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.

Install

npm install dashdash

Active Versions

• 2.x - Version 2.x
  dropped support for node versions earlier than node 10.x. While I don't
  expect to actively break functionality, new changes are no longer tested
  on earlier versions.
• 1.x - This version
  supports back to node version 0.10.x.

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 specs" 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('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: []

See the "examples" dir for a number of starter examples using
some of dashdash's features.

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

Bash completion

Dashdash provides a simple way to create a Bash completion file that you
can place in your "bash_completion.d" directory -- sometimes that is
"/usr/local/etc/bash_completion.d/"). Features:

• Support for short and long opts
• Support for knowing which options take arguments
• Support for subcommands (e.g. 'git log ' to show just options for the
  log subcommand). See
  node-cmdln for
  how to integrate that.
• Does the right thing with "--" to stop options.
• Custom optarg and arg types for custom completions.

Dashdash will return bash completion file content given a parser instance:

var parser = dashdash.createParser({options: options});
console.log( parser.bashCompletion({name: 'mycli'}) );

or directly from a options array of options specs:

var code = dashdash.bashCompletionFromOptions({
    name: 'mycli',
    options: OPTIONS
});

Write that content to "/usr/local/etc/bash_completion.d/mycli" and you will
have Bash completions for mycli. Alternatively you can write it to
any file (e.g. "~/.bashrc") and source it.

You could add a --completion hidden option to your tool that emits the
completion content and document for your users to call that to install
Bash completions.

See examples/ddcompletion.js for a complete
example, including how one can define bash functions for completion of custom
option types. Also see node-cmdln for
how it uses this for Bash completion for full multi-subcommand tools.

• TODO: document specExtra
• TODO: document includeHidden
• TODO: document custom types, function complete\_FOO guide, completionType
• TODO: document argtypes

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). Optional. 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). Optional. 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']}. Leaving the positional arg unprocessed
  is the easiest mistake for the user to recover from.

Option specs

Example using all fields (required fields are noted):

 {
    names: ['file', 'f'],       // Required (one of `names` or `name`).
    type: 'string',             // Required.
    completionType: 'filename',
    env: 'MYTOOL_FILE',
    help: 'Config file to load before running "mytool"',
    helpArg: 'PATH',
    helpWrap: false,
    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
  • date (epoch seconds, e.g. 1396031701, or ISO 8601 format
    YYYY-MM-DD[THH:MM:SS[.sss][Z]], e.g. "2014-03-28T18:35:01.489Z")
  • arrayOfBool
  • arrayOfString
  • arrayOfNumber
  • arrayOfInteger
  • arrayOfPositiveInteger
  • arrayOfDate

  FWIW, these names attempt to match with asserts on
  assert-plus.
  You can add your own custom option types with dashdash.addOptionType.
  See below.

• completionType (String). Optional. This is used for Bash
  completion for an option argument. If not specified,
  then the value of type is used. Any string may be specified, but only the
  following values have meaning:

  • none: Provide no completions.
  • file: Bash's default completion (i.e. complete -o default), which
    includes filenames.
  • Any string FOO for which a function complete_FOO Bash function is
    defined. This is for custom completions for a given tool. Typically
    these custom functions are provided in the specExtra argument to
    dashdash.bashCompletionFromOptions(). See
    "examples/ddcompletion.js" for an example.

• 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
    ...
  

• helpWrap (Boolean). Optional, default true. Set this to false to have
  that option's help not be text wrapped in <parser>.help() output.

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

• hidden (Boolean). Optional, default false. If true, help output will not
  include this option. See also the includeHidden option to
  bashCompletionFromOptions() for Bash completion.

Option group headings

You can add headings between option specs in the options array. To do so,
simply add an object with only a group property -- the string to print as
the heading for the subsequent options in the array. For example:

 var options = [
    {
        group: 'Armament Options'
    },
    {
        names: [ 'weapon', 'w' ],
        type: 'string'
    },
    {
        group: 'General Options'
    },
    {
        names: [ 'help', 'h' ],
        type: 'bool'
    }
];
...

Note: You can use an empty string, {group: ''}, to get a blank line in help
output between groups of options.

Help config

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

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

• indent (Number or String). Default 4. Set to a number (for that many
  spaces) or a string for the literal indent.
• headingIndent (Number or String). Default half length of indent. Set to
  a number (for that many spaces) or a string for the literal indent. This
  indent applies to group heading lines, between normal option lines.
• 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.
• helpWrap (Boolean). Default true. Set to false to have option help
  strings not be textwrapped to the helpCol..maxCol range.
• 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.
• includeDefault (Boolean). Default false. If the option has a default value
  (via the default option spec attribute, or a default on the option's type),
  then a "Default: VALUE" string will be appended to the help string.

Custom option types

Dashdash includes a good starter set of option types that it will parse for
you. However, you can add your own via:

var dashdash = require('dashdash');
dashdash.addOptionType({
    name: '...',
    takesArg: true,
    helpArg: '...',
    parseArg: function (option, optstr, arg) {
        ...
    },
    array: false,  // optional
    arrayFlatten: false,  // optional
    default: ...,   // optional
    completionType: ...  // optional
});

For example, a simple option type that accepts 'yes', 'y', 'no' or 'n' as
a boolean argument would look like:

var dashdash = require('dashdash');

function parseYesNo(option, optstr, arg) {
    var argLower = arg.toLowerCase()
    if (~['yes', 'y'].indexOf(argLower)) {
        return true;
    } else if (~['no', 'n'].indexOf(argLower)) {
        return false;
    } else {
        throw new Error(format(
            'arg for "%s" is not "yes" or "no": "%s"',
            optstr, arg));
    }
}

dashdash.addOptionType({
    name: 'yesno'
    takesArg: true,
    helpArg: '<yes|no>',
    parseArg: parseYesNo
});

var options = {
    {names: ['answer', 'a'], type: 'yesno'}
};
var opts = dashdash.parse({options: options});

See "examples/custom-option-*.js" for other examples.
See the addOptionType block comment in "lib/dashdash.js" for more details.
Please let me know with an
issue if you write a
generally useful one.

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 library.

• 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.

• "commander.js": I wrote
  a critique
  a while back. It seems fine, but last I checked had
  an outstanding bug
  that would prevent me from using it.

License

MIT. See LICENSE.txt.