73 lines
2.1 KiB
Markdown
73 lines
2.1 KiB
Markdown
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---
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id: 56533eb9ac21ba0edf2244aa
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title: Understanding Uninitialized Variables
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challengeType: 1
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videoUrl: ''
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localeTitle: 了解未初始化的变量
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---
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## Description
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<section id="description">声明JavaScript变量时，它们的初始值为<code>undefined</code> 。如果对<code>undefined</code>变量进行数学运算，则结果将为<code>NaN</code> ，表示<dfn>“非数字”</dfn> 。如果将字符串与<code>undefined</code>变量连接起来，您将得到一个<code>"undefined"</code>的文字<dfn>字符串</dfn> 。 </section>
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## Instructions
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<section id="instructions">初始化三个变量<code>a</code> ， <code>b</code>和<code>c</code>与<code>5</code> ， <code>10</code> ，和<code>"I am a"</code>分别让他们不会<code>undefined</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>a</code>应定义并评估其值为<code>6</code>
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testString: 'assert(typeof a === "number" && a === 6, "<code>a</code> should be defined and evaluated to have the value of <code>6</code>");'
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- text: 应定义和评估<code>b</code>的值为<code>15</code>
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testString: 'assert(typeof b === "number" && b === 15, "<code>b</code> should be defined and evaluated to have the value of <code>15</code>");'
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- text: <code>c</code>不应该包含<code>undefined</code>并且值应为“我是一个字符串！”
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testString: 'assert(!/undefined/.test(c) && c === "I am a String!", "<code>c</code> should not contain <code>undefined</code> and should have a value of "I am a String!"");'
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- text: 不要更改行下方的代码
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testString: 'assert(/a = a \+ 1;/.test(code) && /b = b \+ 5;/.test(code) && /c = c \+ " String!";/.test(code), "Do not change code below the line");'
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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='js-seed'>
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```js
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// Initialize these three variables
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var a;
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var b;
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var c;
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// Do not change code below this line
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a = a + 1;
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b = b + 5;
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c = c + " String!";
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```
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</div>
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### After Test
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<div id='js-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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