81 lines
2.9 KiB
Markdown
81 lines
2.9 KiB
Markdown
---
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id: 9d7123c8c441eeafaeb5bdef
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challengeType: 1
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forumTopicId: 301236
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localeTitle: 使用 slice 而不是 splice 从数组中移除元素
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---
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## Description
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<section id='description'>
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使用数组时经常遇到要删除一些元素并保留数组剩余部分的情况。为此，JavaScript 提供了<code>splice</code>方法，它接收两个参数：从哪里开始删除项目的索引，和要删除的项目数。如果没有提供第二个参数，默认情况下是移除到结尾的元素。但<code>splice</code>方法会改变调用它的原始数组。举个例子：
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```js
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var cities = ["Chicago", "Delhi", "Islamabad", "London", "Berlin"];
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cities.splice(3, 1); // Returns "London" and deletes it from the cities array
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// cities is now ["Chicago", "Delhi", "Islamabad", "Berlin"]
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```
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正如我们在上一次挑战中看到的那样，<code>slice</code>方法不会改变原始数组，而是返回一个可以保存到变量中的新数组。回想一下，<code>slice</code>方法接收两个参数，从开始索引开始选取到结束（不包括该元素），并在新数组中返回这些元素。使用<code>slice</code>方法替代<code>splice</code>有助于避免数组变化产生的副作用。
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</section>
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## Instructions
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<section id='instructions'>
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用<code>slice</code>代替<code>splice</code>重写<code>nonMutatingSplice</code>函数。将<code>cities</code>数组长度限制为3，并返回一个仅包含前 3 项的新数组。
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不要改变提供给函数的原始数组。
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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: 应该使用<code>slice</code>方法。
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testString: assert(code.match(/\.slice/g));
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- text: 不能使用<code>splice</code>方法。
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testString: assert(!code.match(/\.splice/g));
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- text: 不能改变<code>inputCities</code>数组。
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testString: assert(JSON.stringify(inputCities) === JSON.stringify(["Chicago", "Delhi", "Islamabad", "London", "Berlin"]));
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- text: "<code>nonMutatingSplice(['Chicago', 'Delhi', 'Islamabad', 'London', 'Berlin'])</code>应返回<code>['Chicago', 'Delhi', 'Islamabad']</code>。"
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testString: assert(JSON.stringify(nonMutatingSplice(["Chicago", "Delhi", "Islamabad", "London", "Berlin"])) === JSON.stringify(["Chicago", "Delhi", "Islamabad"]));
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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 nonMutatingSplice(cities) {
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// Add your code below this line
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return cities.splice(3);
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// Add your code above this line
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}
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var inputCities = ["Chicago", "Delhi", "Islamabad", "London", "Berlin"];
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nonMutatingSplice(inputCities);
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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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function nonMutatingSplice(cities) {
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// Add your code below this line
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return cities.slice(0,3);
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// Add your code above this line
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}
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var inputCities = ["Chicago", "Delhi", "Islamabad", "London", "Berlin"];
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nonMutatingSplice(inputCities);
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```
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</section>
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