122 lines
2.5 KiB
Markdown
122 lines
2.5 KiB
Markdown
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---
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id: 56533eb9ac21ba0edf2244c9
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title: 通過變量訪問對象屬性
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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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dashedName: accessing-object-properties-with-variables
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---
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# --description--
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對對象上使用方括號表示法，還可以訪問對象上作爲變量值存儲的屬性。 當你需要遍歷對象的所有屬性，或者根據一個變量的值查找對應的屬性值時，這種寫法尤其適用。
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以下是一個使用變量來訪問屬性的例子：
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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);
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```
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字符串 `Doberman` 將會出現在控制檯中。
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使用這一概念的另一種情況是：屬性的名字是在程序運行期間動態收集得到的。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");
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console.log(someObj[someProp]);
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```
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`someProp` 的值將爲字符串 `propName`，並且字符串 `John` 將會出現在控制檯中。
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注意，當使用變量名訪問屬性時，我們*沒有*使用引號包裹它，因爲我們正在使用的是變量的*值*，而不是變量的*名字*。
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# --instructions--
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將變量 `playerNumber` 設置爲 `16`。 然後，使用該變量查找玩家的名字，並將其賦值給`player`。
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# --hints--
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`playerNumber` 應該是一個數字
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```js
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assert(typeof playerNumber === 'number');
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```
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變量 `player` 應該是一個字符串
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```js
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assert(typeof player === 'string');
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```
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`player` 的值應該爲字符串 `Montana`
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```js
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assert(player === 'Montana');
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```
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你應該使用方括號訪問 `testObj`
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```js
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assert(/testObj\s*?\[.*?\]/.test(code));
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```
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你不應該直接將值 `Montana` 賦給變量 `player`。
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```js
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assert(!code.match(/player\s*=\s*"|\'\s*Montana\s*"|\'\s*;/gi));
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```
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你應該在你的方括號內使用變量 `playerNumber`。
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```js
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assert(/testObj\s*?\[\s*playerNumber\s*\]/.test(code));
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```
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# --seed--
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## --after-user-code--
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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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## --seed-contents--
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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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# --solutions--
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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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