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0.13.5 • Public • Published

! This is a temporary fork of ow by sindresorhus that merges the .catching() method pr

To install the fork:

$ npm install ow--fork-by-jblew-with-catching

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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));

	// …

//=> ArgumentError: Expected `input` to be of type `string` but received type `number`

//=> 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.


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.


Create a reusable validator.

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

const password = 'foo';

//=> 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));

//=> 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.


Built-in types

Typed arrays

Structured data



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`


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 ${x}`
//=> 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: x > 10,
	message: label => `Expected ${label} to be greater than 10, got ${x}`
//=> ArgumentError: Expected number `input` to be greater than 10, got 1

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


Use a custom validation function that throws error. The function should throw an error if value is invalid or return nothing when value is valid. This allows to use specific validators for complex property or array types.

interface Animal {
	type: string;
	weight: number;

function validateAnimal(animal: Animal) {
	ow(animal.type, 'Animal.type', ow.string.oneOf([ 'dog', 'cat', 'elephant' ]));
	ow(animal.weight, 'Animal.weight', ow.number.finite.positive);

const animals: Animal [] = [
	{ type: 'dog', weight: 5 },
	{ type: 'cat', weight: Number.POSITIVE_INFINITY }

ow(animals, ow.array.ofType(ow.object.catching(animal => validateAnimal(animal as Animal))));
//=> ArgumentError: (array `animals`) (object) Expected number `Animal.weight` to be finite, got Infinity





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