`js
function newUser(name = String, email = String, age = Number) {

newUser.check(arguments)

// Do stuff.
}
`

Failures are clearly indicated:

`
TypeError: newUser(name = String, email = String, age = Number) {...

- name was not of type String. Boolean provided: false

- email was not of type String. Function provided: function find(query=String) ...

- age was not provided.

at newUser (.../index.js:46:14)
at Object. (.../index.js:85:1)
at Module._compile (module.js:624:30)
at Object.Module._extensions..js (module.js:635:10)
at Module.load (module.js:545:32)
at tryModuleLoad (module.js:508:12)
at Function.Module._load (module.js:500:3)
at Function.Module.runMain (module.js:665:10)
`

Performance

`
primitives x 224,658,906 ops/sec ±0.56% (92 runs sampled)
arrays x 130,952,997 ops/sec ±0.67% (90 runs sampled)
objects x 125,843,530 ops/sec ±0.75% (86 runs sampled)
unions x 107,427,725 ops/sec ±0.75% (93 runs sampled)
duck types x 91,229,897 ops/sec ±0.71% (89 runs sampled)
custom classes x 34,945,957 ops/sec ±0.70% (91 runs sampled)
generics x 8,502,684 ops/sec ±0.73% (93 runs sampled)

Mid-2014 15" MacBook Pro
Node.js 10.3.0
`

Supports named and anonymous functions.

`js
function newUser(name=String, email=String, zipcode=Number) {
newUser.check(arguments)
//...
}

async function newUser(name=String, email=String, zipcode=Number) {
newUser.check(arguments)
//...
}

function* newUser(name=String, email=String, zipcode=Number) {
newUser.check(arguments)
//...
}

async function* newUser(name=String, email=String, zipcode=Number) {
newUser.check(arguments)
//...
}
`

Anonymous functions:
`js
const newUser = async function*(name=String, email=String, zipcode=Number) {
newUser.check(arguments)
//...
}
`

To check a method's arguments, use . to check arguments:

`js
class User {
promote(newRole=String, promotedBy=User) {
this.promote.check(arguments)
//...
}
}

const ns = {
newUser: function (name=String, email=String, zipcode=Number) {
ns.newUser.check(arguments)
//...
}
}
`

Constructors are supported, too:
`js
class User {
constructor(name=String, email=String, zipcode=Number) {
User.check(arguments)
//...
}
}
`

Notice: Arrow functions are not supported because they do not bind an arguments object.

Type Support

`js
class User {}

async function authorize(user = User, password = String) {
authorize.check(arguments)
// ...
}
`

Superclass/subclass relationships are supported as well:

`js
class User {}
class Administrator extends User {}

async function authorize(user = User, password = String) {
authorize.check(arguments)
// ...
}

authorize(new Administrator, 'password1') // Pass.
`

"Any" is implicit.

`js
function newUser(name=String, age, phone=String) {
newUser.check(arguments)
//...
}
`

Or to make it explicit, just define Any (doesn't matter what the value is), and use it thus:

`js
let Any

function newUser(name=String, age=Any, phone=String) {
newUser.check(arguments)
//...
}
`

Disjoint/Union Types

`js
function newUser(name=String, age=Number|String) {
newUser.check(arguments)
//...
}
`

You can use null as well, as in: data=Object|null, to permit a falsy value. It's probably best to have the calling function pass a type-consistent falsy value (i.e., an empty string for String, or 0 for Number), and use null only as a disjunct for non-primitive values, since cannot not have falsy values.

Duck Types

`js
function newUser(conf={name:String, age:Number|String}) {
newUser.check(arguments)
//...
}
`

Generics