dashdash

What is dashdash?

The dashdash npm package is a lightweight parser for command-line options. It allows developers to easily define and parse options passed to Node.js scripts, providing a structured way to handle command-line arguments.

What are dashdash's main functionalities?

Option Parsing

This feature allows you to define a set of options and parse them from the command line. The example code demonstrates how to create a parser, define options, and parse them from `process.argv`.

const dashdash = require('dashdash');
const options = [
  { name: 'verbose', type: 'bool', help: 'Print verbose output' },
  { name: 'file', type: 'string', help: 'File to process' }
];
const parser = dashdash.createParser({options: options});
try {
  const opts = parser.parse(process.argv);
  console.log('Options:', opts);
} catch (e) {
  console.error('Error parsing options:', e.message);
  process.exit(1);
}

Help Text Generation

Dashdash can automatically generate help text for the defined options. This code sample shows how to add a help option and print the generated help text when it's used.

const dashdash = require('dashdash');
const options = [
  { name: 'help', type: 'bool', help: 'Print this help and exit.' }
];
const parser = dashdash.createParser({options: options});
if (opts.help) {
  var help = parser.help().trimRight();
  console.log('usage: node myscript.js [OPTIONS]
' + help);
  process.exit(0);
}

Other packages similar to dashdash

commander

Commander is a more feature-rich command-line option parsing library. It provides a high-level API for defining commands, options, and actions, and it's widely used in the Node.js community. Compared to dashdash, commander offers a more expressive syntax and additional features like command-specific help, custom option processing, and subcommands.

yargs

Yargs is another popular command-line option parser that provides a fluent interface for building complex command-line applications. It supports features like command chaining, argument validation, and context-aware help menus. Yargs is more suitable for applications that require a rich command-line interface, while dashdash is simpler and more focused on option parsing.

minimist

Minimist is a minimalistic option parsing library that focuses on simplicity and performance. It does not have the feature set of dashdash, such as typed options or automatic help generation, but it is very lightweight and easy to use for basic argument parsing needs.

README

A light, featureful and explicit option parsing library for node.js.

Why another one? See below. tl;dr: The others I've tried are one of too loosey goosey (not explicit), too big/too many deps, or ill specified. YMMV.

Follow @trentmick for updates to node-dashdash.

Install

npm install dashdash

Usage

var dashdash = require('dashdash');

// Specify the options. Minimally `name` (or `names`) and `type`
// must be given for each.
var options = [
    {
        // `names` or a single `name`. First element is the `opts.KEY`.
        names: ['help', 'h'],
        // See "Option config" below for types.
        type: 'bool',
        help: 'Print this help and exit.'
    }
];

// Shortcut form. As called it infers `process.argv`. See below for
// the longer form to use methods like `.help()` on the Parser object.
var opts = dashdash.parse({options: options});

console.log("opts:", opts);
console.log("args:", opts._args);

Longer Example

A more realistic starter script "foo.js" is as follows. This also shows using parser.help() for formatted option help.

var dashdash = require('./lib/dashdash');

var options = [
    {
        name: 'version',
        type: 'bool',
        help: 'Print tool version and exit.'
    },
    {
        names: ['help', 'h'],
        type: 'bool',
        help: 'Print this help and exit.'
    },
    {
        names: ['verbose', 'v'],
        type: 'arrayOfBool',
        help: 'Verbose output. Use multiple times for more verbose.'
    },
    {
        names: ['file', 'f'],
        type: 'string',
        help: 'File to process',
        helpArg: 'FILE'
    }
];

var parser = dashdash.createParser({options: options});
try {
    var opts = parser.parse(process.argv);
} catch (e) {
    console.error('foo: error: %s', e.message);
    process.exit(1);
}

console.log("# opts:", opts);
console.log("# args:", opts._args);

// Use `parser.help()` for formatted options help.
if (opts.help) {
    var help = parser.help({includeEnv: true}).trimRight();
    console.log('usage: node foo.js [OPTIONS]\n'
                + 'options:\n'
                + help);
    process.exit(0);
}

// ...

Some example output from this script (foo.js):

$ node foo.js -h
# opts: { help: true,
  _order: [ { name: 'help', value: true, from: 'argv' } ],
  _args: [] }
# args: []
usage: node foo.js [OPTIONS]
options:
    --version             Print tool version and exit.
    -h, --help            Print this help and exit.
    -v, --verbose         Verbose output. Use multiple times for more verbose.
    -f FILE, --file=FILE  File to process

$ node foo.js -v
# opts: { verbose: [ true ],
  _order: [ { name: 'verbose', value: true, from: 'argv' } ],
  _args: [] }
# args: []

$ node foo.js --version arg1
# opts: { version: true,
  _order: [ { name: 'version', value: true, from: 'argv' } ],
  _args: [ 'arg1' ] }
# args: [ 'arg1' ]

$ node foo.js -f bar.txt
# opts: { file: 'bar.txt',
  _order: [ { name: 'file', value: 'bar.txt', from: 'argv' } ],
  _args: [] }
# args: []

$ node foo.js -vvv --file=blah
# opts: { verbose: [ true, true, true ],
  file: 'blah',
  _order:
   [ { name: 'verbose', value: true, from: 'argv' },
     { name: 'verbose', value: true, from: 'argv' },
     { name: 'verbose', value: true, from: 'argv' },
     { name: 'file', value: 'blah', from: 'argv' } ],
  _args: [] }
# args: []

Environment variable integration

If you want to allow environment variables to specify options to your tool, dashdash makes this easy. We can change the 'verbose' option in the example above to include an 'env' field:

    {
        names: ['verbose', 'v'],
        type: 'arrayOfBool',
        env: 'FOO_VERBOSE',         // <--- add this line
        help: 'Verbose output. Use multiple times for more verbose.'
    },

then the "FOO_VERBOSE" environment variable can be used to set this option:

$ FOO_VERBOSE=1 node foo.js
# opts: { verbose: [ true ],
  _order: [ { name: 'verbose', value: true, from: 'env' } ],
  _args: [] }
# args: []

Boolean options will interpret the empty string as unset, '0' as false and anything else as true.

$ FOO_VERBOSE= node examples/foo.js                 # not set
# opts: { _order: [], _args: [] }
# args: []

$ FOO_VERBOSE=0 node examples/foo.js                # '0' is false
# opts: { verbose: [ false ],
  _order: [ { key: 'verbose', value: false, from: 'env' } ],
  _args: [] }
# args: []

$ FOO_VERBOSE=1 node examples/foo.js                # true
# opts: { verbose: [ true ],
  _order: [ { key: 'verbose', value: true, from: 'env' } ],
  _args: [] }
# args: []

$ FOO_VERBOSE=boogabooga node examples/foo.js       # true
# opts: { verbose: [ true ],
  _order: [ { key: 'verbose', value: true, from: 'env' } ],
  _args: [] }
# args: []

Non-booleans can be used as well. Strings:

$ FOO_FILE=data.txt node examples/foo.js
# opts: { file: 'data.txt',
  _order: [ { key: 'file', value: 'data.txt', from: 'env' } ],
  _args: [] }
# args: []

Numbers:

$ FOO_TIMEOUT=5000 node examples/foo.js
# opts: { timeout: 5000,
  _order: [ { key: 'timeout', value: 5000, from: 'env' } ],
  _args: [] }
# args: []

$ FOO_TIMEOUT=blarg node examples/foo.js
foo: error: arg for "FOO_TIMEOUT" is not a positive integer: "blarg"

With the includeEnv: true config to parser.help() the environment variable can also be included in help output:

usage: node foo.js [OPTIONS]
options:
    --version             Print tool version and exit.
    -h, --help            Print this help and exit.
    -v, --verbose         Verbose output. Use multiple times for more verbose.
                          Environment: FOO_VERBOSE=1
    -f FILE, --file=FILE  File to process

Parser config

Parser construction (i.e. dashdash.createParser(CONFIG)) takes the following fields:

• options (Array of option specs). Required. See the Option specs section below.

• interspersed (Boolean). Option. Default is true. If true this allows interspersed arguments and options. I.e.:

    node ./tool.js -v arg1 arg2 -h   # '-h' is after interspersed args
  

  Set it to false to have '-h' not get parsed as an option in the above example.

• allowUnknown (Boolean). Option. Default is false. If false, this causes unknown arguments to throw an error. I.e.:

    node ./tool.js -v arg1 --afe8asefksjefhas
  

  Set it to true to treat the unknown option as a positional argument.

  Caveat: When a shortopt group, such as -xaz contains a mix of known and unknown options, the entire group is passed through unmolested as a positional argument.

  Consider if you have a known short option -a, and parse the following command line:

    node ./tool.js -xaz
  

  where -x and -z are unknown. There are multiple ways to interpret this:

  1. -x takes a value: {x: 'az'}
  2. -x and -z are both booleans: {x:true,a:true,z:true}

  Since dashdash does not know what -x and -z are, it can't know if you'd prefer to receive {a:true,_args:['-x','-z']} or {x:'az'}, or {_args:['-xaz']}. Because leaving the positional arg um-molested is the the easiest mistake for the user to recover from.

Option specs

Example using all fields:

{
    names: ['file', 'f'],       // Required (or `name`).
    type: 'string',             // Required.
    env: 'MYTOOL_FILE',
    help: 'Config file to load before running "mytool"',
    helpArg: 'PATH',
    default: path.resolve(process.env.HOME, '.mytoolrc')
}

Each option spec in the options array must/can have the following fields:

• name (String) or names (Array). Required. These give the option name and aliases. The first name (if more than one given) is the key for the parsed opts object.

• type (String). Required. One of:

  • bool
  • string
  • number
  • integer
  • positiveInteger
  • arrayOfBool
  • arrayOfString
  • arrayOfNumber
  • arrayOfInteger
  • arrayOfPositiveInteger

  FWIW, these names attempt to match with asserts on assert-plus.

• env (String or Array of String). Optional. An environment variable name (or names) that can be used as a fallback for this option. For example, given a "foo.js" like this:

    var options = [{names: ['dry-run', 'n'], env: 'FOO_DRY_RUN'}];
    var opts = dashdash.parse({options: options});
  

  Both node foo.js --dry-run and FOO_DRY_RUN=1 node foo.js would result in opts.dry_run = true.

  An environment variable is only used as a fallback, i.e. it is ignored if the associated option is given in argv.

• help (String). Optional. Used for parser.help() output.

• helpArg (String). Optional. Used in help output as the placeholder for the option argument, e.g. the "PATH" in:

    ...
    -f PATH, --file=PATH    File to process
    ...
  

• default. Optional. A default value used for this option, if the option isn't specified in argv.

Help config

The parser.help(...) function is configurable as follows:

Options:
    -w WEAPON, --weapon=WEAPON  Weapon with which to crush. One of: |
                                sword, spear, maul                  |
    -h, --help                  Print this help and exit.           |
^^^^                            ^                                   |
    `-- indent                   `-- helpCol              maxCol ---'

• indent (Number or String). Default 4. Set to a number (for that many spaces) or a string for the literal indent.
• nameSort (String). Default is 'length'. By default the names are sorted to put the short opts first (i.e. '-h, --help' preferred to '--help, -h'). Set to 'none' to not do this sorting.
• maxCol (Number). Default 80. Note that reflow is just done on whitespace so a long token in the option help can overflow maxCol.
• helpCol (Number). If not set a reasonable value will be determined between minHelpCol and maxHelpCol.
• minHelpCol (Number). Default 20.
• maxHelpCol (Number). Default 40.
• includeEnv (Boolean). Default false. If the option has associated environment variables (via the env option spec attribute), then append mentioned of those envvars to the help string.

Why

Why another node.js option parsing lib?

• nopt really is just for "tools like npm". Implicit opts (e.g. '--no-foo' works for every '--foo'). Can't disable abbreviated opts. Can't do multiple usages of same opt, e.g. '-vvv' (I think). Can't do grouped short opts.

• optimist has surprise interpretation of options (at least to me). Implicit opts mean ambiguities and poor error handling for fat-fingering. process.exit calls makes it hard to use as a libary.

• optparse Incomplete docs. Is this an attempted clone of Python's optparse. Not clear. Some divergence. parser.on("name", ...) API is weird.

• argparse Dep on underscore. No thanks just for option processing. find lib | wc -l -> 26. Overkill. Argparse is a bit different anyway. Not sure I want that.

• posix-getopt No type validation. Though that isn't a killer. AFAIK can't have a long opt without a short alias. I.e. no getopt_long semantics. Also, no whizbang features like generated help output.

License

MIT. See LICENSE.txt.

Changelog

1.4.0

• Known issues: #8

• [pull #2, pull #3] Add a allowUnknown: true option on createParser to allow unknown options to be passed through as opts._args instead of parsing throwing an exception (by https://github.com/isaacs).

  See 'allowUnknown' in the README for a subtle caveat.