node package manager
It’s your turn. Help us improve JavaScript. Take the 2017 JavaScript Ecosystem Survey »


dynargs = Dynamic Arguments

Specify and process JavaScript dynamic argument lists simply.

It's common to see functions in JavaScript that accept a list of arguments where some are optional and others can take different types of value (string, array, number, etc.). This module helps to write and process these argument lists in a consistent way.


Unstable: I'm still experimenting with the syntax - suggestions welcome!


var dynargs = require('dynargs');
var T = dynargs.type;
function batchGetItem() {
  var argSpec =
    [ ['tableName', T.string]
    , ['items', T.array]
    , ['attributesToGet', T.optional(T.arrayOf(T.string))]
    , ['consistentRead', T.boolean]
    , ['callback', T.fn]
  var parsed = dynargs.parse(arguments, argSpec);
  if (parsed.attributesToGet) {
    /* ... */
batchGetItem("MyTable", [1, 2, 3], true, function(err, res){/* ... */});

How it works

We convert the arguments into an array and shift the first entry. For each entry in the argument specification (argSpec), we check this argument validates, using the validation function specified. If it validates then we store it to the results, pop another argument and continue. If it doesn't but is optional then we continue. If it doesn't and it isn't optional then we throw a new Error.

Argument specification (argSpec)

This is a simple list of 2-tuples (pairs) where the first value is the name for the argument and the second is a function used to validate it. Functions for arguments that are optional must expose fn.isOptional == true. You can create an optional version of any validator by passing it to dynargs.type.optional().

NOTE: argSpecs could be a lot prettier if JavaScript objects were ordered, but alas we cannot rely on this so we must use arrays to maintain order.


dynargs will throw a new Error if it cannot successfully match the arguments of a function against the argument spec.

Creating your own validation

Each validation function simply takes the value to validate as it's only argument and returns true on success and false on failure, so they're very simple to implement:

function isArray(val) {
  return Array.isArray(val);

Optional arguments' functions need to have the isOptional property set, but you can do this very simply by calling dynargs.type.optional() on your validator:

var optionalArray = dynargs.type.optional(isArray);
// or:
function optionalArray(val) {
  return Array.isArray(val);
optionalArray.isOptional = true;

Built in validators

  • string
  • number
  • boolean
  • array
  • object
  • buffer - a Node.JS buffer (requires Node.JS...)
  • fn - function
  • arrayOf(type)
  • objectOf(type)
  • oneOf(type, type, ...)


Argument is a validator that all entries in the array must conform to.

var arrayOfStrings = dynargs.type.arrayOf(dynargs.type.string);


Argument is a validator that all values in the object must conform to.


Argument must take one of the types given (optional types passed to oneOf are treated as if they are non-optional; though oneOf itself can be optional).

var T = dynargs.type;
var numberOrString = T.oneOf(T.string, T.number);
var optionalNumberOrString = T.optional(T.oneOf(T.string, T.number));