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1.1.1 • Public • Published

A light wrapper around the amazing is type-checking library by Sindre Sorhus designed to throw nice error messages when type assertions fail. You should familiarize yourself with is before using this library.


$ npm i @darkobits/assert-is


assertIs(methodOrMethods: string | array, valueOrQualifier: any, value: ?any) : any

assertIs accepts a string (identifying a method from is) and a value to test. If is returns true, the value is returned. If it returns false, an error is thrown.

import assertIs from '@darkobits/assert-is';

function add (a, b) {
  assertIs('number', a);
  assertIs('number', b);

  return a + b;

add(2, 2)
//=> 4

add(2, 'two')
//=> TypeError('Expected value to be of type "number", got "string".')

// assertIs returns the provided value on success, so we can even get fancy:
const add = (a, b) => assertIs('number', a) + assertIs('number', b);


Some is methods require an additional parameter. For example, inRange accepts a range ([min: number, max: number] or max: number) in addition to the value to check. When using these methods with assertIs, the qualifier always precedes the value.

import assertIs from '@darkobits/assert-is';

// Assert that a number is positive:

assertIs('inRange', [1, Infinity], 42);
//=> 42

assertIs('inRange', [1, Infinity], NaN);
//=> RangeError('Expected value NaN to be between 1 and Infinity.')

Union Types

assertIs can be used to check if a value satisfies at least one of several is predicates.

Note: Predicates which require qualifiers (like inRange and directInstanceOf) are not supported when performing union type assertions.

import assertIs from '@darkobits/assert-is';

function greet (name, age) {
  assertIs('string', name);
  assertIs(['string', 'number'], age);

  return `Hello! My name is ${name}, and I am ${age} years old.`;

greet('Bob', 36)
//=> 'Hello! My name is Bob, and I am 36 years old.'

greet('Alice', 'forty-three')
//=> 'Hello! My name is Alice, and I am forty-three years old.'

greet('Leeroy', NaN)
//=> TypeError('Expected type of value to be either "string" or "number". Got "NaN".')


When assertIs receives fewer arguments than it needs to perform an assertion, it returns a function which accepts the remaining arguments. This allows for the creation of custom, re-usable assertion functions:

import assertIs from '@darkobits/assert-is';

const assertIsString = assertIs('string');

//=> TypeError('Expected value to be of type "string", got "object".')

const assertIsLessThanFive = assertIs('inRange', 5);

const assertIsLessThanFive(10);
//=> RangeError('Expected value 10 to be less than 5.')

class Person { }

const assertIsPerson = assertIs('instanceOf', Person);

assertIsPerson(new Person());
//=> Person

//=> TypeError('Expected value to be an instance of "Person".')

const assertIsIterable = assertIs(['array', 'map', 'set', 'weakMap', 'weakSet']);

assertIsIterable([1, 2, 3]);
//=> [1, 2, 3]

assertIsIterable(function () {});
//=> TypeError('Expected type of value to be either "Array", "Map", "WeakMap" or "WeakSet". Got "Function".')

Additional Assertions

In addition to the methods provided by is, assertIs also provides the following assertions:

instanceOf(Ctor: class | function, instance: any)

Expects a constructor and an object and asserts that the object is an instance of the constructor/class.

import assertIs from '@darkobits/assert-is';

class Person { }

const myPerson = new Person();

assertIs('instanceOf', Person, myPerson);
//=> myPerson

assertIs('instanceOf', Person, 'foo');
//=> TypeError('Expected value to be an instance of "Person", got "string".')

subclassOf(SuperclassCtor: class | function, SubclassCtor: class | function)

Expects two constructors and asserts that the second is a subclass of (re: extends) the first.

import assertIs from '@darkobits/assert-is';

class Person { }

class Teacher extends Person { }

assertIs('subclassOf', Person, Teacher);
//=> Teacher

class Animal { }

assertIs('subclassOf', Person, Animal);
//=> TypeError('Expected value to be a subclass of "Person".')


assertIs provides a fluent API for adding additional contextual information to errors:

import assertIs from '@darkobits/assert-is';
// or:
import { context } from '@darkobits/assert-is';

function add (a, b) {
  const assert = assertIs.context('add');
  // or:
  const assert = context('add');

  // Use assert() just like assertIs():
  assert('number', a);

  // This will throw errors like:
  // TypeError('[add] Expected value to be of type "number", got...');

  // Or, use the 'arg' + 'is' methods to provide additional context about an assertion:
  assert.arg('first argument', a).is('number');

  // This will throw errors like:
  // TypeError('[add] Expected first argument to be of type "number", got...');

  // You can also chain arg/is calls:
  assert.arg('first argument', a).is('number').arg('second argument', b).is('number');

  return a + b;

The signatures for these methods are:

arg(label: string, value: any) : function

You may also use label (an alias to arg) if you prefer. This method always returns an object with is(), which accepts the type or types you expect.

is(typeOrTypes: string | array): AssertionContext

This method always returns the assertion context, meaning you can chain arg + is calls.

See Also / Prior Art



npm i @darkobits/assert-is

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