arity-of

An NPM library that exposes max arity and other metadata for JS functions

This package implements the es-shim API interface. It works in an ES6-supported environment and complies with the spec (TODO this is a lie. Write a proposal).

Concepts

"Arity" refers to the number of parameters a function declares.

function (a, b, c) {} declares 3 parameters so has a minimum and maximum arity of 3.

function (a, b = 0, ...rest) {} declares 3 parameters. It has a minimum arity of 1 since there is no default value for a. It has a maximum arity of 3 since that is the number declared. It uses a rest parameter ...rest so it can actually take additional parameters.

Installation

npm install arity-of

Usage

const arityOf = require('arity-of');

function myFunction(a, b = 0, [c, d], ...e) {
  // ...
}

myFunction.length;   // Minimum arity

const {
  max,               // Maximum arity (including any ...restParameter)
  usesRest,          // True if there is a ...restParameter.
} = arityOf(myFunction);