114 lines
9.4 KiB
Markdown
114 lines
9.4 KiB
Markdown
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---
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id: 5a24c314108439a4d4036187
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title: Use a Ternary Expression for Conditional Rendering
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challengeType: 6
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isRequired: false
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videoUrl: ''
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localeTitle: 使用三元表达式进行条件渲染
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---
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## Description
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<section id="description">在继续使用动态渲染技术之前,最后一种方法是使用内置的JavaScript条件来渲染你想要的东西: <em><strong>三元运算符</strong></em> 。三元运算符通常用作JavaScript中<code>if/else</code>语句的快捷方式。它们不像传统的<code>if/else</code>语句那样健壮,但它们在React开发人员中非常受欢迎。这样做的一个原因是因为JSX是如何编译的, <code>if/else</code>语句不能直接插入到JSX代码中。您可能已经注意到这几个挑战 - 当需要<code>if/else</code>语句时,它总是在<code>return</code>语句<em>之外</em> 。如果要在JSX中实现条件逻辑,三元表达式可能是一个很好的选择。回想一下,三元运算符有三个部分,但是你可以将几个三元表达式组合在一起。这是基本语法: <blockquote>条件? expressionIfTrue:expressionIfFalse </blockquote></section>
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## Instructions
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<section id="instructions">代码编辑器在<code>CheckUserAge</code>组件的<code>render()</code>方法中定义了三个常量。它们被称为<code>buttonOne</code> , <code>buttonTwo</code>和<code>buttonThree</code> 。每个都分配了一个表示按钮元素的简单JSX表达式。首先,使用<code>input</code>和<code>userAge</code>初始化<code>CheckUserAge</code>的状态, <code>userAge</code>其设置为空字符串的值。一旦组件向页面呈现信息,用户就应该有办法与它进行交互。在组件的<code>return</code>语句中,设置一个实现以下逻辑的三元表达式:当页面首次加载时,将提交按钮<code>buttonOne</code>呈现给页面。然后,当用户输入他们的年龄并单击该按钮时,根据年龄呈现不同的按钮。如果用户输入的数字小于<code>18</code> ,则渲染<code>buttonThree</code> 。如果用户输入的数字大于或等于<code>18</code> ,则渲染<code>buttonTwo</code> 。 </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: <code>CheckUserAge</code>组件应使用单个<code>input</code>元素和单个<code>button</code>元素进行渲染。
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testString: 'assert(Enzyme.mount(React.createElement(CheckUserAge)).find("div").find("input").length === 1 && Enzyme.mount(React.createElement(CheckUserAge)).find("div").find("button").length === 1, "The <code>CheckUserAge</code> component should render with a single <code>input</code> element and a single <code>button</code> element.");'
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- text: 应使用<code>userAge</code>属性和<code>input</code>属性初始化<code>CheckUserAge</code>组件的状态,两者都设置为空字符串的值。
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testString: 'assert(Enzyme.mount(React.createElement(CheckUserAge)).state().input === "" && Enzyme.mount(React.createElement(CheckUserAge)).state().userAge === "", "The <code>CheckUserAge</code> component's state should be initialized with a property of <code>userAge</code> and a property of <code>input</code>, both set to a value of an empty string.");'
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- text: 当<code>CheckUserAge</code>组件首次呈现给DOM时, <code>button</code>的内部文本应该是Submit。
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testString: 'assert(Enzyme.mount(React.createElement(CheckUserAge)).find("button").text() === "Submit", "When the <code>CheckUserAge</code> component is first rendered to the DOM, the <code>button</code>'s inner text should be Submit.");'
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- text: 当进入一个数小于18 <code>input</code>元件和<code>button</code>被点击, <code>button</code>的内部文本应该读<code>You Shall Not Pass</code> 。
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testString: 'async () => { const waitForIt = (fn) => new Promise((resolve, reject) => setTimeout(() => resolve(fn()), 250)); const mockedComponent = Enzyme.mount(React.createElement(CheckUserAge)); const initialButton = mockedComponent.find("button").text(); const enter3AndClickButton = () => { mockedComponent.find("input").simulate("change", {target: { value: "3" }}); mockedComponent.find("button").simulate("click"); return waitForIt(() => { mockedComponent.update(); return mockedComponent.find("button").text(); }); }; const enter17AndClickButton = () => { mockedComponent.find("input").simulate("change", {target: { value: "17" }}); mockedComponent.find("button").simulate("click"); return waitForIt(() => { mockedComponent.update(); return mockedComponent.find("button").text(); }); }; const userAge3 = await enter3AndClickButton(); const userAge17 = await enter17AndClickButton(); assert(initialButton === "Submit" && userAge3 === "You Shall Not Pass" && userAge17 === "You Shall Not Pass", "When a number of less than 18 is entered into the <code>input</code> element and the <code>button</code> is clicked, the <code>button</code>'s inner text should read <code>You Shall Not Pass</code>."); }; '
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- text: 当数大于或等于18输入到<code>input</code>元件和<code>button</code>被点击, <code>button</code>的内部文本应该阅读<code>You May Enter</code> 。
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testString: 'async () => { const waitForIt = (fn) => new Promise((resolve, reject) => setTimeout(() => resolve(fn()), 250)); const mockedComponent = Enzyme.mount(React.createElement(CheckUserAge)); const initialButton = mockedComponent.find("button").text(); const enter18AndClickButton = () => { mockedComponent.find("input").simulate("change", {target: { value: "18" }}); mockedComponent.find("button").simulate("click"); return waitForIt(() => { mockedComponent.update(); return mockedComponent.find("button").text(); }); }; const enter35AndClickButton = () => { mockedComponent.find("input").simulate("change", {target: { value: "35" }}); mockedComponent.find("button").simulate("click"); return waitForIt(() => { mockedComponent.update(); return mockedComponent.find("button").text(); }); }; const userAge18 = await enter18AndClickButton(); const userAge35 = await enter35AndClickButton(); assert(initialButton === "Submit" && userAge18 === "You May Enter" && userAge35 === "You May Enter", "When a number greater than or equal to 18 is entered into the <code>input</code> element and the <code>button</code> is clicked, the <code>button</code>'s inner text should read <code>You May Enter</code>."); }; '
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- text: 一旦提交了一个数字,并再次更改了<code>input</code>的值,该<code>button</code>应返回到<code>Submit</code> 。
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testString: 'async () => { const waitForIt = (fn) => new Promise((resolve, reject) => setTimeout(() => resolve(fn()), 250)); const mockedComponent = Enzyme.mount(React.createElement(CheckUserAge)); const enter18AndClickButton = () => { mockedComponent.find("input").simulate("change", {target: { value: "18" }}); mockedComponent.find("button").simulate("click"); return waitForIt(() => { mockedComponent.update(); return mockedComponent.find("button").text(); }); }; const changeInputDontClickButton = () => { mockedComponent.find("input").simulate("change", {target: { value: "5" }}); return waitForIt(() => { mockedComponent.update(); return mockedComponent.find("button").text(); }); }; const enter10AndClickButton = () => { mockedComponent.find("input").simulate("change", {target: { value: "10" }}); mockedComponent.find("button").simulate("click"); return waitForIt(() => { mockedComponent.update(); return mockedComponent.find("button").text(); }); }; const userAge18 = await enter18AndClickButton(); const changeInput1 = await changeInputDontClickButton(); const userAge10 = await enter10AndClickButton(); const changeInput2 = await changeInputDontClickButton(); assert(userAge18 === "You May Enter" && changeInput1 === "Submit" && userAge10 === "You Shall Not Pass" && changeInput2 === "Submit", "Once a number has been submitted, and the value of the <code>input</code> is once again changed, the <code>button</code> should return to reading <code>Submit</code>."); }; '
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- text: 您的代码不应包含任何<code>if/else</code>语句。
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testString: 'assert(new RegExp(/(\s|;)if(\s|\()/).test(Enzyme.mount(React.createElement(CheckUserAge)).instance().render.toString()) === false, "Your code should not contain any <code>if/else</code> statements.");'
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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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const inputStyle = {
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width: 235,
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margin: 5
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}
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class CheckUserAge extends React.Component {
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constructor(props) {
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super(props);
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// change code below this line
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// change code above this line
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this.submit = this.submit.bind(this);
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this.handleChange = this.handleChange.bind(this);
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}
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handleChange(e) {
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this.setState({
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input: e.target.value,
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userAge: "
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});
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}
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submit() {
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this.setState({
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userAge: this.state.input
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});
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}
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render() {
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const buttonOne = <button onClick={this.submit}>Submit</button>;
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const buttonTwo = <button>You May Enter</button>;
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const buttonThree = <button>You Shall Not Pass</button>;
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return (
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<div>
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<h3>Enter Your Age to Continue</h3>
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<input
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style={inputStyle}
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type="number"
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value={this.state.input}
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onChange={this.handleChange} /><br />
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{
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/* change code here */
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}
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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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console.info('after the test');
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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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// solution required
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```
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</section>
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