97 lines
3.1 KiB
Markdown
97 lines
3.1 KiB
Markdown
---
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id: 587d7b7c367417b2b2512b1a
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title: Access Property Names with Bracket Notation
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challengeType: 1
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forumTopicId: 301150
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localeTitle: 使用方括号访问属性名称
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---
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## Description
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<section id='description'>
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在关于对象的第一个挑战中，我们提到可以在方括号符号中用一个变量作为属性名来访问属性值。假设一个超市的收银台的程序中使用了一个<code>foods</code>对象，并且有一些程序逻辑会设置<code>selectedFood</code>，我们需要查询<code>foods</code>对象来检查某种食物是否存在，我们可以这样写检查逻辑：
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```js
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let selectedFood = getCurrentFood(scannedItem);
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let inventory = foods[selectedFood];
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```
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上述代码会先计算<code>selectedFood</code>变量的值，并返回<code>foods</code>对象中以该值命名的属性对应的值，若没有以该值命名的属性则会返回<code>undefined</code>。有时候对象的属性名在运行之前是不确定的，或者我们需要动态地访问对象的属性，这时方括号符号就会很有用。
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</section>
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## Instructions
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<section id='instructions'>
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我们已经定义了一个<code>checkInventory</code>函数，它接受一个被扫描到的商品名作为输入参数。它要返回<code>foods</code>对象中以<code>scannedItem</code>的值命名的属性的值。只有有效的属性名会作为参数传入<code>checkInventory</code>，你在完成挑战时不需处理参数无效的情况。
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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>checkInventory</code>是一个函数
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testString: assert.strictEqual(typeof checkInventory, 'function');
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- text: '<code>foods</code>对象应该只有以下键值对： <code>apples: 25</code> ， <code>oranges: 32</code> ， <code>plums: 28</code> ， <code>bananas: 13</code> ， <code>grapes: 35</code> ， <code>strawberries: 27</code>'
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testString: 'assert.deepEqual(foods, {apples: 25, oranges: 32, plums: 28, bananas: 13, grapes: 35, strawberries: 27});'
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- text: <code>checkInventory("apples")</code>应该返回<code>25</code>
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testString: assert.strictEqual(checkInventory('apples'), 25);
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- text: <code>checkInventory("bananas")</code>应该返回<code>13</code>
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testString: assert.strictEqual(checkInventory('bananas'), 13);
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- text: <code>checkInventory("strawberries")</code>应该返回<code>27</code>
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testString: assert.strictEqual(checkInventory('strawberries'), 27);
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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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let foods = {
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apples: 25,
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oranges: 32,
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plums: 28,
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bananas: 13,
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grapes: 35,
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strawberries: 27
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};
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// do not change code above this line
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function checkInventory(scannedItem) {
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// change code below this line
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}
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// change code below this line to test different cases:
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console.log(checkInventory("apples"));
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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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let foods = {
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apples: 25,
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oranges: 32,
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plums: 28,
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bananas: 13,
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grapes: 35,
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strawberries: 27
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};
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function checkInventory(scannedItem) {
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return foods[scannedItem];
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}
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```
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</section>
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