Latest version : 1.3.2

 Last updated on 2016-11-10T19:37:48.921Z

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Makes sure that JavaScript functions are called with the expected number of parameters.


Just like in any programming language JavaScript functions take parameters. Unlike many other programming languages, JavaScript does not require functions to be called with the defined number of parameters.

For example:

function sum(a, b) {
  return a + b;
sum(1,2); // returns 3
sum(2); // returns NaN

Depending on the situation, it might be preferred that an error is thrown when the wrong number of parameters are passed in.


It's as simple as wrapping your function with ar:

var sum = ar(function (a, b) {
  return a + b;
sum(1,2); // returns 3
sum(2); // throws "Wrong number of parameters. Excpected 2, got 1. Params: a, b."

It's even easier in CoffeeScript (isn't everything?):

sum = ar (a, b) -> a + b
sum(1,2) // returns 3
sum(1,2,3) // throws "Wrong number of parameters. Excpected 2, got 3. Params: a, b."

Installing for Node.js

From the command-line:

npm install arity

From your app:

var ar = require("arity");

Installing for the browser

Download arity.js and put it in your js folder with your project. Then include it like so:

<script src="js/arity.js"></script>

You now have access to the ar function.


npm install # Installs mocha
make test


Created by Jon Abrams.

Inspired by the rethinkDB's implementation.

I Found out about it from this blog post written by Andrew Berls.


Use it however you like, a warranty is neither implied nor provided.