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geekpi
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[#]: subject: "How I build command-line apps in JavaScript"
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[#]: via: "https://opensource.com/article/21/11/javascript-command-line-apps"
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[#]: author: "Seth Kenlon https://opensource.com/users/seth"
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[#]: collector: "lujun9972"
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[#]: translator: "geekpi"
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[#]: reviewer: " "
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[#]: publisher: " "
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[#]: url: " "
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How I build command-line apps in JavaScript
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======
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Including options for your users is an important feature for any
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application, and Commander.js makes it easy to do. What's your favorite
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JavaScript command-line builder? Take our poll.
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![Javascript code close-up with neon graphic overlay][1]
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JavaScript is a language developed for the web, but its usefulness has gone far beyond just the Internet. Thanks to projects like Node.js and Electron, JavaScript is as much a general-purpose scripting language as a browser component. There are JavaScript libraries specially designed to build command-line interfaces. Yes, you can run JavaScript in your terminal.
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Now, when you enter a command into your terminal, there are generally [options][2], also called _switches_ or _flags_, that you can use to modify how the command runs. This is a useful convention defined by the [POSIX specification][3], so as a programmer, it's helpful to know how to detect and parse the options. To get this functionality from JavaScript, it's useful to use a library designed to make it easy to build command-line interfaces. My favorite is [Commander.js][4]. It's easy, it's flexible, and it's intuitive.
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### Installing node
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To use the Commander.js library, you must have Node.js installed. On Linux, you can install Node using your package manager. For example, on Fedora, CentOS, Mageia, and others:
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```
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`$ sudo dnf install nodejs`
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```
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On Windows and macOS, you can [download installers from the nodejs.org website][5].
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### Installing Commander.js
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To install Commander.js, use the `npm` command:
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```
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`$ npm install commander`
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```
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### Adding a library to your JavaScript code
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In JavaScript, you can use the `require` keyword to include (or import, if you're used to Python) a library into your code. Create a file called `example.js` and open it in your favorite text editor. Add this line to the top to include the Commander.js library:
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```
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`const { program } = require('commander');`
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```
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### Option parsing in JavaScript
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The first thing you must do to parse options is to define the valid options your application can accept. The Commander.js library lets you define both short and long options, along with a helpful message to clarify the purpose of each.
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```
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program
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.description('A sample application to parse options')
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.option('-a, --alpha', 'Alpha')
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.option('-b, --beta <VALUE>', 'Specify a VALUE', 'Foo');
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```
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The first option, which I've called `--alpha` (`-a` for short), is a Boolean switch: It either exists or it doesn't. It takes no arguments. The second option, which I've called `--beta` (`-b` for short), accepts an argument and even specifies a default value when you've provided nothing.
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### Accessing the command-line data
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Once you've defined valid options, you can reference the values using the long option name:
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```
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program.parse();
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const options = program.opts();
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console.log('Options detected:');
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if (options.alpha) console.log('alpha');
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const beta = !options.beta ? 'no' : options.beta;
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console.log('beta is: %s', beta);
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```
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### Run the application
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Try running it with the `node` command, first with no options:
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```
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$ node ./example.js
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Options detected:
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beta is: Foo
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```
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The default value for `beta `gets used in the absence of an override from the user.
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Run it again, this time using the options:
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```
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$ node ./example.js --beta hello --alpha
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Options detected:
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alpha
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beta is: hello
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```
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This time, the test script successfully detected the option `--alpha`, and the `--beta` option with the value provided by the user.
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### Option parsing
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Here's the full demonstration code for your reference:
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```
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const { program } = require('commander');
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program
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.description('A sample application to parse options')
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.option('-a, --alpha', 'Alpha')
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.option('-b, --beta <VALUE>', 'Specify a VALUE', 'Foo');
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program.parse();
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const options = program.opts();
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console.log('Options detected:');
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console.log(typeof options);
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if (options.alpha) console.log(' * alpha');
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const beta = !options.beta ? 'no' : options.beta;
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console.log(' * beta is: %s', beta);
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```
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There are further examples in the project's [Git repository][4].
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Including options for your users is an important feature for any application, and Commander.js makes it easy to do. There are other libraries aside from Commander.js, but I find this one easy and quick to use. What's your favorite JavaScript command-line builder?
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--------------------------------------------------------------------------------
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via: https://opensource.com/article/21/11/javascript-command-line-apps
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作者：[Seth Kenlon][a]
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选题：[lujun9972][b]
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译者：[译者ID](https://github.com/译者ID)
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校对：[校对者ID](https://github.com/校对者ID)
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本文由 [LCTT](https://github.com/LCTT/TranslateProject) 原创编译，[Linux中国](https://linux.cn/) 荣誉推出
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[a]: https://opensource.com/users/seth
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[b]: https://github.com/lujun9972
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[1]: https://opensource.com/sites/default/files/styles/image-full-size/public/lead-images/code_javascript.jpg?itok=60evKmGl (Javascript code close-up with neon graphic overlay)
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[2]: https://opensource.com/article/21/8/linux-terminal
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[3]: https://opensource.com/article/19/7/what-posix-richard-stallman-explains
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[4]: https://github.com/tj/commander.js
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[5]: https://nodejs.org/en/download
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@ -0,0 +1,166 @@
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[#]: subject: "How I build command-line apps in JavaScript"
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[#]: via: "https://opensource.com/article/21/11/javascript-command-line-apps"
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[#]: author: "Seth Kenlon https://opensource.com/users/seth"
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[#]: collector: "lujun9972"
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[#]: translator: "geekpi"
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[#]: reviewer: " "
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[#]: publisher: " "
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[#]: url: " "
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我如何用 JavaScript 构建命令行应用
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======
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为你的用户提供选项是任何应用的一个重要功能，而 Commander.js 使它变得容易做到。你最喜欢的 JavaScript
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命令行构建器是什么？参加我们的投票吧。
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![Javascript code close-up with neon graphic overlay][1]
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JavaScript 是一种为 Web 开发的语言，但它的用处已经远远超出了互联网的范畴。由于 Node.js 和 Electron 这样的项目，JavaScript 既是一种通用的脚本语言，也是一种浏览器组件。有专门设计的 JavaScript 库来构建命令行界面。是的，你可以在你的终端中运行 JavaScript。
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现在，当你在终端中输入一个命令时，一般都有[选项][2]，也叫_开关_或_标志_，你可以用来修改命令的运行方式。这是由 [POSIX 规范][3]定义的一个有用的惯例，所以作为一个程序员，知道如何检测和解析这些选项是很有帮助的。要从 JavaScript 获得此功能，使用旨在简化构建命令行界面的库很有用。我最喜欢的是 [Commander.js][4]。它很简单，很灵活，而且很直观。
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### 安装 node
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要使用 Commander.js 库，你必须安装 Node.js。在 Linux 上，你可以用你的包管理器安装 Node。例如，在 Fedora、CentOS、Mageia 和其他系统上：
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```
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`$ sudo dnf install nodejs`
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```
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在 Windows 和 macOS 上，你可以[从 nodejs.org 网站下载安装程序][5]。
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### 安装 Commander.js
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要安装 Commander.js，请使用 `npm` 命令：
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```
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`$ npm install commander`
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```
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### 在你的 JavaScript 代码中添加一个库
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在 JavaScript 中，你可以使用 `require` 关键字在你的代码中包含（或者导入，如果你习惯于 Python）一个库。创建一个名为 `example.js` 的文件，并在你喜欢的文本编辑器中打开它。在顶部添加这一行，以包括 Commander.js 库：
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```
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`const { program } = require('commander');`
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```
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### JavaScript 中的选项解析
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要解析选项，你必须做的第一件事是定义你的应用可以接受的有效选项。Commander.js 库可以让你定义短选项和长选项，同时还有一个有用的信息来澄清每个选项的目的。
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```
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program
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.description('A sample application to parse options')
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.option('-a, --alpha', 'Alpha')
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.option('-b, --beta <VALUE>', 'Specify a VALUE', 'Foo');
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```
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第一个选项，我称之为 `--alpha`（简写 `-a`），是一个布尔型开关：它要么存在，要么不存在。它不需要任何参数。第二个选项，我称之为 `--beta`（简写 `-b`），接受一个参数，甚至在你没有提供任何参数的情况下指定一个默认值。
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### 访问命令行数据
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当你定义了有效的选项，你就可以使用长的选项名称来引用这些值：
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```
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program.parse();
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const options = program.opts();
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console.log('Options detected:');
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if (options.alpha) console.log('alpha');
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const beta = !options.beta ? 'no' : options.beta;
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console.log('beta is: %s', beta);
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```
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### 运行应用
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试着用 `node` 命令来运行它，首先不使用选项：
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```
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$ node ./example.js
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Options detected:
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beta is: Foo
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```
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在用户没有覆盖的情况下，`beta` 的默认值被使用。
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再次运行它，这次使用选项：
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```
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$ node ./example.js --beta hello --alpha
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Options detected:
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alpha
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beta is: hello
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```
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这次，测试脚本成功检测到了选项 `--alpha`，以及用户提供的 `--beta` 选项的值。
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### 选项解析
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下面是完整的演示代码供你参考：
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```
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const { program } = require('commander');
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program
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.description('A sample application to parse options')
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.option('-a, --alpha', 'Alpha')
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.option('-b, --beta <VALUE>', 'Specify a VALUE', 'Foo');
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program.parse();
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const options = program.opts();
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console.log('Options detected:');
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console.log(typeof options);
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if (options.alpha) console.log(' * alpha');
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const beta = !options.beta ? 'no' : options.beta;
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console.log(' * beta is: %s', beta);
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```
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在项目的 [Git 仓库][4]中还有更多例子。
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对任何应用来说，包括用户的选项都是一个重要的功能，而 Commander.js 使它很容易做到。除了 Commander.js，还有其他库，但我觉得这个库使用起来很方便快捷。你最喜欢的 JavaScript 命令行构建器是什么？
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--------------------------------------------------------------------------------
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via: https://opensource.com/article/21/11/javascript-command-line-apps
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作者：[Seth Kenlon][a]
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选题：[lujun9972][b]
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译者：[geekpi](https://github.com/geekpi)
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校对：[校对者ID](https://github.com/校对者ID)
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||||
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||||
本文由 [LCTT](https://github.com/LCTT/TranslateProject) 原创编译，[Linux中国](https://linux.cn/) 荣誉推出
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[a]: https://opensource.com/users/seth
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[b]: https://github.com/lujun9972
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[1]: https://opensource.com/sites/default/files/styles/image-full-size/public/lead-images/code_javascript.jpg?itok=60evKmGl (Javascript code close-up with neon graphic overlay)
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[2]: https://opensource.com/article/21/8/linux-terminal
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[3]: https://opensource.com/article/19/7/what-posix-richard-stallman-explains
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[4]: https://github.com/tj/commander.js
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[5]: https://nodejs.org/en/download
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