0.6.3 • Public • Published

typ: Type predicates and assertions for Node

This Node module is meant to provide a unified place to ask and assert about all the built-in JavaScript and core Node types.

"Typ" is German for "type". Also, as of this writing, both "type" and "types" were taken in the npm module registry. The name was picked to be both memorable and short, the latter in order to encourage it to be used liberally.

Building and Installing

npm install typ

Or grab the source. As of this writing, this module has no dependencies, so once you have the source, there's nothing more to do to "build" it.


npm test


node ./test/test.js

API Details


This module provides some convenient constants. With regards to the string constants, the author finds it handy to use them instead of quoted strings, since that makes typos cause more blatant errors.


This is what's returned by typeof when given a boolean value.


This is what's returned by typeof when given a function value.


This is what's returned by typeof when given a numeric value.


This is what's returned by typeof when given an object value.


This is what's returned by typeof when given a string value.


This is what's returned by typeof when given undefined.


This is the object prototype of array instances.


This is the object prototype of function instances.


This is the default object prototype.

Assertion Functions

This module defines both predicates and assertions for a set of types that covers the standard core JavaScript and Node types. For any given type name, the predicate is called isName() and the assertion is called assertName(). A predicate simply returns a boolean indicating whether or not its value argument is of the appropriate type, and an assertion does nothing other than throw a descriptive message if its value argument is not of the expected type.

Predicates all take a single argument, value, the value to test. Assertions all take two arguments, value as with the predicate, and an optional message to replace the default message to report in an error in the case of failure. The message if specified may contain any number of instances of %s, which will be substituted with the default message. %% is substituted with a single %. No other % substitution is done.

The following run-down indicates the meaning of the various types, as far as this module is concerned. If being a particular type implies also being some other type(s), then that fact is indicated by an "implies" line. (Note: Since all values but undefined are considered defined, that fact isn't listed.)

array: isArray(value), assertArray(value, message)

Implies: object

Arrays are what you get when you use the [...] array literal syntax or the new Array(...) constructor.

boolean: isBoolean(value), assertBoolean(value, message)

The only two booleans are true and false. Notably, Boolean objects are not considered to be booleans here.

buffer: isBuffer(value), assertBuffer(value, message)

Implies: object

Buffers are Node's standard ordered-list-of-bytes type, created with the new Buffer(...) constructor and used all over the place in Node.

date: isDate(value), assertDate(value, message)

Implies: object

Dates represent moments in time. They can be created with the new Date(...) constructor.

defined: isDefined(value), assertDefined(value, message)

All values other than undefined are defined.

error: isError(value), assertError(value, message)

Implies: object

Errors are the standard exception values in JavaScript. They can be created by using the new Error(...) constructor as well as sub-class constructors.

function: isFunction(value), assertFunction(value, message)

Implies: object

Functions are the things in JavaScript that do work. They can be created by using the function... definition and literal syntax, as well as with the new Function(...) constructor.

int: isInt(value), assertInt(value, message)

Implies: number

An int is an integer value, which is to say a number with no fractional part. As far as this module is concerned, there is no range limit on the ints (that is, an int doesn't have to fit in 32 bits, for example).

Notably, neither positive nor negative Infinity qualifies as an int.

In addition, as a strange edge case, "negative zero" is also not considered to be an int. (You can produce a "negative zero" value in JavaScript with the expression -1e-1000. You can differentiate it from plain old regular zero by dividing 1 by it and observing that the result is -Infinity.)

map: isMap(value), assertMap(value, message)

Implies: object

A map is any object that behaves like a simple map-type collection. In particular, a map's prototype must be the default object prototype, and none of a map's bindings may be dynamic properties. That is, getter and setter functions disqualify an object from being considered a map.

null: isNull(value), assertNull(value, message)

The only value that is null is null per se.

nullish: isNullish(value), assertNullish(value, message)

The only two values that are considered to be "nullish" are null and undefined.

number: isNumber(value), assertNumber(value, message)

A number is, well, a numeric value. Numbers are what result from using number literals (like 123) and are returned, for example, from the methods on the built-in Math object.

The values Infinity and -Infinity are considered to be numbers. The special value NaN is alson considered to be a number, despite the direct expansion of its name to "Not a Number".

Notably, Number objects are not considered numbers here.

object: isObject(value), assertObject(value, message)

An object is an arbitrary mapping of string keys to values. They can be created in any number of ways (and if you need more description than that, you should find a good intro book on JavaScript).

Notably, null is not considered to be an object (even though typeof null == "object").

regexp: isRegExp(value), assertRegExp(value, message)

Implies: object

A regexp is an object that represents a "regular expression". They can be created by using the /.../ literal syntax or the new RegExp(...) constructor.

string: isString(value), assertString(value, message)

A string is an ordered sequence of characters. They can be created by using the '...' literal syntax and are produced by many standard JavaScript functions.

Notably, String objects are not considered strings here.

uint: isUInt(value), assertUInt(value, message)

Implies: int, number

A uint is an unsigned integer, also known as a whole number. That is, it's anything that's an int which is also non-negative. 0 is notably a uint.

undefined: isUndefined(value), assertUndefined(value, message)

The only value that is undefined is undefined.

Notably, null is defined, not undefined.

Miscellaneous Functions

hasDefaultPrototype(obj) -> boolean

Returns true if and only if the given object's prototype is the default one. That is, this is just a convenient way to say:

Object.getPrototypeOf(obj) === OBJECT_PROTOTYPE

hasOwnProperty(obj, name) -> boolean

This is a safe version of the per-object hasOwnProperty() method. You should use this any time you can't be 100% sure that the object you're checking won't possibly have a binding for "hasOwnProperty".

To Do

  • Figure out something to do.


Questions, comments, bug reports, and pull requests are all welcome. Submit them at the project on GitHub.

Bug reports that include steps-to-reproduce (including code) are the best. Even better, make them in the form of pull requests that update the test suite. Thanks!


Dan Bornstein (personal website), supported by The Obvious Corporation.

Thanks to Jeremy Stanley, Dan Pupius, Mike Fleming, and Sho Kuwamoto for suggestions.


Copyright 2012 The Obvious Corporation.

Licensed under the Apache License, Version 2.0. See the top-level file LICENSE.txt and (http://www.apache.org/licenses/LICENSE-2.0).




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