115 lines
2.8 KiB
Markdown
115 lines
2.8 KiB
Markdown
---
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id: 56533eb9ac21ba0edf2244c9
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challengeType: 1
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videoUrl: 'https://scrimba.com/c/cnQyKur'
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forumTopicId: 16165
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title: 通过变量访问对象属性
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---
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## Description
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<section id='description'>
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中括号操作符的另一个使用方式是访问赋值给变量的属性。当你需要遍历对象的属性列表或访问查找表（lookup tables）时，这种方式极为有用。
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这有一个使用变量来访问属性的例子：
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```js
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var dogs = {
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Fido: "Mutt", Hunter: "Doberman", Snoopie: "Beagle"
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};
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var myDog = "Hunter";
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var myBreed = dogs[myDog];
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console.log(myBreed); // "Doberman"
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```
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使用此概念的另一种方法是在程序执行期间动态收集属性名称，如下所示：
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```js
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var someObj = {
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propName: "John"
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};
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function propPrefix(str) {
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var s = "prop";
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return s + str;
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}
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var someProp = propPrefix("Name"); // someProp now holds the value 'propName'
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console.log(someObj[someProp]); // "John"
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```
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提示：当我们通过变量名访问属性的时候，不需要给变量名包裹引号。因为实际上我们使用的是变量的值，而不是变量的名称。
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</section>
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## Instructions
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<section id='instructions'>
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使用变量<code>playerNumber</code>，通过中括号操作符找到<code>testObj</code>中<code>playerNumber</code>为<code>16</code>的值。然后把名字赋给变量<code>player</code>。
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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>playerNumber</code>应该是一个数字。
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testString: assert(typeof playerNumber === 'number');
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- text: 变量<code>player</code>应该是一个字符串。
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testString: assert(typeof player === 'string');
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- text: <code>player</code>点值应该是 "Montana"。
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testString: assert(player === 'Montana');
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- text: 你应该使用中括号访问<code>testObj</code>。
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testString: assert(/testObj\s*?\[.*?\]/.test(code));
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- text: 你不应该直接将<code>Montana</code>赋给<code>player</code>。
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testString: assert(!code.match(/player\s*=\s*"|\'\s*Montana\s*"|\'\s*;/gi));
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- text: 你应该在中括号中使用<code>playerNumber</code>变量。
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testString: assert(/testObj\s*?\[\s*playerNumber\s*\]/.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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// Setup
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var testObj = {
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12: "Namath",
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16: "Montana",
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19: "Unitas"
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};
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// Only change code below this line;
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var playerNumber; // Change this Line
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var player = testObj; // Change this Line
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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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if(typeof player !== "undefined"){(function(v){return v;})(player);}
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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 testObj = {
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12: "Namath",
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16: "Montana",
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19: "Unitas"
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};
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var playerNumber = 16;
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var player = testObj[playerNumber];
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```
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</section>
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