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Light-weight fork of 'optimist' for option parsing.

optimist^H pessimist

This is trentm's fork of substack/node-optimist. Pessimist is a node.js library for option parsing for people who like optimist, but are a little retentive than substack. Basically I'm making some tweaks. Install it:

npm install pessimist


  • Don't add '[...]' markers for type, default, and required in option help strings. The self.favors object allows a way to get these back. TODO: expose an API to set those appropriately.

  • Hide "boolean option takes literal true/false argument" surprise behind favors.trueFalseBool.

  • Correct handling of 'string' options: they must have a provided argument. With the old behaviour you could get this: Say '-f' is type:string, then '-fvv' will return f:true. That's just wrong. That should be an error. Now it is an error.

  • Support for arrayOfBool option type to allow counting multiple uses of a boolean option. E.g.:

      var argv = require('pessimist').arrayOfBoolean('v').argv;

    results in:

      $ node foo.js
      { _: [], '$0': 'node ./foo.js' }
      $ node foo.js -v
      { _: [], '$0': 'node ./foo.js', v: [ true ] }
      $ node foo.js -vv
      { _: [], '$0': 'node ./foo.js', v: [ true, true ] }
  • Add .strict() to error on usage of undefined options:

      $ cat foo.js
      var argv = require('pessimist')
      $ node foo.js -v
      { _: [], '$0': 'node ./foo.js', v: true }
      $ node foo.js -x
      Error: illegal option: "x"


A pessmist Argv parser has a favors object. This is empty by default. Set the following keys to true to get the associated favor:

  • favors.trueFalseBool This will enable a "boolean" type option taking a string argument that is a literal "true" or "false". Personally this is surprising behaviour. If I have a '-v' boolean and my tool takes an argument that could be the string "false", then this won't do what you expect:

      node foo.js -v false   # 'v' will be false and args will be empty!
  • favors.helpRequired

  • favors.helpDefault

  • favors.helpType


Optimist is a node.js library for option parsing for people who hate option parsing. More specifically, this module is for people who like all the --bells and -whistlz of program usage but think optstrings are a waste of time.

With optimist, option parsing doesn't have to suck (as much).

build status


With Optimist, the options are just a hash! No optstrings attached.


#!/usr/bin/env node
var argv = require('optimist').argv;
if (argv.rif - 5 * argv.xup > 7.138) {
    console.log('Buy more riffiwobbles');
else {
    console.log('Sell the xupptumblers');

$ ./xup.js --rif=55 --xup=9.52
Buy more riffiwobbles

$ ./xup.js --rif 12 --xup 8.1
Sell the xupptumblers

This one's optimistic.

But wait! There's more! You can do short options:


#!/usr/bin/env node
var argv = require('optimist').argv;
console.log('(%d,%d)', argv.x, argv.y);

$ ./short.js -x 10 -y 21

And booleans, both long and short (and grouped):


#!/usr/bin/env node
var util = require('util');
var argv = require('optimist').argv;
if (argv.s) {
    util.print( ? 'Le chat dit: ' : 'The cat says: ');
    ( ? 'miaou' : 'meow') + (argv.p ? '.' : '')

$ ./bool.js -s
The cat says: meow

$ ./bool.js -sp
The cat says: meow.

$ ./bool.js -sp --fr
Le chat dit: miaou.

And non-hypenated options too! Just use argv._!


#!/usr/bin/env node
var argv = require('optimist').argv;
console.log('(%d,%d)', argv.x, argv.y);

$ ./nonopt.js -x 6.82 -y 3.35 moo
[ 'moo' ]

$ ./nonopt.js foo -x 0.54 bar -y 1.12 baz
[ 'foo', 'bar', 'baz' ]

Plus, Optimist comes with .usage() and .demand()!


#!/usr/bin/env node
var argv = require('optimist')
    .usage('Usage: $0 -x [num] -y [num]')
console.log(argv.x / argv.y);

$ ./divide.js -x 55 -y 11

$ node ./divide.js -x 4.91 -z 2.51
Usage: node ./divide.js -x [num] -y [num]

  -x  [required]
  -y  [required]

Missing required arguments: y



#!/usr/bin/env node
var argv = require('optimist')
    .default('x', 10)
    .default('y', 10)
console.log(argv.x + argv.y);

$ ./default_singles.js -x 5


#!/usr/bin/env node
var argv = require('optimist')
    .default({ x : 10, y : 10 })
console.log(argv.x + argv.y);

$ ./default_hash.js -y 7

And if you really want to get all descriptive about it...


#!/usr/bin/env node
var argv = require('optimist')

$ ./boolean_single.js -v foo bar baz
[ 'bar', 'baz', 'foo' ]


#!/usr/bin/env node
var argv = require('optimist')
console.dir([ argv.x, argv.y, argv.z ]);

$ ./boolean_double.js -x -z one two three
[ true, false, true ]
[ 'one', 'two', 'three' ]

Optimist is here to help...

You can describe parameters for help messages and set aliases. Optimist figures out how to format a handy help string automatically.


#!/usr/bin/env node
var argv = require('optimist')
    .usage('Count the lines in a file.\nUsage: $0')
    .alias('f', 'file')
    .describe('f', 'Load a file')
var fs = require('fs');
var s = fs.createReadStream(argv.file);
var lines = 0;
s.on('data', function (buf) {
    lines += buf.toString().match(/\n/g).length;
s.on('end', function () {

$ node line_count.js
Count the lines in a file.
Usage: node ./line_count.js

  -f, --file  Load a file  [required]

Missing required arguments: f

$ node line_count.js --file line_count.js

$ node line_count.js -f line_count.js


By itself,


will use process.argv array to construct the argv object.

You can pass in the process.argv yourself:

require('optimist')([ '-x', '1', '-y', '2' ]).argv

or use .parse() to do the same thing:

require('optimist').parse([ '-x', '1', '-y', '2' ])

The rest of these methods below come in just before the terminating .argv.

.alias(key, alias)

Set key names as equivalent such that updates to a key will propagate to aliases and vice-versa.

Optionally .alias() can take an object that maps keys to aliases.

.default(key, value)

Set argv[key] to value if no option was specified on process.argv.

Optionally .default() can take an object that maps keys to default values.


If key is a string, show the usage information and exit if key wasn't specified in process.argv.

If key is a number, demand at least as many non-option arguments, which show up in argv._.

If key is an Array, demand each element.

.describe(key, desc)

Describe a key for the generated usage information.

Optionally .describe() can take an object that maps keys to descriptions.

.options(key, opt)

Instead of chaining together .alias().demand().default(), you can specify keys in opt for each of the chainable methods.

For example:

var argv = require('optimist')
    .options('f', {
        alias : 'file',
        default : '/etc/passwd',

is the same as

var argv = require('optimist')
    .alias('f', 'file')
    .default('f', '/etc/passwd')

Optionally .options() can take an object that maps keys to opt parameters.


Set a usage message to show which commands to use. Inside message, the string $0 will get interpolated to the current script name or node command for the present script similar to how $0 works in bash or perl.


Check that certain conditions are met in the provided arguments.

If fn throws or returns false, show the thrown error, usage information, and exit.


Interpret key as a boolean. If a non-flag option follows key in process.argv, that string won't get set as the value of key.

If key never shows up as a flag in process.arguments, argv[key] will be false.

If key is an Array, interpret all the elements as booleans.


Tell the parser logic not to interpret key as a number or boolean. This can be useful if you need to preserve leading zeros in an input.

If key is an Array, interpret all the elements as strings.


Format usage output to wrap at columns many columns.


Return the generated usage string.


Print the usage data using fn for printing.


Parse args instead of process.argv. Returns the argv object.


Get the arguments as a plain old object.

Arguments without a corresponding flag show up in the argv._ array.

The script name or node command is available at argv.$0 similarly to how $0 works in bash or perl.

parsing tricks

stop parsing

Use -- to stop parsing flags and stuff the remainder into argv._.

$ node examples/reflect.js -a 1 -b 2 -- -c 3 -d 4
{ _: [ '-c', '3', '-d', '4' ],
  '$0': 'node ./examples/reflect.js',
  a: 1,
  b: 2 }

negate fields

If you want to explicity set a field to false instead of just leaving it undefined or to override a default you can do --no-key.

$ node examples/reflect.js -a --no-b
{ _: [],
  '$0': 'node ./examples/reflect.js',
  a: true,
  b: false }


Every argument that looks like a number (!isNaN(Number(arg))) is converted to one. This way you can just net.createConnection(argv.port) and you can add numbers out of argv with + without having that mean concatenation, which is super frustrating.


If you specify a flag multiple times it will get turned into an array containing all the values in order.

$ node examples/reflect.js -x 5 -x 8 -x 0
{ _: [],
  '$0': 'node ./examples/reflect.js',
    x: [ 5, 8, 0 ] }

dot notation

When you use dots (.s) in argument names, an implicit object path is assumed. This lets you organize arguments into nested objects.

 $ node examples/reflect.js --foo.quux=5
 { _: [],
   '$0': 'node ./examples/reflect.js',
     foo: { bar: { baz: 33 }, quux: 5 } }


With npm, just do: npm install optimist

or clone this project on github:

git clone

To run the tests with expresso, just do:


inspired By

This module is loosely inspired by Perl's Getopt::Casual.