A light, featureful and explicit option parsing library.

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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.
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.
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);

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);

// ...

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]
    --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',
   [ { 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 "FOOVERBOSE" 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: []

$ 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]
    --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 "bashcompletion.d" directory -- sometimes that is "/usr/local/etc/bashcompletion.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/bashcompletion.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"](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:
  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');
    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));

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