Function argument validation for humans

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

$ npm install ow

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`

Complete API documentation

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

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.

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

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`

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`

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));

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);

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.

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

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

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

• arrayBuffer
• dataView

• nan
• nullOrUndefined
• iterable
• typedArray

The following predicates are available on every type.

Inverts the following predicate.

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

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

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`

• Sindre Sorhus
• Sam Verschueren

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

MIT