ow

Function argument validation for humans

Highlights

• Expressive chainable API
• Lots of built-in validations
• Supports custom validations
• Automatic label inference in Node.js
• Written in TypeScript

Install

$ npm install ow

Usage

import ow from 'ow';

const unicorn = input => {
	ow(input, ow.string.minLength(5));

	// …
};

unicorn(3);
//=> ArgumentError: Expected `input` to be of type `string` but received type `number`

unicorn('yo');
//=> ArgumentError: Expected string `input` to have a minimum length of `5`, got `yo`

We can also match the shape of an object.

import ow from 'ow';

const unicorn = {
	rainbow: '?',
	stars: {
		value: '?'
	}
};

ow(unicorn, ow.object.exactShape({
	rainbow: ow.string,
	stars: {
		value: ow.number
	}
}));
//=> ArgumentError: Expected property `stars.value` to be of type `number` but received type `string` in object `unicorn`

Note: If you intend on using ow for development purposes only, use require('ow/dev-only') instead of the usual import 'ow', and run the bundler with NODE_ENV set to production (e.g. $ NODE_ENV="production" parcel build index.js). This will make ow automatically export a shim when running in production, which should result in a significantly lower bundle size.

API

Complete API documentation

ow(value, predicate)

Test if value matches the provided predicate. Throws an ArgumentError if the test fails.

ow(value, label, predicate)

Test if value matches the provided predicate. Throws an ArgumentError with the specified label if the test fails.

The label is automatically inferred in Node.js but you can override it by passing in a value for label. The automatic label inference doesn't work in the browser.

ow.isValid(value, predicate)

Returns true if the value matches the predicate, otherwise returns false.

ow.create(predicate)

Create a reusable validator.

const checkPassword = ow.create(ow.string.minLength(6));

const password = 'foo';

checkPassword(password);
//=> ArgumentError: Expected string `password` to have a minimum length of `6`, got `foo`

ow.create(label, predicate)

Create a reusable validator with a specific label.

const checkPassword = ow.create('password', ow.string.minLength(6));

checkPassword('foo');
//=> ArgumentError: Expected string `password` to have a minimum length of `6`, got `foo`

ow.any(...predicate[])

Returns a predicate that verifies if the value matches at least one of the given predicates.

ow('foo', ow.any(ow.string.maxLength(3), ow.number));

ow.optional.{type}

Makes the predicate optional. An optional predicate means that it doesn't fail if the value is undefined.

ow(1, ow.optional.number);

ow(undefined, ow.optional.number);

ow.{type}

All the below types return a predicate. Every predicate has some extra operators that you can use to test the value even more fine-grained.

Primitives

• undefined
• null
• string
• number
• boolean
• symbol

Built-in types

• array
• function
• buffer
• object
• regExp
• date
• error
• promise
• map
• set
• weakMap
• weakSet

Typed arrays

• int8Array
• uint8Array
• uint8ClampedArray
• int16Array
• uint16Array
• int32Array
• uint32Array
• float32Array
• float64Array

Structured data

• arrayBuffer
• dataView

Miscellaneous

• nan
• nullOrUndefined
• iterable
• typedArray

Predicates

The following predicates are available on every type.

not

Inverts the following predicate.

ow(1, ow.number.not.infinite);

ow('', ow.string.not.empty);
//=> ArgumentError: Expected string to not be empty, got ``

is(fn)

Use a custom validation function. Return true if the value matches the validation, return false if it doesn't.

ow(1, ow.number.is(x => x < 10));

ow(1, ow.number.is(x => x > 10));
//=> ArgumentError: Expected `1` to pass custom validation function

Instead of returning false, you can also return a custom error message which results in a failure.

const greaterThan = (max: number, x: number) => {
	return x > max