is

sindresorhus/is is an open-source project by sindresorhus: Type check values. Indexed here so it can be found — not resold.

It is free. Get it from the upstream repository: https://github.com/sindresorhus/is

From the project's own README (excerpt, reproduced for discovery under its MIT license):

is

> Type check values

For example, is.string('🦄') //=> true

Highlights • Written in TypeScript • Extensive use of type guards • Supports type assertions • Aware of generic type parameters (use with caution) • Actively maintained • Install

Usage

Assertions perform the same type checks, but throw an error if the type does not match.

Assertions (except assertAll and assertAny) also support an optional custom error message.

And with TypeScript:

Named exports

Named exports allow tooling to perform tree-shaking, potentially reducing bundle size by including only code from the methods that are used.

Every method listed below is available as a named export. Each method is prefixed by either is or assert depending on usage.

For example:

API

is(value)

Returns the type of value.

Primitives are lowercase and object types are camelcase.

Example: • 'undefined' • 'null' • 'string' • 'symbol' • 'Array' • 'Function' • 'Object'

This method is also exported as detect. You can import it like this:

Note: It will throw an error if you try to feed it object-wrapped primitives, as that's a bad practice. For example new String('foo').

is.{method}

All the below methods accept a value and return a boolean for whether the value is of the desired type.

Primitives

.undefined(value) .null(value)

.string(value) .number(value)

Note: is.number(NaN) returns false. This intentionally deviates from typeof behavior to increase user-friendliness of is type checks.

.boolean(value) .symbol(value) .bigint(value)

Built-in types

.array(value, assertion?)

Returns true if value is an array and all of its items match the assertion (if provided).

.arrayOf(predicate)

Returns a type guard that checks if value is an array where every item matches the predicate.…