Simple argument parsing
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arg is yet another command line option parser.


Use Yarn or NPM to install.

$ yarn add arg


$ npm install arg


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)})

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).

const arg = require('arg');

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

For example:

$ node ./hello.js --port=1234 -n 'My name' foo bar --tag qux --tag=qix -- --foobar
// hello.js
const arg = require('arg');

const args = arg({
	// Types
	'--help':    Boolean,
	'--version': Boolean,
	'--port':    Number,      // --port <number> or --port=<number>
	'--name':    String,      // --name <string> or --name=<string>
	'--tag':     [String],    // --tag <string> or --tag=<string>

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

	_: ["foo", "bar", "--foobar"],
	'--port': 1234,
	'--name': "My name",
	'--tag': ["qux", "qix"]

The values for each key=>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 operatons or for supporting -v multiple times, etc.)

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


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


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:

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

results in:

const args = {
	_: ['hello'],
	'--foo': 'world'


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:

const arg = require('arg');

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

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

results in:

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



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

// cli.js
try {
  require('arg')({ '--hi': String });
} catch (err) {
  if (err.code === 'ARG_UNKNOWN_OPTION') {
  } else {
    throw err;
node cli.js --extraneous true
Unknown or unexpected option: --extraneous


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