Building command line applications with JavaScript

Preface:

JavaScript is a language developed for the Web, but its usefulness extends far beyond the Internet. JavaScript is both a general-purpose scripting language and a browser component, thanks to projects like Node.js and Electron . There are JavaScript libraries specifically designed for building command-line interfaces. Yes, you can run JavaScript in your terminal.

Now, when you type a command into the terminal, there are generally options, also called switches or flags, that you can use to modify the way the command runs. This is a useful convention defined by the POSIX specification, so as a programmer it is helpful to know how to detect and parse these 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 . It's simple, flexible, and intuitive.

1. Install node

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 other systems:

$ sudo dnf install nodejs

On Windows and macOS , we can download the installer from the nodejs.org website.

2. Install Commander.js

To install Commander.js, use the npm command:

$ npm install commander

3. Add a library to your JavaScript code

In JavaScrip , you can include (or import, if you're used to Python) a library in your code using require keyword. Create a file called example.js and open it in your favorite text editor. Add this line at the top to include the Commander.js library:

const { program } = require('commander');

4. Option parsing in JavaScript

To parse options, the first thing you must do is 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 clarifying the purpose of each option.

program
  .description('A sample application to parse options')
  .option('-a, --alpha', 'Alpha')
  .option('-b, --beta <VALUE>', 'Specify a VALUE', 'Foo');

The first option, which I call alpha ( -a for short), is a boolean switch: it is either present or it is not. It does not require any parameters. The second option, which I call beta ( -b for short), accepts a single argument and even specifies a default value if you don't provide any.

5. Access command line data

Once you have defined valid options, you can refer to the values ​​using the long option names:

program.parse();

const options = program.opts();
console.log('Options detected:');

if (options.alpha) console.log('alpha');
 
const beta = !options.beta ? 'no' : options.beta;
console.log('beta is: %s', beta);

6. Run the application

Try running it with the node command, without options first:

$ node ./example.js 
Options detected: 
beta is: Foo

In the absence of an override by the user, beta defaults are used.

Run it again, this time using the options:

$ node ./example.js --beta hello --alpha
Options detected: 
alpha
beta is: hello

This time, the test script successfully detected the option alpha , and the value provided by the user for the beta option.

7. Option analysis

Here is the complete demo code for your reference:

const { program } = require('commander');

program
  .description('A sample application to parse options')
  .option('-a, --alpha', 'Alpha')
    .option('-b, --beta <VALUE>', 'Specify a VALUE', 'Foo');

program.parse();

const options = program.opts();
console.log('Options detected:');

console.log(typeof options);

if (options.alpha) console.log(' * alpha');
const beta = !options.beta ? 'no' : options.beta;
console.log(' * beta is: %s', beta);

There are many more examples in the project's Git repository.

Including user options is an important feature for any application, and Commander.js makes it easy to do. There are other libraries besides Commander.js , but I find this one quite convenient and quick to use. What is your favorite command-line builder for JavaScript ?

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