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Javascript arity simplified.


The ability to pass a variable number of parameters to functions is a nice feature of javascript, but it can sometimes lead to boilerplate code at the top of a method to determine which parameters you're actually working with. Something like

function (url, params, options, callback) {
  if (typeof params === 'function') {
    callback = params, params = {}, options = {};
  } else if (typeof options === 'function') {
    callback = options, options = {};
  // . . . now for actual function logic

Varity ("variable-arity") handles this for you. And it can do some other handy things to.


npm install varity --save

Basic Use

When you require('varity'), you'll get a wrapper function back. This wrapper accepts a set of expectations and the function to wrap, then handles boilerplate arity shenanigans for you. If you tell varity you're expecting a string and a number, but only pass it a number, varity will fill in the string argument with undefined. No need to test if (typeof options === 'function'). There are several ways to tell varity what you're expecting.

With types

var $ = require('varity');
var wrapped = $(Array, Function, function(list, callback) {
  // list is guaranteed to be an array or undefined
  // callback will always be a function or undefined
// For example
wrapped([1, 2, 3]); // the function will be called with [1, 2, 3] and undefined
// or
wrapped(function() {}); // undefined, function() {}
// any additional params will also be passed on
wrapped([1, 2, 3], function() {}, { foo: 'bar' }) // [1, 2, 3], function() {}, { foo: 'bar' }
// in fact, varity is smart enough to handle this:
wrapped([1, 2, 3], { foo: 'bar' }) // [1, 2, 3], undefined, { foo: 'bar' }
// and even
wrapped({ foo: 'bar'}) // undefined, undefined, { foo: 'bar'}

You can pass any object type (use null for Null and undefined for Undefined - though there's not much value in expecting these in functions). You can also pass custom types.

function Foo () {}
var wrapped = $(Foo, function(foo) {
  // We have a foo!

With an array

var wrapped = $(['String', 'Function'], function(name, callback) {
  // . . .

This let's you compile argument lists on the fly if necessary.

var args = ['String'];
if (opts.async) {
var wrapped = $.apply($, [args, function(path, cb){
  // Do neat stuff

With a string

The first two methods are useful, but also lengthy. The real value in varity is in string abbreviations. To keep calls to varity short, all built in types have one character analogs that can be passed collectively as a single string:

var wrapped = $('ssf', function(fname, lname, callback) {
  // . . . 

The following abbreviations are currently recognized by varity:

  • s: String
  • f: Function
  • o: Object
  • a: Array
  • 1: Number
  • b: Boolean
  • r: RegExp
  • d: Date
  • N: NaN
  • n: Null
  • u: Undefined
  • A: Arguments
  • i: Infinity
  • e: Error
  • E: Element
  • $: jQuery

Additionally, strings can have array wrappers ([s]) and ors (|). Any letter wrapped in [] tells varity to wrap the arg (if it matches that type) in an array. | says that either of two types are acceptable. These can also be combined.

var wrapped = $('[s]', function(list) {
  // When called with a string, arguments will be an array with that string as the first and only item
var wrapped = $('1|b', function(isTruthy) {
  // Either a number or boolean can be passed
var wrapped = $('[s]|a', function(list) {
  // Either a string (which will be wrapped) or an array can be passed
var wrapped = $('[s]|[1]', function(list) {
  // Either a string or number can be passed, but in either case, it will be wrapped as an array


Knowing that options will never be a function is nice, but you might still need to check for definedness before doing something with a parameter:

var wrapped = $('af', function(list, cb) {
  cb(list.concat(['foo', 'bar']);
wrapped(function(newList) { /* . . . */ }); // Uh, oh. Cannot call method concat of undefined.

Thus, varity has some flags, indicated with symbols, that tell it to handle awkward conditions, such as the above, gracefully.

Populate: +

Tells varity to return a default of the given type so that you don't have to worry about calling type specific methods.

var wrapped = $('+a', function(list) {
  list.push('something'); // list will ALWAYS be an array

The built in defaults are as follows (though you can override them - more on that later).

  • String
  • Function
  • Object
  • Array
  • Number
  • Boolean
  • RegExp
  • Date
(function() {
  return new Date();
  • NaN
  • Null
  • Undefined
  • Arguments:
  return arguments;
  • Infinity
2/0 // because I like the number 2
  • Error:
(function() {
  return new Error();
  • Element:
(function() {
  if (typeof window !== 'undefined') {
    return window.document;
  } else {
    return '<div></div>';
  • jQuery:
(function() {
  if (typeof $ !== 'undefined') {
    return $(document);
  } else {
    return [];

Optional: -

Normally, if you pass two of the same type next to each other, Varity will assign the first parameter that matches that type to the first argument and leave the second undefined.

var wrapped = $('oo', function(options, data) {
  // If only one object is passed, it will be passed as "options" and "data" will be undefined

The optional flag reverses this behavior. (Alternatively, you could just reverse the parameters: function(data, options).)

var wrapped = $('-oo', function(options, data) {
  // Now if only one object is passed, it'll be set to "data", leaving "options" undefined.

You can also combine populate with optional:

var wrapped = $('-+oo', function(obj1, obj2) {
  // If passed only {foo: 'bar'}, arguments will be {}, {foo: 'bar'}

Non-empty: _

The non-empty flag tells varity to treat "empty" parameters as if they were undefined. This isn't that useful unless you change the defaults (more below). If you tell varity to populate a type with some other default and use the _ flag, varity will replace an empty type (e.g. {}, [], function() {}, etc.) with the default of that type.

$.populate('Object', {
  dataType: 'json',
  method: 'put'
var wrapped = $('s_o', function(url, opts) {
  // see below for more about populate
wrapped('', {}); // Provides '' and { dataType: 'json', method: 'put' }

Required: *

Marks a parameter as required. If that parameter is not passed, Varity will throw an exception.

var wrapped = $('*so', function(name, options) {
  // . . .
wrapped({ async: true }); // throws

Extend: &

For matching types, the actual argument will be extended with additional defaults. For objects, this means a deep object extend; for arrays, concat; for strings, joining with " "; and for functions, calling _.compose (with the default function coming first in the composition). This is an excellent way to handle option extension.

$.extend('Object', { async: true }); // See "Helpers" below
var wrapped = $('&o', function(options) { });
wrapped({ path: '/foo/bar' }); // options will equal { async: true, path: '/foo/bar' }
$.extend('Array', [ 'Chuck' ]);
var wrapped = $('&a', function(ppl) { });
wrapped([ 'Sue', 'Douglas' ]); // ppl will equal [ 'Sue', 'Douglas', 'Chuck' ]
$.extend('String', 'Please try again.');
var wrapped = $('&s', function(message) { });
wrapped('We were unable to update your profile.'); // message will equal 'We were unable to update your profile. Please try again.'
$.extend('Function', function(name) { return 'My name is ' + name; });
var wrapped = $('&f', function(fn) { });
wrapped(function(message) { console.log(message); }); // when "fn" is called with "Tim", "My name is Tim" will be logged


Varity also a couple helper methods for changing options.


Use varity.configure for one time, initial setup. All calls to varity() after that will use whatever options you pass. You can pass the following options to varity.configure:

  • letters - add custom abbreviations or override default ones
  • symbols - additional symbols and their corresponding functions
  • defaults - override built in defaults or provide defaults for custom types
  • populate - turn on populate for all types (with true) or a set of types (with an array) so that you don't have to use the + flag
var $ = require('varity');
  letters: {
    '~': 'Foo',
    'a': 'Array',
    'g': 'Arguments' // If you don't like dealing with captials 
  symbols: {
    '!': function(arg, context) {
      return !!arg;
  defaults: {
    'Object': {
      jsonp: true,
      method: 'get',
      data: {
        user: localStorage.get('user')
    'Foo': function() {
      return new Foo('my foo param');
  populate: true // Always populate ALL types
   * OR
   * populate: ['Object', 'Array', 'Foo']
   * to always populate ONLY these types

Note that these options will be used for EVERY call to varity. If you need to undo these options, you can call varity.reset(), which will restore the defaults. However, any already wrapped functions will still have the custom options.

There are also simplified helpers that set one-time options:


$.letters('q', 'Quux');


$.symbols('!', function(arg, context) {
  return !!arg;


$.defaults('Array', [1, 2, 3]);


$.populate('Array', [1, 2, 3]);
// or
// or

Unlike the others, which simply set the corresponding option, varity.populate both adds the type to the populate list and calls $.defaults when called with two arguments (since, for a one-time option, it's essentially implied that the populating thing should be used). When called with only a type, that type is added to the populate list, but no additional default is set. When called with true, all types will be populated by default.


$.extend('Object', { async: true, path: '/foo/bar' });

Custom Symbols

Each symbol corresponds to a function that receives the current argument and a context. The context looks like this:

  symbols: ['-', '+'], // a list of symbols passed with this argument
  types: ['String'], // a list of types to match against
  wrapType: 'array' // indicates whether [] or | were used in the string

In addition, the varity object is passed as the this context, so you can access things like this.args, which has the full set of arguments passed to varity. You can create custom functionality by adding symbols and manipulating one or both of these parameters. Use this to whatever destructive ends you see fit. Note that symbol operations are called in the order in which they're passed, which can make a difference. For instance, if you want to use + and - together, you should always pass - first, since, after + runs, the current argument will never be undefined (well, unless the expected type is undefined). The example in the configure section above coerces results to booleans. You can take a look at the existing symbol operations to get an idea of how to use the context object.

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