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Editing: README.md

# Arg

`arg` is an unopinionated, no-frills CLI argument parser.

## Installation

```bash
npm install arg
```

## Usage

`arg()` takes either 1 or 2 arguments:

1. Command line specification object (see below)
2. Parse options (_Optional_, defaults to `{permissive: false, argv: process.argv.slice(2), stopAtPositional: false}`)

It returns an object with any values present on the command-line (missing options are thus
missing from the resulting object). Arg performs no validation/requirement checking - we
leave that up to the application.

All parameters that aren't consumed by options (commonly referred to as "extra" parameters)
are added to `result._`, which is _always_ an array (even if no extra parameters are passed,
in which case an empty array is returned).

```javascript
const arg = require('arg');

// `options` is an optional parameter
const args = arg(
	spec,
	(options = { permissive: false, argv: process.argv.slice(2) })
);
```

For example:

```console
$ node ./hello.js --verbose -vvv --port=1234 -n 'My name' foo bar --tag qux --tag=qix -- --foobar
```

```javascript
// hello.js
const arg = require('arg');

const args = arg({
	// Types
	'--help': Boolean,
	'--version': Boolean,
	'--verbose': arg.COUNT, // Counts the number of times --verbose is passed
	'--port': Number, // --port <number> or --port=<number>
	'--name': String, // --name <string> or --name=<string>
	'--tag': [String], // --tag <string> or --tag=<string>

// Aliases
	'-v': '--verbose',
	'-n': '--name', // -n <string>; result is stored in --name
	'--label': '--name' // --label <string> or --label=<string>;
	//     result is stored in --name
});

console.log(args);
/*
{
	_: ["foo", "bar", "--foobar"],
	'--port': 1234,
	'--verbose': 4,
	'--name': "My name",
	'--tag': ["qux", "qix"]
}
*/
```

The values for each key=&gt;value pair is either a type (function or [function]) or a string (indicating an alias).

- In the case of a function, the string value of the argument's value is passed to it,
  and the return value is used as the ultimate value.

- In the case of an array, the only element _must_ be a type function. Array types indicate
  that the argument may be passed multiple times, and as such the resulting value in the returned
  object is an array with all of the values that were passed using the specified flag.

- In the case of a string, an alias is established. If a flag is passed that matches the _key_,
  then the _value_ is substituted in its place.

Type functions are passed three arguments:

1. The parameter value (always a string)
2. The parameter name (e.g. `--label`)
3. The previous value for the destination (useful for reduce-like operations or for supporting `-v` multiple times, etc.)

This means the built-in `String`, `Number`, and `Boolean` type constructors "just work" as type functions.

Note that `Boolean` and `[Boolean]` have special treatment - an option argument is _not_ consumed or passed, but instead `true` is
returned. These options are called "flags".

For custom handlers that wish to behave as flags, you may pass the function through `arg.flag()`:

```javascript
const arg = require('arg');

const argv = [
	'--foo',
	'bar',
	'-ff',
	'baz',
	'--foo',
	'--foo',
	'qux',
	'-fff',
	'qix'
];

function myHandler(value, argName, previousValue) {
	/* `value` is always `true` */
	return 'na ' + (previousValue || 'batman!');
}

const args = arg(
	{
		'--foo': arg.flag(myHandler),
		'-f': '--foo'
	},
	{
		argv
	}
);

console.log(args);
/*
{
	_: ['bar', 'baz', 'qux', 'qix'],
	'--foo': 'na na na na na na na na batman!'
}
*/
```

As well, `arg` supplies a helper argument handler called `arg.COUNT`, which equivalent to a `[Boolean]` argument's `.length`
property - effectively counting the number of times the boolean flag, denoted by the key, is passed on the command line..
For example, this is how you could implement `ssh`'s multiple levels of verbosity (`-vvvv` being the most verbose).

```javascript
const arg = require('arg');

const argv = ['-AAAA', '-BBBB'];

const args = arg(
	{
		'-A': arg.COUNT,
		'-B': [Boolean]
	},
	{
		argv
	}
);

console.log(args);
/*
{
	_: [],
	'-A': 4,
	'-B': [true, true, true, true]
}
*/
```

### Options

If a second parameter is specified and is an object, it specifies parsing options to modify the behavior of `arg()`.

#### `argv`

If you have already sliced or generated a number of raw arguments to be parsed (as opposed to letting `arg`
slice them from `process.argv`) you may specify them in the `argv` option.

For example:

```javascript
const args = arg(
	{
		'--foo': String
	},
	{
		argv: ['hello', '--foo', 'world']
	}
);
```

results in:

```javascript
const args = {
	_: ['hello'],
	'--foo': 'world'
};
```

#### `permissive`

When `permissive` set to `true`, `arg` will push any unknown arguments
onto the "extra" argument array (`result._`) instead of throwing an error about
an unknown flag.

For example:

```javascript
const arg = require('arg');

const argv = [
	'--foo',
	'hello',
	'--qux',
	'qix',
	'--bar',
	'12345',
	'hello again'
];

const args = arg(
	{
		'--foo': String,
		'--bar': Number
	},
	{
		argv,
		permissive: true
	}
);
```

results in:

```javascript
const args = {
	_: ['--qux', 'qix', 'hello again'],
	'--foo': 'hello',
	'--bar': 12345
};
```

#### `stopAtPositional`

When `stopAtPositional` is set to `true`, `arg` will halt parsing at the first
positional argument.

For example:

```javascript
const arg = require('arg');

const argv = ['--foo', 'hello', '--bar'];

const args = arg(
	{
		'--foo': Boolean,
		'--bar': Boolean
	},
	{
		argv,
		stopAtPositional: true
	}
);
```

results in:

```javascript
const args = {
	_: ['hello', '--bar'],
	'--foo': true
};
```

### Errors

Some errors that `arg` throws provide a `.code` property in order to aid in recovering from user error, or to
differentiate between user error and developer error (bug).

##### ARG_UNKNOWN_OPTION

If an unknown option (not defined in the spec object) is passed, an error with code `ARG_UNKNOWN_OPTION` will be thrown:

```js
// cli.js
try {
	require('arg')({ '--hi': String });
} catch (err) {
	if (err.code === 'ARG_UNKNOWN_OPTION') {
		console.log(err.message);
	} else {
		throw err;
	}
}
```

```shell
node cli.js --extraneous true
Unknown or unexpected option: --extraneous
```

# FAQ

A few questions and answers that have been asked before:

### How do I require an argument with `arg`?

Do the assertion yourself, such as:

```javascript
const args = arg({ '--name': String });

if (!args['--name']) throw new Error('missing required argument: --name');
```

# License

Released under the [MIT License](LICENSE.md).

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