freeCodeCamp/remove-elements-from-an-array-using-slice-instead-of-splice.md at 873fce02a2d2124f3b76ce3f317fb29b34abb542

Oliver Eyton-Williams dec409a4bd fix: s/localeTitle/title/g

2020-10-06 23:10:08 +05:30

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id, challengeType, forumTopicId, title

id	challengeType	forumTopicId	title
9d7123c8c441eeafaeb5bdef	1	301236	使用 slice 而不是 splice 从数组中移除元素

Description

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splice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splice方法会改变调用它的原始数组。举个例子：

var cities = ["Chicago", "Delhi", "Islamabad", "London", "Berlin"];
cities.splice(3, 1); // Returns "London" and deletes it from the cities array
// cities is now ["Chicago", "Delhi", "Islamabad", "Berlin"]

正如我们在上一次挑战中看到的那样，slice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，slice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slice方法替代splice有助于避免数组变化产生的副作用。

Instructions

用slice代替splice重写nonMutatingSplice函数。将cities数组长度限制为3，并返回一个仅包含前 3 项的新数组。不要改变提供给函数的原始数组。

Tests

tests:
  - text: 应该使用<code>slice</code>方法。
    testString: assert(code.match(/\.slice/g));
  - text: 不能使用<code>splice</code>方法。
    testString: assert(!code.match(/\.splice/g));
  - text: 不能改变<code>inputCities</code>数组。
    testString: assert(JSON.stringify(inputCities) === JSON.stringify(["Chicago", "Delhi", "Islamabad", "London", "Berlin"]));
  - text: "<code>nonMutatingSplice(['Chicago', 'Delhi', 'Islamabad', 'London', 'Berlin'])</code>应返回<code>['Chicago', 'Delhi', 'Islamabad']</code>。"
    testString: assert(JSON.stringify(nonMutatingSplice(["Chicago", "Delhi", "Islamabad", "London", "Berlin"])) === JSON.stringify(["Chicago", "Delhi", "Islamabad"]));

Challenge Seed

function nonMutatingSplice(cities) {
  // Add your code below this line
  return cities.splice(3);

  // Add your code above this line
}
var inputCities = ["Chicago", "Delhi", "Islamabad", "London", "Berlin"];
nonMutatingSplice(inputCities);

Solution

function nonMutatingSplice(cities) {
  // Add your code below this line
  return cities.slice(0,3);
  // Add your code above this line
}
var inputCities = ["Chicago", "Delhi", "Islamabad", "London", "Berlin"];
nonMutatingSplice(inputCities);

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