ow

What is ow?

The 'ow' npm package is a powerful and expressive validation library for Node.js. It allows developers to validate arguments and inputs in a concise and readable manner. 'ow' provides a variety of built-in validators and supports custom validation logic, making it a versatile tool for ensuring data integrity in applications.

What are ow's main functionalities?

Basic Type Validation

This feature allows you to validate the type of a variable. In this example, the function `validateString` checks if the input is a string. If the input is not a string, an error is thrown.

const ow = require('ow');

const validateString = (input) => {
  ow(input, ow.string);
};

validateString('Hello, World!'); // Passes
validateString(123); // Throws an error

Complex Object Validation

This feature allows you to validate the shape and properties of an object. In this example, the function `validateUser` checks if the user object has a `name` property that is a string with a minimum length of 3 and an `age` property that is a positive integer.

const ow = require('ow');

const validateUser = (user) => {
  ow(user, ow.object.exactShape({
    name: ow.string.minLength(3),
    age: ow.number.integer.positive
  }));
};

validateUser({ name: 'Alice', age: 25 }); // Passes
validateUser({ name: 'Al', age: -5 }); // Throws an error

Custom Validation

This feature allows you to create custom validation logic. In this example, the function `validateEvenNumber` checks if the input is an even number. If the input is not an even number, an error is thrown with a custom message.

const ow = require('ow');

const validateEvenNumber = (input) => {
  ow(input, ow.number.validate(n => ({
    validator: n % 2 === 0,
    message: 'Expected an even number'
  })));
};

validateEvenNumber(4); // Passes
validateEvenNumber(3); // Throws an error

Other packages similar to ow

joi

Joi is a powerful schema description language and data validator for JavaScript. It allows you to create blueprints or schemas for JavaScript objects to ensure validation of key information. Compared to 'ow', Joi offers a more extensive API and is widely used in the industry for complex validation tasks.

yup

Yup is a JavaScript schema builder for value parsing and validation. It is similar to Joi but is often preferred for its simplicity and ease of use, especially in React applications. Yup provides a fluent API for object schema validation and is highly customizable.

validator

Validator is a library of string validators and sanitizers. It is less comprehensive than 'ow' but is highly efficient for validating and sanitizing strings. Validator is often used for simple validation tasks such as checking email formats, URLs, and other string-based validations.

README

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