Function argument validation for humans

For schema validation, I recommend zod.

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 import ow from 'ow/dev-only' instead of the usual import ow from '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 includes TypeScript type guards, so using it will narrow the type of previously-unknown values.

function (input: unknown) {
	input.slice(0, 3) // Error, Property 'slice' does not exist on type 'unknown'

	ow(input, ow.string)

	input.slice(0, 3) // OK
}

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.

Predicate docs.

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
• sharedArrayBuffer

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 || `Expected \`${x}\` to be greater than \`${max}\``;
};

ow(5, ow.number.is(x => greaterThan(10, x)));
//=> ArgumentError: Expected `5` to be greater than `10`

validate(fn)

Use a custom validation object. The difference with is is that the function should return a validation object, which allows more flexibility.

ow(1, ow.number.validate(value => ({
	validator: value > 10,
	message: `Expected value to be greater than 10, got ${value}`
})));
//=> ArgumentError: (number) Expected value to be greater than 10, got 1

You can also pass in a function as message value which accepts the label as argument.

ow(1, 'input', ow.number.validate(value => ({
	validator: value > 10,
	message: label => `Expected ${label} to be greater than 10, got ${value}`
})));
//=> ArgumentError: Expected number `input` to be greater than 10, got 1

message(string | fn)

Provide a custom message:

ow('🌈', 'unicorn', ow.string.equals('🦄').message('Expected unicorn, got rainbow'));
//=> ArgumentError: Expected unicorn, got rainbow

You can also pass in a function which receives the value as the first parameter and the label as the second parameter and is expected to return the message.

ow('🌈', ow.string.minLength(5).message((value, label) => `Expected ${label}, to have a minimum length of 5, got \`${value}\``));
//=> ArgumentError: Expected string, to be have a minimum length of 5, got `🌈`

It's also possible to add a separate message per validation:

ow(
	'1234',
	ow.string
		.minLength(5).message((value, label) => `Expected ${label}, to be have a minimum length of 5, got \`${value}\``)
		.url.message('This is no url')
);
//=> ArgumentError: Expected string, to be have a minimum length of 5, got `1234`

ow(
	'12345',
	ow.string
		.minLength(5).message((value, label) => `Expected ${label}, to be have a minimum length of 5, got \`${value}\``)
		.url.message('This is no url')
);
//=> ArgumentError: This is no url

This can be useful for creating your own reusable validators which can be extracted to a separate npm package.

TypeScript

Requires TypeScript 4.7 or later.

Ow includes a type utility that lets you to extract a TypeScript type from the given predicate.

import ow, {Infer} from 'ow';

const userPredicate = ow.object.exactShape({
	name: ow.string
});

type User = Infer<typeof userPredicate>;

Related

• @sindresorhus/is - Type check values
• ngx-ow - Angular form validation on steroids