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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.


Fork the latest source from github, or else use NPM:

npm install flags


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
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