Improved JavaScript Type Testing

A robust alternative to JavaScript's built-in type testing.

See the test page for examples.

Version 4 is not backwards compatible.

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This module uses an expanded set of type names and related descriptors to simplify common tests of values. Basic types do not distinguish between primitives and objects, but descriptors primitive or object can be used to determine that aspect. For example, 5 and new Number(5) are both number, but 5 is primitive and new Number(5) is object.

The is() function returns an IsType object describing its argument.

Syntax:

is(value)

IsType Object

Member	Description
.type	The type of value (using this module's type names).
.of(class)	Tests if value was an instance of class.
.all(...descriptors)	Takes a list of descriptor names as arguments. Returns true if all of them applied to value.
.any(...descriptors)	Takes a list of descriptor names as arguments. Returns true if any of them applied to value.
[descriptor]	Descriptors are listed in the table below. Each descriptor property is a boolean that is true if it applied to value.

Enumerable properties of the is function are string values of the name of each descriptor. These can be used in the .all() and .any() methods instead of string literals. For example, is(value).all("number", "object") is equivalent to is(value).all(is.number, is.object).

Type Names and Related Descriptors

Descriptor	Type Name	Primitive Values	Instances Of Classes
defined		not undefined	Object
undefined	yes	undefined
primitive		not an instance of Object
object	yes		Object
objectish		null	Object
null	yes	null
nullish		undefined, null
boolean	yes	false, true
false		false
true		true
falsy		undefined, null, false, 0n, NaN, 0, ""
truthy		not falsy	Object
symbol	yes	a Symbol
bigint	yes	0n, 5n
numberish		0, 5, Infinity, NaN	Number
nan	yes	NaN	Number with value NaN
number	yes	0, 5, Infinity	Number excluding those with value NaN
real		0, 5	Number with a real number value
infinite		Infinity	Number with an infinite value
string	yes	"", "foo"	String
array	yes	[], [1,2]	Array
map	yes		Map
set	yes		Set
weakmap	yes		WeakMap
weakset	yes		WeakSet
empty		"", []	String or Array of length == 0, Map or Set of size == 0
nonempty		not empty	String, Array, Map, or Set that is not empty
date	yes		Date
error	yes		Error
function	yes		Function
promise	yes		Promise
regex	yes		Regex

Note

Note that JavaScript doesn't always treat mathematical expressions of undefined or indeterminate form as you might expect. For example, 1/0 is an undefined form, but JavaScript evaluates it as Infinity.