Command Line Flags in TypeScript

Here’s the translation of the Go code to TypeScript, along with explanations in Markdown format suitable for Hugo:

Command-line flags are a common way to specify options for command-line programs. For example, in wc -l the -l is a command-line flag.

TypeScript doesn’t have built-in support for command-line flag parsing, so we’ll use a popular library called yargs to implement our example command-line program.

First, install the yargs package:

npm install yargs

Now, let’s create our TypeScript program:

import yargs from 'yargs';
import { hideBin } from 'yargs/helpers';

const argv = yargs(hideBin(process.argv))
  .option('word', {
    type: 'string',
    description: 'a string',
    default: 'foo'
  })
  .option('numb', {
    type: 'number',
    description: 'a number',
    default: 42
  })
  .option('fork', {
    type: 'boolean',
    description: 'a boolean',
    default: false
  })
  .option('svar', {
    type: 'string',
    description: 'a string var',
    default: 'bar'
  })
  .parse();

console.log('word:', argv.word);
console.log('numb:', argv.numb);
console.log('fork:', argv.fork);
console.log('svar:', argv.svar);
console.log('tail:', argv._);

In this TypeScript version:

1. We import yargs and hideBin helper function.
2. We use yargs to define our command-line options, similar to the flag declarations in the original code.
3. The parse() method is called to process the command-line arguments.
4. We access the parsed values directly from the argv object.

To run this TypeScript program, you’ll need to compile it first:

tsc command-line-flags.ts

Then you can run the compiled JavaScript:

node command-line-flags.js -word=opt -numb=7 --fork --svar=flag
word: opt
numb: 7
fork: true
svar: flag
tail: []

If you omit flags, they automatically take their default values:

node command-line-flags.js -word=opt
word: opt
numb: 42
fork: false
svar: bar
tail: []

You can provide trailing positional arguments after any flags:

node command-line-flags.js -word=opt a1 a2 a3
word: opt
numb: 42
fork: false
svar: bar
tail: [ 'a1', 'a2', 'a3' ]

To get automatically generated help text, use the --help flag:

node command-line-flags.js --help
Options:
  --help     Show help                                             [boolean]
  --version  Show version number                                   [boolean]
  --word     a string                                [string] [default: "foo"]
  --numb     a number                                   [number] [default: 42]
  --fork     a boolean                               [boolean] [default: false]
  --svar     a string var                            [string] [default: "bar"]

If you provide a flag that wasn’t specified, the program will print an error message:

node command-line-flags.js --wat
Unknown argument: wat

This TypeScript version provides similar functionality to the original program, using the yargs library to handle command-line argument parsing.