69 lines
2.8 KiB
Markdown
69 lines
2.8 KiB
Markdown
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---
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id: 587d7b87367417b2b2512b41
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title: Declare a Read-Only Variable with the const Keyword
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challengeType: 1
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videoUrl: ''
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localeTitle: 使用const关键字声明只读变量
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---
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## Description
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<section id="description"> <code>let</code>不是声明变量的唯一新方法。在ES6中，您还可以使用<code>const</code>关键字声明变量。 <code>const</code>拥有所有的真棒功能， <code>let</code>具有，与额外的奖励，使用声明的变量<code>const</code>是只读的。它们是一个常量值，这意味着一旦变量被赋值为<code>const</code> ，就无法重新赋值。 <blockquote> “严格使用” <br> const FAV_PET =“Cats”; <br> FAV_PET =“狗”; //返回错误</blockquote>如您所见，尝试重新分配使用<code>const</code>声明的变量将引发错误。您应该始终使用<code>const</code>关键字命名您不想重新分配的变量。当您意外尝试重新分配一个旨在保持不变的变量时，这会有所帮助。命名常量时的常见做法是使用全部大写字母，单词用下划线分隔。 </section>
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## Instructions
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<section id="instructions">更改代码，以便使用<code>let</code>或<code>const</code>声明所有变量。如果希望变量更改，请使用<code>let</code> ;如果希望变量保持不变，请使用<code>const</code> 。此外，重命名用<code>const</code>声明的变量以符合常规做法，这意味着常量应该全部大写。 </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>var</code>在您的代码中不存在。
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testString: 'getUserInput => assert(!getUserInput("index").match(/var/g),"<code>var</code> does not exist in your code.");'
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- text: <code>SENTENCE</code>应该是用<code>const</code>声明的常量变量。
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testString: 'getUserInput => assert(getUserInput("index").match(/(const SENTENCE)/g), "<code>SENTENCE</code> should be a constant variable declared with <code>const</code>.");'
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- text: <code>i</code>应该以<code>let</code>来宣布。
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testString: 'getUserInput => assert(getUserInput("index").match(/(let i)/g), "<code>i</code> should be declared with <code>let</code>.");'
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- text: 应更改<code>console.log</code>以打印<code>SENTENCE</code>变量。
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testString: 'getUserInput => assert(getUserInput("index").match(/console\.log\(\s*SENTENCE\s*\)\s*;?/g), "<code>console.log</code> should be adjusted to print the variable <code>SENTENCE</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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function printManyTimes(str) {
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"use strict";
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// change code below this line
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var sentence = str + " is cool!";
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for(var i = 0; i < str.length; i+=2) {
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console.log(sentence);
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}
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// change code above this line
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}
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printManyTimes("freeCodeCamp");
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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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