Improved JavaScript Type Testing

A robust alternative to type testing in vanilla JavaScript. Uses an expanded set of type names to simplify common tests.

See the test page for examples.

Version 4 is not backwards compatible.

Syntax

is(value)

The return value is an object that describes the argument.

Member	Description
.type	The type name of value.
.of(class)	Tests if value is an instance of class.
.all(...descriptors)	Takes descriptor names as arguments. Returns true if all of them apply to value.
.any(...descriptors)	Takes descriptor names as arguments. Returns true if any of them apply to value.
[descriptor]	Each descriptor property is a boolean that is true if it applies to value.

Enumerable properties of is are string constants of all the descriptor names. These can be used in the .all() and .any() methods instead of string literals. For example, these are equivalent:

is(value).all("number", "object")

is(value).all(is.number, is.object)

Types and Descriptors

Types in this module do not distinguish between primitives and objects. For example, 5 and new Number(5) are both of type "number".

A descriptor is a boolean value that is true if it applies to the value being tested. Type names are included as descriptors. For example, 5 is associated with the primitive, number, and finite descriptors, among others.

Descriptor	Type Name	Primitive Values	Instances Of Classes
defined		not undefined	Object
undefined	yes	undefined
primitive		any primitive value
object	yes1		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 a falsy value	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
real2		0, 5	Number with a finite number value
finite		0, 5	Number with a finite 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 with .length === 0, Map or Set with .size === 0
nonempty		"foo", [1,2]	String or Array with .length > 0, Map or Set with .size > 0
date	yes		Date
error	yes		Error
function	yes		Function
promise	yes		Promise
regex	yes		Regex

Footnotes

1. The "object" type is only used for a value if no other type is appropriate. ↩

2. Deprecated. Use finite instead. ↩