85 lines
2.1 KiB
Markdown
85 lines
2.1 KiB
Markdown
---
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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: 'https://scrimba.com/c/cBa2JAL'
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forumTopicId: 18335
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localeTitle: 理解未初始化的变量
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---
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## Description
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<section id='description'>
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当 JavaScript 中的变量被声明的时候，程序内部会给它一个初始值<code>undefined</code>。当你对一个值为<code>undefined</code>的变量进行运算操作的时候，算出来的结果将会是<code>NaN</code>，<code>NaN</code>的意思是<dfn>"Not a Number"</dfn>。当你用一个值是<code>undefined</code>的变量来做字符串拼接操作的时候，它会输出字符串<code>"undefined"</code>。
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</section>
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## Instructions
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<section id='instructions'>
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定义 3 个变量<code>a</code>、<code>b</code>、<code>c</code>，并且分别给他们赋值：<code>5</code>、<code>10</code>、<code>"I am a"</code>，这样它们值就不会是<code>undefined</code>了。
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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: <code>a</code>应该被定义，并且值为<code>6</code>。
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testString: assert(typeof a === 'number' && a === 6);
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- text: <code>b</code>应该被定义，并且值为<code>15</code>。
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testString: assert(typeof b === 'number' && b === 15);
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- text: <code>c</code>的值不能包含<code>undefined</code>，应该为 "I am a String!"。
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testString: assert(!/undefined/.test(c) && c === "I am a String!");
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- text: 不要修改第二条注释下的代码。
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testString: assert(/a = a \+ 1;/.test(code) && /b = b \+ 5;/.test(code) && /c = c \+ " String!";/.test(code));
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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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(function(a,b,c){ return "a = " + a + ", b = " + b + ", c = '" + c + "'"; })(a,b,c);
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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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var a = 5;
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var b = 10;
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var c = "I am a";
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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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</section>
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