[](https://travis-ci.org/sindresorhus/ow) [](https://codecov.io/gh/sindresorhus/ow) [](https://cdn.jsdelivr.net/npm/ow/dist/index.js) [](https://packagephobia.now.sh/result?p=ow) > Function argument validation for humans ## Highlights - Expressive chainable API - Lots of built-in validations - Supports custom validations - Written in TypeScript ## Install ``` $ npm install ow ``` ## Usage ```ts import ow from 'ow'; const unicorn = input => { ow(input, ow.string.minLength(5)); // … }; unicorn(3); //=> ArgumentError: Expected argument to be of type `string` but received type `number` unicorn('yo'); //=> ArgumentError: Expected string to have a minimum length of `5`, got `yo` ``` ## API [Complete API documentation](https://sindresorhus.com/ow) ### ow(value, predicate) Test if `value` matches the provided `predicate`. Throws an `ArgumentError` if the test fails. ### ow.isValid(value, predicate) Returns `true` if the value matches the predicate, otherwise returns `false`. ### ow.create(predicate) Create a reusable validator. ```ts const checkPassword = ow.create(ow.string.minLength(6)); checkPassword('foo'); //=> ArgumentError: Expected string 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. ```ts ow('foo', ow.any(ow.string.maxLength(3), ow.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`](https://sindresorhus.com/ow/interfaces/ow.html#undefined) - [`null`](https://sindresorhus.com/ow/interfaces/ow.html#null) - [`string`](https://sindresorhus.com/ow/classes/stringpredicate.html) - [`number`](https://sindresorhus.com/ow/classes/numberpredicate.html) - [`boolean`](https://sindresorhus.com/ow/classes/booleanpredicate.html) - [`symbol`](https://sindresorhus.com/ow/interfaces/ow.html#symbol) #### Built-in types - [`array`](https://sindresorhus.com/ow/classes/arraypredicate.html) - [`function`](https://sindresorhus.com/ow/interfaces/ow.html#function) - [`buffer`](https://sindresorhus.com/ow/interfaces/ow.html#buffer) - [`object`](https://sindresorhus.com/ow/classes/objectpredicate.html) - [`regExp`](https://sindresorhus.com/ow/interfaces/ow.html#regexp) - [`date`](https://sindresorhus.com/ow/classes/datepredicate.html) - [`error`](https://sindresorhus.com/ow/classes/errorpredicate.html) - [`promise`](https://sindresorhus.com/ow/interfaces/ow.html#promise) - [`map`](https://sindresorhus.com/ow/classes/mappredicate.html) - [`set`](https://sindresorhus.com/ow/classes/setpredicate.html) - [`weakMap`](https://sindresorhus.com/ow/classes/weakmappredicate.html) - [`weakSet`](https://sindresorhus.com/ow/classes/weaksetpredicate.html) #### Typed arrays - [`int8Array`](https://sindresorhus.com/ow/interfaces/ow.html#int8Array) - [`uint8Array`](https://sindresorhus.com/ow/interfaces/ow.html#uint8Array) - [`uint8ClampedArray`](https://sindresorhus.com/ow/interfaces/ow.html#uint8ClampedArray) - [`int16Array`](https://sindresorhus.com/ow/interfaces/ow.html#int16Array) - [`uint16Array`](https://sindresorhus.com/ow/interfaces/ow.html#uint16Array) - [`int32Array`](https://sindresorhus.com/ow/interfaces/ow.html#in32Array) - [`uint32Array`](https://sindresorhus.com/ow/interfaces/ow.html#uin32Array) - [`float32Array`](https://sindresorhus.com/ow/interfaces/ow.html#float32Array) - [`float64Array`](https://sindresorhus.com/ow/interfaces/ow.html#float64Array) #### Structured data - [`arrayBuffer`](https://sindresorhus.com/ow/interfaces/ow.html#arraybuffer) - [`dataView`](https://sindresorhus.com/ow/interfaces/ow.html#dataview) #### Miscellaneous - [`nan`](https://sindresorhus.com/ow/interfaces/ow.html#nan) - [`nullOrUndefined`](https://sindresorhus.com/ow/interfaces/ow.html#nullorundefined) - [`iterable`](https://sindresorhus.com/ow/interfaces/ow.html#iterable) - [`typedArray`](https://sindresorhus.com/ow/interfaces/ow.html#typedarray) ### Predicates The following predicates are available on every type. #### not Inverts the following predicate. ```ts ow(1, ow.number.not.infinite); ow('', ow.string.not.empty); //=> ArgumentError: [NOT] Expected string to be empty, got `` ``` #### is(fn) Use a custom validation function. Return `true` if the value matches the validation, return `false` if it doesn't. ```ts 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. ```ts 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` ``` #### label(string) This assigns a custom label to be used in any error messages. It is useful for making error messages more user-friendly by including the name of the variable which failed validation. This predicate does not add any additional validation. ```ts ow('', ow.string.nonEmpty); //=> ArgumentError: Expected string to not be empty ow('', ow.string.label('foo').nonEmpty); //=> ArgumentError: Expected string `foo` to not be empty ``` ## Maintainers - [Sindre Sorhus](https://github.com/sindresorhus) - [Sam Verschueren](https://github.com/SamVerschueren) ## Related - [@sindresorhus/is](https://github.com/sindresorhus/is) - Type check values - [ngx-ow](https://github.com/SamVerschueren/ngx-ow) - Angular form validation on steroids ## License MIT