1.0.4 • Public • Published


    Command-line option parser

    NPM version Build Status

    NPM installation

    Inspired by the extremly well-known argp C library, this module parses GNU-style command-line options. Help, usage and version messages are automatically generated and line-wrapped at 80 columns. The module checks for errors, can be easily adapted to your needs thanks to its evented system and it also works when Node.js is in debug mode. The module is uncached and each property is deleted once all the input parameters have been parsed, so there's no memory footprint.

    This module it's made for you if you want:

    • Robust solution that reads GNU-style options.
    • Command configuration.
    • Basic error checking.
    • Nice help messages without caring about indentation, multilines, etc.
    • Zero memory footprint.

    A common configuration looks like this:

    //If you're not going to use multiple parser instances or you don't need to
    //reuse a parser, don't cache the module, this will guarantee a zero memory
    var argv = require ("argp").createParser ({ once: true })
        .description ("Sample app.")
        .email ("a@b.c")
        .body ()
            //The object and argument definitions and the text of the --help message
            //are configured at the same time
            .text (" Arguments:")
            .argument ("arg", { description: "Sample argument" })
            .text ("\n Options:")
            .option ({ short: "o", long: "opt", description: "Sample option" })
            .help ()
            .version ("v1.2.3")
        .argv ();
    console.log (argv);
    $ node script.js
      opt: false,
      help: false,
      version: false,
      arg: false
    $ node script.js --help
    Usage: script [options] [arguments]
    Sample app.
      arg                         Sample argument
      -o, --opt                   Sample option
      -h, --help                  Display this help message and exit
      -v, --version               Output version information and exit
    Report bugs to <a@b.c>.

    If you have a package.json file you can take from it the description, email and version using the readPackage() function. Take into account that this function calls a synchronous fs operation. Doesn't really matter because this module is one of the first things you're going to execute in your program.

    var argv = require ("argp")
        //If no path is provided, it tries to read the "./package.json" file
        .readPackage ("path/to/package.json")
        .body ()
            .text (" Arguments:")
            .argument ("arg", { description: "Sample argument" })
            .text ("\n Options:")
            .option ({ short: "o", long: "opt", description: "Sample option" })
            .help ()
        .argv ();




    What's new in v1?

    Two things will break your code:

    • Parser instances are introduced. To create one you need to call to createParser(). In most cases you only need to append .createParser ({ once: true }), for example:

      var argv = require ("argp").createParser ({ once: true })
          argv ();

      The once option will uncache the module when argv() is called.

    • The second argument from the end event is removed.


      .on ("end", function (argv, fns){
        fns.printHelp ();
        fns.printUsage ();
        fns.printVersion (); ();


      .on ("end", function (argv){
        this.printHelp ();
        this.printUsage ();
        this.printVersion (); ();

    Quick examples with no configuration

    If an option has not been defined the type of the value is converted automatically from a string to a number, boolean, null or undefined.

    By default the parser doesn't allow undefined arguments and options because you typically want to have an absolute control over the calls to your program in order to avoid unexpected behaviours. Allowing undefined arguments and options is as simple as this:

    var argv = require ("argp").createParser ({ once: true })
        .allowUndefinedArguments ()
        .allowUndefinedOptions ()
        .argv ();
    $ node script.js -a -b -c
    { a: true, b: true, c: true }
    $ node script.js a b c
    { a: true, b: true, c: true }
    $ node script.js -abc null
    { a: true, b: true, c: null }
    $ node script.js --a --b 1 --d=e
    { a: true, b: 1, d: "e" }
    $ node script.js --no-a b
    { a: false, b: true }
    $ node script.js --a -- -b --c d
    { a: true"-b"true"--c"true, d: true }

    Configuring options

    Example: options.js.


    1. By default the options are flags. If the option requires a value, the metavar property must be defined. This property is a string and can be seen when the --help and --usage messages are printed.
    $ node script.js --help
      o, --opt=STR               Sample option

    Where STR is the metavar property.

    1. By default, the value of the options is a string. Configure the type property if the value is a number, boolean (rarely used, use a flag instead) or array (comma-separated values and multiple assignments).
    2. Each option has an id which is used to store the value in the final object. This id is the long name. If the long name has not been defined then the id is the short name.
    .option ({ short: "a", long: "aa" })
    //{ aa: false }
    .option ({ long: "aa" })
    //{ aa: false }
    .option ({ short: "a" })
    //{ a: false }
    1. Mandatory options (aka required) are not implemented because options are optional. Use a command if you need mandatory parameters.

    Common properties between flags and options with a value:

    • description - String
      The description.
    • hidden - Boolean
      If true, the option is not displayed in the --help and --usage messages. Default is false.
    • long - String
      The long name. Cannot contain white spaces.
    • short - String
      The short name. It must be an alphanumeric character.


    • negate - Boolean
      If true, the flag is negated. The default value is true and it becomes false when the short name or the negated long name (eg. --no-flag) is present.

      .option ({ short: "a", long: "aaa" })
      .option ({ short: "b", long: "bbb", negate: true })
      $ node script.js
      { aaa: false, bbb: true }
      $ node script.js -a -b
      { aaa: true, bbb: false }
      $ node script.js --aaa --bbb
      { aaa: true, bbb: true }
      $ node script.js --no-aaa --no-bbb
      { aaa: false, bbb: false }

    Options with a value:

    • aliases - Array
      An alias it's a long-name option that points to another option.

      .body ()
          .option ({ long: "name", aliases: ["foo", "bar"] })
          .help ()
          .usage ()
      $ node script.js --foo
      { name: true }
      $ node script.js --usage
      Usage: script [--name|--foo|--bar] [-h|--help] [--usage]
      $ node script.js --help
      Usage: script [options]
            --name, --foo, --bar
        -h, --help                  Display this help message and exit
            --usage                 Display a short usage message and exit

      The options() function returns an object with all the configured options:

        name: { ... },
        foo: { ... },
        bar: { ... },
        help: { ... },
        usage: { ... }

      Where name, foo and bar point to the same object:

      .on ("start", function (){
        var options = this.options ();
        assert.ok ( === && ===;
    • choices - Array
      The input value must be one of these choices. If the option is optional, the choices property is ignored.

      .option ({ long: "opt", metavar: "NUM", type: Number, choices: [1, 2, 3] })
      $ node script.js --opt=1
      { opt: 1 }
      $ node script.js --opt=7 # Error! 

      When default and choices are defined in the same option the default value doesn't need to match any choice:

      .option ({ long: "opt", metavar: "STR", default: "d", choices: ["a", "b", "c"] })
      $ node script.js
      { opt: "d" }
    • default - Object
      The default value.

      .option ({ long: "name", metavar: "STR", default: "bar", optional: true })
      $ node script.js
      { name: "bar" }
      $ node script.js --name
      { name: "bar" }
      $ node script.js --name foo
      { name: "foo" }
    • metavar - String
      Must be defined if the option requires a value. If metavar is not defined, the option is a flag. The string is used when the --help and --usage messages are printed.

      .option ({ long: "name", metavar: "STR" })
      $ node script.js --help
    • optional - Boolean
      If true, the value is optional. Default is false. If the option doesn't receive any value the default value is set and it depends on the default and type properties.

      Types and default values:

      • String: null
      • Number: 0
      • Array: []
      • Boolean: false
      .option ({ long: "name1", metavar: "STR", optional: true })
      .option ({ long: "name2", metavar: "STR", optional: true, type: String })
      .option ({ long: "name3", metavar: "NUM", optional: true, type: Number })
      .option ({ long: "name4", metavar: "ARR", optional: true, type: Array })
      //Boolean type is rarely used, use a flag instead
      .option ({ long: "name5", metavar: "BOOL", optional: true, type: Boolean })
      $ node script.js --name1 --name2 --name3 --name4 --name5
      { name1: null, name2: null, name3: 0, name4: [], name5: false }
      $ node script.js --name1 foo --name2 bar --name3 12 --name4 -12.34,foo,true --name4 false --name5 true
      { name1: "foo", ame2: "bar", name3: 12, name4: [-12.34, "foo"truefalse], name5: true }
    • reviver - Function
      The function is executed when the option is parsed. It is similar to the reviver parameter of the JSON.parse() function. This is the right place where you can validate the input data and fail() if it's not valid. For example, if the option requires a number you can validate the range here.

      .option ({ long: "name", metavar: "STR", reviver: function (value){
        return value + "bar";
      $ node script.js --name foo
      { name: "foobar" }
      .option ({ long: "opt", metavar: "NUM", type: Number,
          reviver: function (value){
        //The "value" parameter is already a number
        if (value < 1 || value > 3){
 ("Option 'opt': Invalid range.");
        return value;
    • type - String | Number | Boolean | Array
      The type of the value. Default is String.

      If the type is an Array, comma-separated values are automatically stored in an array and each element is converted to the type it represents. Multiple assignments are also supported.

      .option ({ long: "name", metavar: "ARR", type: Array })
      $ node script.js --name 1,true,foo
      { name: [1, true"foo"] }
      $ node script.js --name 1 --name true --name foo
      { name: [1, true"foo"] }
      $ node script.js --name 1,2 --name true,false --name foo,bar
      { name: [1, 2, truefalse"foo""bar"] }

    Configuring arguments

    Example: arguments.js.

    An argument is an individual name like login, reset, build, etc. They are basically flags.


    • description - String
      The description.
    • hidden - Boolean
      If true, the option is not displayed in the --help and --usage messages. Default is false.

    Note: synopsis and trailing properties can be also configured but they have meaning only with commands.

    .argument ("arg1")
    .argument ("arg2", { description: "foo" })
    .argument ("arg3", { description: "bar", hidden: true })
    $ node script.js arg1
    { arg1: true, arg2: false, arg3: false }
    $ node script.js --help
      arg1                        foo
      arg2                        bar

    Note that an argument is just a flag but it can be present without any prefixed hyphen. It is a flag with more weight, it is used to denote important actions.

    As you can see, the arguments are also stored in a hash like regular options, undefined arguments inclusive. For example:

    var argv = require ("argp").createParser ({ once: true })
        .allowUndefinedArguments ()
        .allowUndefinedOptions ()
        .argv ();
    console.log (argv);
    $ node script.js a b c
    { a: true, b: true, c: true }

    This feature is different from other cli parsers that store the arguments in an array. If you need to read undefined arguments and save them in an array you can use the events. For more details: to-upper-case.js

    Configuring commands

    A command is an argument followed by other arguments and options. NPM is an example:

    npm config set <key> [<value>]
    npm install [<package>...] -g

    config is a command and set an argument with 2 trailing arguments: minimum 1, maximum 2.
    install is a command with infinite trailing arguments: minimum 0, maximum Infinity. -g is an option which only applies to the install command.

    If you have a very few commands you can configure them in the same file (commands.js example), but you typically want to modularize them, one command per file. Then you should check the npm.js example.

    The commands are configured exactly the same way as the Argp instance with only one difference: argument() accepts 2 new properties:

    • synopsis - String
      The string replaces the argument name in the --help and --usage messages.

      .argument ("set", { description: "Sample argument" });
          set                         Sample argument
      .argument ("set", { synopsis: "set <key> [<value>]", description: "Sample argument" });
          set <key> [<value>]         Sample argument
    • trailing - Object
      Configures how many trailing arguments must follow this argument.

      There are 3 properties: eq, min and max. eq cannot be used with min or max. If min and max are used, by default min is 0 and max is Infinity. A trailing object without any of these 3 properties defaults to min 0 and max Infinity.

      Some examples:

      • 2 trailing arguments required: cmd arg <x> <x>.

        .argument ("arg", { trailing: { eq: 2 } })
      • 1 required, 1 optional: cmd arg <x> [<x>].

        .argument ("arg", { trailing: { min 1, max: 2 } })
      • 1 optional: cmd arg [<x>].

        .argument ("arg", { trailing: { max: 1 } })
      • 1 required, infinite optional: cmd arg <x> [<x>...].

        .argument ("arg", { trailing: { min: 1 } })
      • Infinite: cmd arg [<x>...].

        .argument ("arg", { trailing: {} })
        //Same as trailing: { min: 0, max: Infinity }
      • No trailing arguments: cmd arg.

        .argument ("arg")
      • Multiple arguments with trailing in the same line. Argument arg1 with 1 required, and argument arg2 with infinite trailing arguments: cmd arg1 <x> arg2 [<y>...]. Note that writing cmd arg1 <x> arg2 [<y>...] is not the same as cmd arg2 [<y>...] arg1 <x>. In the latter case, arg1 <x> will be eaten by the trailing arguments of arg2.

         .argument ("arg1", { trailing: { eq: 1 } })
         .argument ("arg2", { trailing: {} })

    Real examples

    • brainfuck: Interpreter for the Brainfuck esoteric language.
    • ntftp: Streaming TFTP client and server.

    module.createParser([options]) : Argp

    Returns a new Argp instance.


    • once - Boolean
      Set it to true if you want to uncache the whole module when argv() finishes. This will guarantee a zero memory footprint. Default is false.


    The module returns an instance of Argp. It inherits from an EventEmitter.

    The parser follows the GNU-style rules: -a, -abc, --a, --no-a, --a=b, --, etc. Long-option abbreviation is also supported.

    If you don't want to configure anything simply require the module, allow undefined arguments and options and call to argv().

    var argv = require ("argp").createParser ({ once: true })
        .allowUndefinedArguments ()
        .allowUndefinedOptions ()
        .argv ();

    Note: If no configuration is provided you cannot join a short name with its value in the same token, eg: -Dvalue, all the characters following a hyhen are interpreted as individual flags.


    With the event system you can fully adapt this module to yours needs. Examples: to-upper-case.js, mkdir.js.




    Emitted when an argument is found.


    • argv - Object
      The final object.
    • argument - String
      The name of the argument.
    • ignore - Function
      When the function is called the parser ignores the argument, hence it isn't stored in the final object.


    Emitted when all the options and arguments have been parsed.


    • argv - Object
      The final object.


    Emitted when an error occurs. If you don't listen for error events, the message will be printed to stderr and the process will exit. If you attach an error handler and an error occurs, argv() returns null.

    You typically want to listen for error events when you need to do something with the error and then quit the process, or simply because you want to continue executing the process (maybe because you are executing a shell prompt) and display the error in the console.

    .on ("error", function (error){
      doSomethingWith (error);
      //This will end the process (error);

    For example, the ntftp module uses two parsers. One is the main parser. If something is not correct, it prints the error to stderr and finishes. The second parser is used when the program is executing a shell prompt. It is being reused and the errors are simply printed to console.


    Emitted when an option is found.


    • argv - Object
      The final object.
    • option - String
      The name of the option.
    • value - String
      The value of the option after calling the reviver, if any.
    • long - Boolean
      True if the option is a long name, otherwise false.
    • ignore - Function
      When the function is called the parser ignores the argument, hence it isn't stored in the final object.


    Emitted just before the parser begins to read the cli parameters.


    • argv - Object
      The final object. The default values are already set.

    Argp#allowUndefinedArguments() : Argp

    Allows undefined arguments.

    Argp#allowUndefinedOptions() : Argp

    Allows undefined options.

    Argp#arguments() : Object

    Returns the configured arguments. Look at the internal-data.js example for further details.

    Argp#argv([input]) : Object | null

    Parses the process.argv array or the given input array.

    If you don't need to reuse the parser and want a zero memory footprint, you shouldn't cache the module and the parser instance. Remember to also set once to true.

    var argv = require ("argp").createParser ({ once: true })
        argv ();

    If you need to reuse a parser, you probably want to listen for error events. If an error occurs, argv() returns null.

    var argp = require ("argp");
    var parser = argp.createParser ()
        .on ("error", ...)
    var argv;
    argv = parser.argv (["-a", "--b", ...]);
    argv = parser.argv (["c", "d", ...]);

    If you pass an input array, then the functions printHelp(), printUsage() and printVersion() will print the message but won't terminate the process.

    Argp#body() : Argp

    Returns a Body instance.

    Argp#columns(columns) : Argp

    Sets the maximum line length. By default lines are wrapped at 80 columns.

    Argp#command(name[, configuration]) : Command

    Configures a command. A command it's like a new fresh cli program. It behaves exactly like an Argp. See Configuring commands.

    Argp#commands() : Object

    Returns the configured commands.

    Look at the internal-data.js example for further details.

    Argp#description(str) : Argp

    Sets a description. The description is printed at the start of the --help message, after the "Usage" lines.

    Argp#email(str) : Argp

    Sets a contact email. The email is printed at the end of the --help message.

    Argp#exitStatus(code) : Argp

    Sets the exit code that will be used is some methods. Default is 1.

    Argp#fail(str[, code]) : undefined

    Prints a message to the stderr and exits with the given code number or if not given, uses the code configured with exitStatus() or if not configured, exits with code 1.

    Argp#main() : Argp

    Returns de main Argp instance. It's a no-op function, just for a better visual organization when configuring commands.

    Look at the npm.js example for further details.

    Argp#options([filter]) : Object

    Returns the configured options. filter is an object which can be used to return the options that have a short name or a long name.

    .options ()
    .options ({ short: true })
    .options ({ long: true })

    Look at the internal-data.js example for further details.

    Argp#printHelp([code]) : undefined

    Prints the help message and exits with the given code number or if not given, uses the code configured with exitStatus() or if not configured, exits with code 0.

    If argv() is called with an input array, the process doesn't exit.

    Argp#printUsage([code]) : undefined

    Prints the usage message and exits with the given code number or if not given, uses the code configured with exitStatus() or if not configured, exits with code 0.

    If argv() is called with an input array, the process doesn't exit.

    Argp#printVersion([code]) : undefined

    Prints the version message (if it was configured) and exits with the given code number or if not given, uses the code configured with exitStatus() or if not configured, exits with code 0.

    If argv() is called with an input array, the process doesn't exit.

    Argp#readPackage([path][, options]) : Argp

    Reads a package.json file and configures the parser with the description, email and version. If no path is provided it tries to read the ./package.json path. The description, email and version labels can be ignored individually with the options parameter. For example, if you only want to ignore the email and read the description and version:

    .readPackage ("path/to/package.json", { email: false })

    Options are: description, version, email. By default they are true. Set them to false to ignore them.

    Argp#usages(usages) : Argp

    Changes the "Usage" line from the --help and --usage messages. usages is an array of strings.

    Look at the custom-usages.js example for further details.

    Argp#sort() : Argp

    If sort() is enabled, the options are parsed before the arguments, if not, the options and arguments are parsed in the same order they come.


    The Body instance is returned by calling Argp#body(). All the following functions (except argv(), command() and main()) print a message in the same order they are configured, this allows you to fully customize the --help message very easily.

    Look at fully-descriptive-help.js for further details.


    Body#argument(name[, configuration]) : Body

    Defines an argument. See Configuring arguments.

    Body#argv([input]) : Object

    Same as Argp#argv().

    Body#columns(column1, column2) : Body

    Prints a line with 2 columns. The first column shouldn't contain line breaks (\n). This functionality is used to print the options and arguments.

    Body#command(name[, configuration]) : Command

    Same as Argp#command().

    Body#help([options]) : Body

    Enables the -h, --help option. The short option can be disabled using the options parameter.

    .help ({ short: false })
    $ node script.js --help
          --help                  Display this help message and exit

    Body#main() : Argp

    Same as Argp#main().

    Body#option(o) : Body

    Defines an option. See Configuring options.

    Body#text(str[, prefix]) : Body

    Prints a text message. By default it's line-wrapped at 80 columns and supports multilines (line breaks, \n). The prefix is a string that is printed before each line. It's mainly used to indent the text with some spaces.

    Body#usage() : Body

    Enables the --usage option.

    Body#version(str[, options]) : Body

    Enables the -v, --version option. str is the text to print when the option is called. The short option can be disabled using the options parameter.

    .version ("v1.2.3", { short: false })
    $ node script.js --help
          --version               Output version information and exit


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