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Flag handling library for node.js.
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This is a flags library for use with node.js. Flag definitions can be distributed across multiple files, as long as they are defined before flags.parse() is called.


var flags = require('flags');

flags.defineString('name', 'Billy Noone', 'Your name');
flags.defineInteger('age', 21, 'Your age in whole years');
flags.defineNumber('height', 1.80, 'Your height in meters');
flags.defineStringList('pets', []);
flags.defineMultiString('hobby', []);


// ====

var info = [];
info.push('Name : ' + flags.get('name'));
info.push('Age : ' + flags.get('age'));
info.push('Height : ' + flags.get('height') + '"');
info.push('Pets : ' + flags.get('pets').join(', '));
info.push('Hobbies : \n  ' + flags.get('hobby').join('\n  '));


Then on the command line:

node example.js --name='Your Name' --age 43  --height=1.234 --pets=fred,bob --hobby biking --hobby=snowboarding

Passing Flags

  • Flag names should be prefixed with two dashes: e.g. --flagname
  • Values can be separated from the name with either an equal sign or a space: e.g. --flagname=flagvalue or --flagname flagvalue
  • Complex string flags should be quoted: e.g. --flag="some flag with spaces"
  • Additional non-flag arguments can be passed by adding -- before the subsequent args. The remaining args will be returned from flags.parse() as an array, e.g. --one --two -- other stuff here

Defining Flags

To define flags, use one of the defineX functions exported by the flags module:

flags.defineString - Takes the raw input from the command line.

flags.defineBoolean - Usually doesn't take a value, passing --flag will set the corresponding flag to true. Also supported are --noflag to set it to false and --flag=true or --flag=false or --flag=0 or --flag=1 or --flag=f or --flag=t

flags.defineInteger - Must take a value and will be cast to a Number. Passing a non-integer arg will throw.

flags.defineNumber - Must take a value and will be cast to a number. Passing an arg that evaluates to NaN will throw.

flags.defineStringList - Takes a comma separated argument list and returns an array as it's value.

flags.defineMultiString - Same as defineString but allows multiple flags to be passed. All values will be returned in an array.

All the define methods take the same arguments:

flags.defineX(name, opt_default, opt_description);

name - The flag's name.
opt_default - [optional] The default value if not specified on the command line.
opt_description - [optional] Description to show in the help text.

The methods return a Flag object that exposes the following methods, for additional configuration:

flag.setDefault({*} defaultValue) - Sets the flag's default value.
flag.setDescription({string} description) - Sets the flag's description field.
flag.setValidator({function(string)} validator) - Sets a function for validating the input, should throw if the input isn't valid.
flag.setSecret({boolean} secret) - If set to true then the flag won't show up in the help text.

These setters return the flag instance so they can be chained:

    setDescription('A test flag').
    setValidator(function(inp) {
      if (inp.substr(0, 1) != 'e') {
        throw Error('Flag must start with an "e"');

Querying Flag Values

A flag's value can be queried by either calling flags.get('flagname') or by querying the flags object directly flags.FLAGS.flagname.get().

The flag object also contains the following properties you may be interested in:


By default flags.parse uses process.argv and slices off the first 2 elements. For tests you can pass a predefined set of arguments as an array:

flags.parse(['--flag', '--nofood', '--foo=bar']);

If you want to change flags between test cases, you may call:



  • Support --flagsfile
  • Support multi space separated flags, e.g. --files file1 file2 file3
  • Set up for npm install
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