the string <code>React Rocks!</code>.
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Click the button and watch the rendered state update. Don't worry if you don't fully understand how the click handler code works at this point. It's covered in upcoming challenges.
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</section>
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## Tests
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<section id='tests'>
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```yml
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tests:
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- text: 'The state of <code>MyComponent</code> should initialize with the key value pair <code>{ name: Initial State }</code>.'
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testString: 'assert(Enzyme.mount(React.createElement(MyComponent)).state(''name'') === ''Initial State'');'
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- text: <code>MyComponent</code> should render an <code>h1</code> header.
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testString: assert(Enzyme.mount(React.createElement(MyComponent)).find('h1').length === 1);
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- text: The rendered <code>h1</code> header should contain text rendered from the component's state.
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testString: 'async () => { const waitForIt = (fn) => new Promise((resolve, reject) => setTimeout(() => resolve(fn()), 250)); const mockedComponent = Enzyme.mount(React.createElement(MyComponent)); const first = () => { mockedComponent.setState({ name: ''TestName'' }); return waitForIt(() => mockedComponent.html()); }; const firstValue = await first(); assert(/<h1>TestName<\/h1>/.test(firstValue)); };'
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- text: Calling the <code>handleClick</code> method on <code>MyComponent</code> should set the name property in state to equal <code>React Rocks!</code>.
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testString: 'async () => { const waitForIt = (fn) => new Promise((resolve, reject) => setTimeout(() => resolve(fn()), 250)); const mockedComponent = Enzyme.mount(React.createElement(MyComponent)); const first = () => { mockedComponent.setState({ name: ''Before'' }); return waitForIt(() => mockedComponent.state(''name'')); }; const second = () => { mockedComponent.instance().handleClick(); return waitForIt(() => mockedComponent.state(''name'')); }; const firstValue = await first(); const secondValue = await second(); assert(firstValue === ''Before'' && secondValue === ''React Rocks!''); };'
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```
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</section>
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## Challenge Seed
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<section id='challengeSeed'>
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<div id='jsx-seed'>
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```jsx
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class MyComponent extends React.Component {
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constructor(props) {
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super(props);
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this.state = {
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name: 'Initial State'
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};
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this.handleClick = this.handleClick.bind(this);
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}
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handleClick() {
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// change code below this line
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// change code above this line
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}
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render() {
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return (
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<div>
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<button onClick={this.handleClick}>Click Me</button>
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<h1>{this.state.name}</h1>
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</div>
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);
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}
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};
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```
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</div>
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### After Test
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<div id='jsx-teardown'>
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```js
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ReactDOM.render(<MyComponent />, document.getElementById('root'))
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```
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</div>
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</section>
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## Solution
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<section id='solution'>
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```js
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class MyComponent extends React.Component {
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constructor(props) {
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super(props);
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this.state = {
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name: 'Initial State'
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};
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this.handleClick = this.handleClick.bind(this);
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}
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handleClick() {
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// change code below this line
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this.setState({
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name: 'React Rocks!'
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});
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// change code above this line
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}
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render() {
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return (
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<div>
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<button onClick = {this.handleClick}>Click Me</button>
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<h1>{this.state.name}</h1>
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</div>
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);
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}
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};
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```
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</section>
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