0.9.4 • Public • Published


Optionator is a JavaScript/Node.js option parsing and help generation library used by eslint, Grasp, LiveScript, esmangle, escodegen, and many more.

For an online demo, check out the Grasp online demo.

About · Usage · Settings Format · Argument Format


The problem with other option parsers, such as yargs or minimist, is they just accept all input, valid or not. With Optionator, if you mistype an option, it will give you an error (with a suggestion for what you meant). If you give the wrong type of argument for an option, it will give you an error rather than supplying the wrong input to your application.

$ cmd --halp
Invalid option '--halp' - perhaps you meant '--help'?

$ cmd --count str
Invalid value for option 'count' - expected type Int, received value: str.

Other helpful features include reformatting the help text based on the size of the console, so that it fits even if the console is narrow, and accepting not just an array (eg. process.argv), but a string or object as well, making things like testing much easier.


Optionator uses type-check and levn behind the scenes to cast and verify input according the specified types.

MIT license. Version 0.9.4

npm install optionator

For updates on Optionator, follow me on twitter.

Optionator is a Node.js module, but can be used in the browser as well if packed with webpack/browserify.


require('optionator'); returns a function. It has one property, VERSION, the current version of the library as a string. This function is called with an object specifying your options and other information, see the settings format section. This in turn returns an object with three properties, parse, parseArgv, generateHelp, and generateHelpForOption, which are all functions.

var optionator = require('optionator')({
    prepend: 'Usage: cmd [options]',
    append: 'Version 1.0.0',
    options: [{
        option: 'help',
        alias: 'h',
        type: 'Boolean',
        description: 'displays help'
    }, {
        option: 'count',
        alias: 'c',
        type: 'Int',
        description: 'number of things',
        example: 'cmd --count 2'

var options = optionator.parseArgv(process.argv);
if (options.help) {

parse(input, parseOptions)

parse processes the input according to your settings, and returns an object with the results.

  • input - [String] | Object | String - the input you wish to parse
  • parseOptions - {slice: Int} - all options optional
    • slice specifies how much to slice away from the beginning if the input is an array or string - by default 0 for string, 2 for array (works with process.argv)

Object - the parsed options, each key is a camelCase version of the option name (specified in dash-case), and each value is the processed value for that option. Positional values are in an array under the _ key.

parse(['node', 't.js', '--count', '2', 'positional']); // {count: 2, _: ['positional']}
parse('--count 2 positional');                         // {count: 2, _: ['positional']}
parse({count: 2, _:['positional']});                   // {count: 2, _: ['positional']}


parseArgv works exactly like parse, but only for array input and it slices off the first two elements.

  • input - [String] - the input you wish to parse

See "returns" section in "parse"



generateHelp produces help text based on your settings.

  • helpOptions - {showHidden: Boolean, interpolate: Object} - all options optional
    • showHidden specifies whether to show options with hidden: true specified, by default it is false
    • interpolate specify data to be interpolated in prepend and append text, {{key}} is the format - eg. generateHelp({interpolate:{version: '0.4.2'}}), will change this append text: Version {{version}} to Version 0.4.2

String - the generated help text

generateHelp(); /*
"Usage: cmd [options] positional

  -h, --help       displays help
  -c, --count Int  number of things

Version  1.0.0


generateHelpForOption produces expanded help text for the specified with optionName option. If an example was specified for the option, it will be displayed, and if a longDescription was specified, it will display that instead of the description.

  • optionName - String - the name of the option to display

String - the generated help text for the option

generateHelpForOption('count'); /*
"-c, --count Int
description: number of things
example: cmd --count 2

Settings Format

When your require('optionator'), you get a function that takes in a settings object. This object has the type:

  prepend: String,
  append: String,
  options: [{heading: String} | {
    option: String,
    alias: [String] | String,
    type: String,
    enum: [String],
    default: String,
    restPositional: Boolean,
    required: Boolean,
    overrideRequired: Boolean,
    dependsOn: [String] | String,
    concatRepeatedArrays: Boolean | (Boolean, Object),
    mergeRepeatedObjects: Boolean,
    description: String,
    longDescription: String,
    example: [String] | String
  helpStyle: {
    aliasSeparator: String,
    typeSeparator: String,
    descriptionSeparator: String,
    initialIndent: Int,
    secondaryIndent: Int,
    maxPadFactor: Number
  mutuallyExclusive: [[String | [String]]],
  concatRepeatedArrays: Boolean | (Boolean, Object), // deprecated, set in defaults object
  mergeRepeatedObjects: Boolean, // deprecated, set in defaults object
  positionalAnywhere: Boolean,
  typeAliases: Object,
  defaults: Object

All of the properties are optional (the Maybe has been excluded for brevities sake), except for having either heading: String or option: String in each object in the options array.

Top Level Properties

  • prepend is an optional string to be placed before the options in the help text
  • append is an optional string to be placed after the options in the help text
  • options is a required array specifying your options and headings, the options and headings will be displayed in the order specified
  • helpStyle is an optional object which enables you to change the default appearance of some aspects of the help text
  • mutuallyExclusive is an optional array of arrays of either strings or arrays of strings. The top level array is a list of rules, each rule is a list of elements - each element can be either a string (the name of an option), or a list of strings (a group of option names) - there will be an error if more than one element is present
  • concatRepeatedArrays see description under the "Option Properties" heading - use at the top level is deprecated, if you want to set this for all options, use the defaults property
  • mergeRepeatedObjects see description under the "Option Properties" heading - use at the top level is deprecated, if you want to set this for all options, use the defaults property
  • positionalAnywhere is an optional boolean (defaults to true) - when true it allows positional arguments anywhere, when false, all arguments after the first positional one are taken to be positional as well, even if they look like a flag. For example, with positionalAnywhere: false, the arguments --flag --boom 12 --crack would have two positional arguments: 12 and --crack
  • typeAliases is an optional object, it allows you to set aliases for types, eg. {Path: 'String'} would allow you to use the type Path as an alias for the type String
  • defaults is an optional object following the option properties format, which specifies default values for all options. A default will be overridden if manually set. For example, you can do default: { type: "String" } to set the default type of all options to String, and then override that default in an individual option by setting the type property

Heading Properties

  • heading a required string, the name of the heading

Option Properties

  • option the required name of the option - use dash-case, without the leading dashes
  • alias is an optional string or array of strings which specify any aliases for the option
  • type is a required string in the type check format, this will be used to cast the inputted value and validate it
  • enum is an optional array of strings, each string will be parsed by levn - the argument value must be one of the resulting values - each potential value must validate against the specified type
  • default is a optional string, which will be parsed by levn and used as the default value if none is set - the value must validate against the specified type
  • restPositional is an optional boolean - if set to true, everything after the option will be taken to be a positional argument, even if it looks like a named argument
  • required is an optional boolean - if set to true, the option parsing will fail if the option is not defined
  • overrideRequired is a optional boolean - if set to true and the option is used, and there is another option which is required but not set, it will override the need for the required option and there will be no error - this is useful if you have required options and want to use --help or --version flags
  • concatRepeatedArrays is an optional boolean or tuple with boolean and options object (defaults to false) - when set to true and an option contains an array value and is repeated, the subsequent values for the flag will be appended rather than overwriting the original value - eg. option g of type [String]: -g a -g b -g c,d will result in ['a','b','c','d']

You can supply an options object by giving the following value: [true, options]. The one currently supported option is oneValuePerFlag, this only allows one array value per flag. This is useful if your potential values contain a comma.

  • mergeRepeatedObjects is an optional boolean (defaults to false) - when set to true and an option contains an object value and is repeated, the subsequent values for the flag will be merged rather than overwriting the original value - eg. option g of type Object: -g a:1 -g b:2 -g c:3,d:4 will result in {a: 1, b: 2, c: 3, d: 4}
  • dependsOn is an optional string or array of strings - if simply a string (the name of another option), it will make sure that that other option is set, if an array of strings, depending on whether 'and' or 'or' is first, it will either check whether all (['and', 'option-a', 'option-b']), or at least one (['or', 'option-a', 'option-b']) other options are set
  • description is an optional string, which will be displayed next to the option in the help text
  • longDescription is an optional string, it will be displayed instead of the description when generateHelpForOption is used
  • example is an optional string or array of strings with example(s) for the option - these will be displayed when generateHelpForOption is used

Help Style Properties

  • aliasSeparator is an optional string, separates multiple names from each other - default: ' ,'
  • typeSeparator is an optional string, separates the type from the names - default: ' '
  • descriptionSeparator is an optional string , separates the description from the padded name and type - default: ' '
  • initialIndent is an optional int - the amount of indent for options - default: 2
  • secondaryIndent is an optional int - the amount of indent if wrapped fully (in addition to the initial indent) - default: 4
  • maxPadFactor is an optional number - affects the default level of padding for the names/type, it is multiplied by the average of the length of the names/type - default: 1.5

Argument Format

At the highest level there are two types of arguments: named, and positional.

Name arguments of any length are prefixed with -- (eg. --go), and those of one character may be prefixed with either -- or - (eg. -g).

There are two types of named arguments: boolean flags (eg. --problemo, -p) which take no value and result in a true if they are present, the falsey undefined if they are not present, or false if present and explicitly prefixed with no (eg. --no-problemo). Named arguments with values (eg. --tseries 800, -t 800) are the other type. If the option has a type Boolean it will automatically be made into a boolean flag. Any other type results in a named argument that takes a value.

For more information about how to properly set types to get the value you want, take a look at the type check and levn pages.

You can group single character arguments that use a single -, however all except the last must be boolean flags (which take no value). The last may be a boolean flag, or an argument which takes a value - eg. -ba 2 is equivalent to -b -a 2.

Positional arguments are all those values which do not fall under the above - they can be anywhere, not just at the end. For example, in cmd -b one -a 2 two where b is a boolean flag, and a has the type Number, there are two positional arguments, one and two.

Everything after an -- is positional, even if it looks like a named argument.

You may optionally use = to separate option names from values, for example: --count=2.

If you specify the option NUM, then any argument using a single - followed by a number will be valid and will set the value of NUM. Eg. -2 will be parsed into NUM: 2.

If duplicate named arguments are present, the last one will be taken.

Technical About

optionator is written in LiveScript - a language that compiles to JavaScript. It uses levn to cast arguments to their specified type, and uses type-check to validate values. It also uses the prelude.ls library.

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