142 lines
5.9 KiB
Markdown
142 lines
5.9 KiB
Markdown
---
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id: 587d7b8e367417b2b2512b5d
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challengeType: 1
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forumTopicId: 301241
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title: 了解使用命令式编程的危害
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---
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## Description
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<section id='description'>
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函数式编程是一种好习惯，它能让代码管理更简单，不受隐藏 bug 影响。在我们开始函数式编程之前，为了更好的突显可能遇到的问题，我们先看看命令式编程。
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类似在英语（和许多其他语言）中，命令式时态用于给出命令，编程中的命令式是给计算机一组语句来执行任务。
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这些语句通常会改变程序的状态，例如更新全局变量，典型的例子就是写一个 <code>for</code> 循环，它给出了迭代数组索引的精确方向。
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相反，函数式编程是声明式编程的一种形式，通过调用方法或函数来告诉计算机要做什么。
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JavaScript 提供了许多处理常见任务的方法，所以你无需写出计算机应如何执行它们。例如，你可以用 <code>map</code> 函数替代上面提到的 <code>for</code> 循环来处理数组迭代。这有助于避免语义错误，如调试章节介绍的"Off By One Errors"。
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考虑这样的场景：你正在浏览器中浏览网页，并想操作你打开的标签。下面我们来试试用面向对象的思路来描述这种情景。
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窗口对象由选项卡组成，通常会打开多个窗口。窗口对象中每个打开网站的标题都保存在一个数组中。在对浏览器进行了如打开新标签、合并窗口、关闭标签之类的操作后，你需要输出所有打开的标签。关掉的标签将从数组中删除，新打开的标签（为简单起见）则添加到数组的末尾。
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代码编辑器中显示了此功能的实现，其中包含 <code>tabOpen()</code>，<code>tabClose()</code>，和 <code>join()</code> 函数。tabs数组是窗口对象的一部分用于储存打开页面的名称。
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</section>
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## Instructions
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<section id='instructions'>
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在编辑器中运行代码。它使用了有副作用的方法，导致输出错误。打开标签的最终列表应该是 <code>['FB', 'Gitter', 'Reddit', 'Twitter', 'Medium', 'new tab', 'Netflix', 'YouTube', 'Vine', 'GMail', 'Work mail', 'Docs', 'freeCodeCamp', 'new tab']</code> 但输出会略有不同。
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修改 <code>Window.prototype.tabClose</code> 使其删除正确的标签。
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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>finalTabs.tabs</code> 应该是 <code>['FB', 'Gitter', 'Reddit', 'Twitter', 'Medium', 'new tab', 'Netflix', 'YouTube', 'Vine', 'GMail', 'Work mail', 'Docs', 'freeCodeCamp', 'new tab']</code>
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testString: assert.deepEqual(finalTabs.tabs, ['FB', 'Gitter', 'Reddit', 'Twitter', 'Medium', 'new tab', 'Netflix', 'YouTube', 'Vine', 'GMail', 'Work mail', 'Docs', 'freeCodeCamp', 'new tab'])
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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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// tabs is an array of titles of each site open within the window
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var Window = function(tabs) {
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this.tabs = tabs; // we keep a record of the array inside the object
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};
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// When you join two windows into one window
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Window.prototype.join = function (otherWindow) {
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this.tabs = this.tabs.concat(otherWindow.tabs);
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return this;
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};
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// When you open a new tab at the end
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Window.prototype.tabOpen = function (tab) {
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this.tabs.push('new tab'); // let's open a new tab for now
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return this;
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};
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// When you close a tab
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Window.prototype.tabClose = function (index) {
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// Only change code below this line
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var tabsBeforeIndex = this.tabs.splice(0, index); // get the tabs before the tab
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var tabsAfterIndex = this.tabs.splice(index + 1); // get the tabs after the tab
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this.tabs = tabsBeforeIndex.concat(tabsAfterIndex); // join them together
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// Only change code above this line
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return this;
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};
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// Let's create three browser windows
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var workWindow = new Window(['GMail', 'Inbox', 'Work mail', 'Docs', 'freeCodeCamp']); // Your mailbox, drive, and other work sites
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var socialWindow = new Window(['FB', 'Gitter', 'Reddit', 'Twitter', 'Medium']); // Social sites
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var videoWindow = new Window(['Netflix', 'YouTube', 'Vimeo', 'Vine']); // Entertainment sites
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// Now perform the tab opening, closing, and other operations
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var finalTabs = socialWindow
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.tabOpen() // Open a new tab for cat memes
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.join(videoWindow.tabClose(2)) // Close third tab in video window, and join
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.join(workWindow.tabClose(1).tabOpen());
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console.log(finalTabs.tabs);
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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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// tabs is an array of titles of each site open within the window
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var Window = function(tabs) {
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this.tabs = tabs; // we keep a record of the array inside the object
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};
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// When you join two windows into one window
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Window.prototype.join = function (otherWindow) {
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this.tabs = this.tabs.concat(otherWindow.tabs);
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return this;
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};
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// When you open a new tab at the end
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Window.prototype.tabOpen = function (tab) {
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this.tabs.push('new tab'); // let's open a new tab for now
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return this;
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};
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// When you close a tab
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Window.prototype.tabClose = function (index) {
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var tabsBeforeIndex = this.tabs.slice(0, index); // get the tabs before the tab
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var tabsAfterIndex = this.tabs.slice(index + 1); // get the tabs after the tab
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this.tabs = tabsBeforeIndex.concat(tabsAfterIndex); // join them together
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return this;
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};
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// Let's create three browser windows
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var workWindow = new Window(['GMail', 'Inbox', 'Work mail', 'Docs', 'freeCodeCamp']); // Your mailbox, drive, and other work sites
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var socialWindow = new Window(['FB', 'Gitter', 'Reddit', 'Twitter', 'Medium']); // Social sites
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var videoWindow = new Window(['Netflix', 'YouTube', 'Vimeo', 'Vine']); // Entertainment sites
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// Now perform the tab opening, closing, and other operations
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var finalTabs = socialWindow
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.tabOpen() // Open a new tab for cat memes
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.join(videoWindow.tabClose(2)) // Close third tab in video window, and join
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.join(workWindow.tabClose(1).tabOpen());
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```
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</section>
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