Variadic Function (vfn)

Lets you specify a “rest parameter” that’s not at the end.

Installation

Requires Node.js 6.0.0 or above.

npm i vfn

API

The module exports a single function.

Parameters

1. Object argument (or a value for arg):
   • Optional: arg (positive integer): The zero-based index of which parameter in fn’s parameters list should be the “rest parameter.” Defaults to 0.
   • Optional: oo (boolean): If true, an optional options argument at the end of the parameter list will be ignored. (This will only work if you can be sure that none of your other arguments will be plain objects.) Defaults to false.
2. fn (function): The function that has a parameter you want to convert into a “rest parameter.”

Return Value

A wrapper function that turns all excess arguments into an array which is passed to fn at parameter index arg.

Example

JavaScript supports “rest parameters,” but only at the end. Anything else will throw an error:

function func (...a, b, c) {} // Uncaught SyntaxError: Rest parameter must be last formal parameter

The vfn module lets you accomplish this. The index of a in the parameters list is 0, so we pass 0 as the first argument to vfn():

const vfn = require('vfn')

const func = vfn(0, function (a, b, c) {})

func(1, 2, 3, 4, 5) // a = [1, 2, 3]; b = 4; c = 5
func(1, 2, 3) // a = [1]; b = 2; c = 3
func('test', 'example') // a = []; b = 'test'; c = 'example'
func('hello world') // a = []; b = 'hello world'; c = undefined

With Optional Options Argument

Normally, vfn does not support optional parameters, because there’s no way to tell which arguments go to the optional parameter and which go to the variadic parameter. Therefore, vfn ignores optional parameters altogether. Take for example the common design pattern of including an optional options parameter at the end of the parameter list:

const vfn = require('vfn')

const func = vfn(0, function (a, b, c, {option} = {}) {})

func(1, 2, 3, 4, 5) // a = [1, 2, 3]; b = 4; c = 5; option = undefined
func(1, 2, 3, 4, 5, {option: 123}) // a = [1, 2, 3, 4]; b = 5; c = {option: 123}; option = undefined

As you can see, the first call works as expected, because the optional options parameter is ignored completely. But it continues to be ignored when we introduce the options object in the second call. vfn collapses one too many arguments into a and puts the options object into c.

To make use of an optional options argument, set the oo flag when calling vfn:

const vfn = require('vfn')

const func = vfn({arg: 0, oo: true}, function (a, b, c, {option} = {}) {})

func(1, 2, 3, 4, 5) // a = [1, 2, 3]; b = 4; c = 5; option = undefined
func(1, 2, 3, 4, 5, {option: 123}) // a = [1, 2, 3]; b = 4; c = 5; option = 123

With oo set, vfn will recognize that a plain object at the end belongs to an optional ending parameter.

This only works if your options parameter is optional and is at the end, and if you can be sure that none of your other arguments will be plain objects.

Related

For more projects like this, check out @lamansky/fn.