awesome-type-check

A JavaScript library that provides runtime type checks / schema validation for JavaScript data.

This library generates types that are on JSON schema format and this makes the types easy to parse and generate documentation from. You can use a JSON schema validator like ajv (and ajv-keywords) to validate the types if you like but this library has validation logic built in (with a subset of the JSON schema rules).

This library provides:

A thin layer of syntactic sugar on top of JSON schema
Extensions to JSON schema - essentially the addition of a validate function - that allows us to do typeof/instanceof checks as well as any custom validation that we need for our types. JSON schema is great for validating JSON data but JSON data only has six different types (object, array, string, number, boolean, and null). In JavaScript we typically need to validate more types and this library aims to help with that.

Use Cases for this Library

Form and model validation
Type checking options objects and other function arguments
Validating types of properties passed to React components
Ability to generate API documentation from types. Types are just JSON schema data structures with validate functions

Installation

npm install awesome-type-check

Usage

const {typeErrors, TypeError, ObjectType, StringType, Enum, Required} = require('awesome-type-check')
 
const Username = StringType({minLength: 3, maxLength: 50, pattern: '^[a-z0-9_-]+$'})
const User = ObjectType({
    name: 'string',
    tags: ['string'],
    username: Required(Username),
    status: Enum(['active', 'inactive']),
    bonus: (v) => typeof v === 'number' && v > 0
})
 
const user = {
  name: 'Joe',
  tags: ['admin', 'vip'],
  username: 'j',
  status: 'foobar'
}
 
const errors = typeErrors(User, user)
 
errors.length // => 2
errors.every(e => e instanceof TypeError) // => true
errors[0].message // => 'must have at least 3 characters but had only 1'
errors[0].path // => ['username']
errors[1].message // => 'must be one of: active, inactive'
errors[1].path // => ['status']

What is a Type?

A type can be specified as:

A string that represents a type returned by the typeOf function, i.e. number, string, boolean, function, object, array etc. The typeOf function used by this library is essentially the built-in JavaScript typeof with a few extensions such as null, undefined, NaN, Infinity, array, date, error, and regexp. A string type can also have the value any which will validate against all values. Within an ObjectType you can add an exclamation mark to the type of a value to indicate that the corresponding key is required, i.e. number!. You can specify multiple types in a string by separating them by pipes, i.e. string|number.
An array containing a single type, i.e. ['string']. This will validate all values that are arrays where all items are of the given type (syntactic sugar for ArrayType).
A validate function. The validate function can either be a predicate that returns true or false or a function that returns undefined or errors. If the validate function returns true or undefined then the data is considered valid and otherwise it is considered invalid. Errors are typically an array of TypeError objects.
A JSON schema object that optionally contains a validate function

In addition to the types listed above you can create your own types by using custom validate functions.

Options

All built-in types take an options argument and the following options are shared across all types:

title - the name of the type, for documentation purposes
description - a description of the type, for documentation purposes
isRequired - used to indicate that the corresponding key in an object is required (equivalent to (Required)[#required])

Validating Function Arguments

const {assertType, assertOptions, Enum} = require('awesome-type-check')
 
function area (length, options = {}) {
  assertType('number', length)
  assertOptions(options, {
    type: Enum(['square', 'circle'])
  })
  if (options.type === 'square') {
    return length * length
  } else {
    return Math.PI * Math.pow((length/2), 2)
  }
}
 
Math.round(area(5)) // => 20
area(5, {type: 'square'}) // => 25
Math.round(area(5, {type: 'circle'})) // => 20
area(5, {type: 'foobar'}) // => throws /must be one of: square, circle/

Basic Types Represented as Strings

Here is an example of a typeOf type represented as a string:

const {typeErrors, isValid} = require('awesome-type-check')
const Bonus = 'number'
 
typeErrors(Bonus, 123) // => undefined
isValid(Bonus, 123) // => true
typeErrors(Bonus, 'foobar')[0].message // => 'must be of type number but was string'
isValid(Bonus, 'foobar') // => false

You can allow for multiple types by separating them by a pipe (equivalent to AnyOf):

const {typeErrors} = require('awesome-type-check')
const Bonus = 'number|string'
 
typeErrors(Bonus, 123) // => undefined
typeErrors(Bonus, 'foobar') // => undefined
typeErrors(Bonus, true)[0].message // => 'must be of type number|string but was boolean'

String types can be converted to JSON schema objects with typeObject:

const {typeObject} = require('awesome-type-check')
const MyNumber = typeObject('number')
 
typeof MyNumber // => 'object'
MyNumber // => {type: 'number', title: 'number', description: 'TypeOf(number)', arg: 'number'}
typeof MyNumber.validate // => 'function'

TypeOf

Basic types with additional metadata can be created with the TypeOf function:

const {typeErrors, isValid, TypeOf} = require('awesome-type-check')
const Bonus = TypeOf('number', {title: 'Bonus', description: 'Amount of bonus points for a user'})
typeErrors(Bonus, 123) // => undefined
isValid(Bonus, 123) // => true
typeErrors(Bonus, 'foobar')[0].message // => 'must be of type Bonus (number) but was string'
Bonus // => {type: 'number', title: 'Bonus', description: 'Amount of bonus points for a user', arg: 'number'}

You can also provide TypeOf with an array of types (equivalent to AnyOf(types)):

const {typeErrors, TypeOf} = require('awesome-type-check')
const Bonus = TypeOf(['number', 'boolean'])
typeErrors(Bonus, 123) // => undefined
typeErrors(Bonus, false) // => undefined
typeErrors(Bonus, 'foobar')[0].message // => 'must be of type number|boolean but was string'

Custom Validate Functions

Here is an example of a predicate validate function:

const {typeErrors, isValid} = require('awesome-type-check')
const isEven = (v) => typeof v === 'number' && v % 2 === 0
typeErrors(isEven, 2) // => undefined
isValid(isEven, 2) // => true
isValid(isEven, 3) // => false

Validate functions that are anonymous JavaScript predice functions (that return true/false) are opaque in the sense that they don't have a name (which can be useful for documentation and errors) and they don't provide a specific/custom error message. If you need to generate documentation from your types or if you are targeting end users you probably want your validate functions to be named JavaScript functions (or be created with Validate, se below) and you want them to return a useful error message. Here is the same type as above implemented as a validate function that returns undefined or errors:

const {typeErrors, isValid, TypeError} = require('awesome-type-check')
const isEven = (v) => {
  if (typeof v === 'number' && v % 2 === 0) {
    return undefined
  } else {
    return [new TypeError(isEven, v, 'must be an even number')]
  }
}
typeErrors(isEven, 2) // => undefined
typeErrors(isEven, 3)[0].message // => 'must be an even number'
isValid(isEven, 2) // => true
isValid(isEven, 3) // => false

Here is an example of using the Validate function to create the same type:

const {typeErrors, isValid, TypeError, Validate} = require('awesome-type-check')
const isEven = (v) => {
  if (typeof v === 'number' && v % 2 === 0) {
    return undefined
  } else {
    return [new TypeError(isEven, v, 'must be an even number')]
  }
}
const IsEven = Validate(isEven, {title: 'IsEven', description: 'an even number'})
typeErrors(IsEven, 2) // => undefined
typeErrors(IsEven, 3)[0].message // => 'must be an even number'
isValid(IsEven, 2) // => true
isValid(IsEven, 3) // => false

Validating Nested Data

You can use ObjectType and ArrayType to validate nested data:

const {typeErrors, ObjectType, ArrayType} = require('awesome-type-check')
const User = ObjectType({
  username: 'string!',
  items: ArrayType(ObjectType({
    name: 'string!',
    createdAt: 'date!'
  }))
})
 
typeErrors(User, {username: 'joe'}) // => undefined
const errors = typeErrors(User, {username: 123, items: [{name: 'foo'}, {name: 123, createdAt: new Date()}]})
errors.length // => 3
errors[0].path // => ['username']
errors[0].message // => 'must be of type string but was number'
errors[1].path // => ['items', 0]
errors[1].message // => 'is missing the following required keys: createdAt'
errors[2].path // => ['items', 1, 'name']
errors[2].message // => 'must be of type string but was number'

See NestedObject below for a slightly nicer syntax for nested data.

NestedObject

NestedObject is a wrapper around ObjectType that provides some syntactic sugar for validating nested data. Any object literals nested in the structure provided to NestedObject will be wrapped by the ObjectType function (i.e. interpreted as object properties) unless they contain a validate property with a value of type function (i.e. any nested built-in types will be preserved):

const {typeErrors, NestedObject} = require('awesome-type-check')
const User = NestedObject({
  username: 'string!',
  items: [{
    name: 'string!',
    createdAt: 'date!'
  }]
})
 
typeErrors(User, {username: 'joe'}) // => undefined
const errors = typeErrors(User, {username: 123, items: [{name: 'foo'}, {name: 123, createdAt: new Date()}]})
errors.length // => 3
errors[0].path // => ['username']
errors[0].message // => 'must be of type string but was number'
errors[1].path // => ['items', 0]
errors[1].message // => 'is missing the following required keys: createdAt'
errors[2].path // => ['items', 1, 'name']
errors[2].message // => 'must be of type string but was number'

TypeError

On validation failure the typeErrors method will return an array of TypeError objects with these properties:

stack - a stacktrace to help you figure out where in your code validation failed
message - an error message
type - the type definition (JSON schema object) for which validation failed
value - the data for which validation failed
path - if validation failed inside an object or array (or a nested combination of them) the path will show you exactly where in the data structure validation failed
code - an error category/classification, i.e. maxLength if a string is too long, or typeof if the data type was wrong etc.

StringType

Use StringType to validate string values and optionally provide minLength, maxLength, and pattern options:

const {typeErrors, StringType} = require('awesome-type-check')
const Username = StringType({minLength: 3, maxLength: 50, pattern: '^[a-z0-9_-]+$'})
typeErrors(Username, 'foobar') // => undefined
typeErrors(Username, 123)[0].message // => 'must be of type StringType but was number'
typeErrors(Username, 'fo')[0].message // => 'must have at least 3 characters but had only 2'
typeErrors(Username, '!').map(e => e.message) // => ['must have at least 3 characters but had only 1', 'must match pattern ^[a-z0-9_-]+$']
Username // => { type: 'string', title: 'StringType', minLength: 3, maxLength: 50, pattern: '^[a-z0-9_-]+$', description: 'String with minimum length 3 and maximum length 50 and pattern ^[a-z0-9_-]+$',}

NumberType

Validates number values with optional minimum and maximum restrictions:

const {typeErrors, NumberType} = require('awesome-type-check')
const Score = NumberType({minimum: 0, maximum: 100})
typeErrors(Score, 0) // => undefined
typeErrors(Score, 10) // => undefined
typeErrors(Score, 100) // => undefined
typeErrors(Score, 'foobar')[0].message // => 'must be of type NumberType but was string'
typeErrors(Score, -1)[0].message // => 'must be at least 0 was only -1'
typeErrors(Score, 101)[0].message // 'must be no more than 100 but was 101'
Score // => {type: 'number', title: 'NumberType', minimum: 0, maximum: 100, description: 'Number with minimum length 0 and maximum length 100'}

BoolType

Validate that a value is true or false, equivalent to TypeOf('boolean':

const {typeErrors, BoolType, TypeOf} = require('awesome-type-check')
const Active = BoolType()
typeErrors(Active, false) // => undefined
typeErrors(Active, true) // => undefined
typeErrors(Active, {})[0].message // => 'must be of type boolean but was object'
typeErrors(TypeOf('boolean'), false) // => undefined
typeErrors(TypeOf('boolean'), true) // => undefined
typeErrors(TypeOf('boolean'), {})[0].message // => 'must be of type boolean but was object'
Active // => {type: 'boolean', title: 'boolean', description: 'TypeOf(boolean)', arg: 'boolean'}

NullType

Validates that a value is null. Equivalent to TypeOf('null'):

const {typeErrors, NullType} = require('awesome-type-check')
typeErrors(NullType(), null) // => undefined
typeErrors(NullType(), undefined)[0].message // => 'must be of type null but was undefined'
typeErrors(NullType(), [])[0].message // => 'must be of type null but was array'

ObjectType

Use ObjectType to validate objects, accepts JSON schema equivalent options required and additionalProperties:

const {typeErrors, ObjectType, ArrayType, Required} = require('awesome-type-check')
const User = ObjectType({
  username: Required('string'),
  score: 'number'
})
 
typeErrors(User, {username: 'joe'}) // => undefined
const errors = typeErrors(User, {username: 123, score: true})
errors.length // => 2
errors[0].path // => ['username']
errors[0].message // => 'must be of type string but was number'
errors[1].path // => ['score']
errors[1].message // => 'must be of type number but was boolean'

ExactObject

Use ExactObject to validate objects where no additional properties other than those specified are allowed. Syntactic sugar for ObjectType(properties, {addtionalProperties: false}):

const {typeErrors, ExactObject} = require('awesome-type-check')
const User = ExactObject({
  username: 'string'
})
 
typeErrors(User, {username: 'joe'}) // => undefined
const errors = typeErrors(User, {userName: 'joe'})
errors.length // => 1
errors[0].message // => 'has the following invalid keys: userName'

ObjectOf

Use ObjectOf to validate that all values of an object must be of a certain type. Syntactic sugar for ObjectType({}, {patternProperties: {'.*': valueType}}):

const {typeErrors, ObjectOf} = require('awesome-type-check')
const Callbacks = ObjectOf('function')
 
typeErrors(Callbacks, {onClick: function () {}}) // => undefined
const errors = typeErrors(Callbacks, {onClick: []})
errors.length // => 1
errors[0].message // => 'must be of type function but was array'

ArrayType

Use ArrayType to validate that a value must be an array. Options are minItems and maxItems:

const {typeErrors, ArrayType} = require('awesome-type-check')
const Numbers = ArrayType('number', {minItems: 2, maxItems: 5})
 
typeErrors(Numbers, [1, 2]) // => undefined
const errors = typeErrors(Numbers, ['foobar'])
errors.length // => 2
errors[0].message // => 'must be of type number but was string'
errors[1].message // => 'must have at least 2 items but had only 1'

There is also syntactic sugar available that allows you to use an array literal to represent an ArrayType:

const {typeErrors} = require('awesome-type-check')
const Numbers = ['number']
 
typeErrors(Numbers, [1, 2]) // => undefined
typeErrors(Numbers, [1, 'foobar'])[0].message // => 'must be of type number but was string'

Enum

Use Enum to check that a value is in a given set of values:

const {typeErrors, Enum} = require('awesome-type-check')
const Status = Enum(['active', 'inactive'])
 
typeErrors(Status, 'active') // => undefined
typeErrors(Status, 'inactive') // => undefined
typeErrors(Status, 'foobar')[0].message // => 'must be one of: active, inactive'

InstanceOf

Use InstanceOf to check that a value has a certain constructor (or class):

const {typeErrors, InstanceOf} = require('awesome-type-check')
function Car(make, model) {
  this.make = make
  this.model = model
}
 
typeErrors(InstanceOf(Car), new Car('volvo', 'v90')) // => undefined
typeErrors(InstanceOf(Date), new Date) // => undefined
typeErrors(InstanceOf(RegExp), new Date)[0].message // => 'must be instanceof RegExp'

Required

You can use Required to mark a key in an ObjectType as required:

const {typeErrors, ObjectType, Required} = require('awesome-type-check')
const User = ObjectType({
  username: Required('string'),
  bio: 'string'
})
 
typeErrors(User, {username: 'joe'}) // => undefined
typeErrors(User, {})[0].message // => 'is missing the following required keys: username'

You can also mark keys with types represented as strings as required by adding an exclamation mark:

const {typeErrors, ObjectType} = require('awesome-type-check')
const User = ObjectType({
  username: 'string!',
  bio: 'string'
})
 
typeErrors(User, {username: 'joe'}) // => undefined
typeErrors(User, {})[0].message // => 'is missing the following required keys: username'

AllOf

Use AllOf to check that a value must validate against all of the given types (intersection type):

const {typeErrors, NumberType, AllOf} = require('awesome-type-check')
const PositiveNumber = NumberType({minimum: 0})
const DivisibleByTen = (v) => v % 10 === 0 ? undefined : 'must be divisible by 10'
const Score = AllOf([PositiveNumber, DivisibleByTen])
 
typeErrors(Score, 0) // => undefined
typeErrors(Score, 10) // => undefined
typeErrors(Score, 15)[0].message // => 'must be divisible by 10'
typeErrors(Score, 'foobar')[0].message // => 'must be of type NumberType but was string'

AnyOf

Use AnyOf to check that a value must validate against at least one of the given types (union type):

const {typeErrors, NumberType, AnyOf} = require('awesome-type-check')
const PositiveNumber = NumberType({minimum: 0})
const DivisibleByTen = (v) => v % 10 === 0 ? undefined : 'must be divisible by 10'
const Score = AnyOf([PositiveNumber, DivisibleByTen])
 
typeErrors(Score, 0) // => undefined
typeErrors(Score, 3) // => undefined
typeErrors(Score, -10) // => undefined
typeErrors(Score, -3)[0].message // => 'must be of type AnyOf(NumberType, DivisibleByTen)'
typeErrors(Score, 'foobar')[0].message // => 'must be of type AnyOf(NumberType, DivisibleByTen)'

TODO

ESLint
Unit test for NestedObject
Add error toJSON test (i.e. check JSON.parse(JSON.stringify(error)))
Always preserve constructor type name (ObjectType, ArrayType etc.) in constructor property?
Add ajv schema validation to nested type test
Add comparison to other libraries in RAEDME (prop-types, superstruct, joi etc.)
Improve generic message if you can extract title
Create single ES5/UMD file with Babel/Rollup for client side use? Try https://www.pikapkg.com/blog/introducing-pika-pack
Remove @pika/pack from package.json and from pkg if we can't get it to work properly
Create a JSFiddle with unpkg (https://medium.com/cameron-nokes/the-30-second-guide-to-publishing-a-typescript-package-to-npm-89d93ff7bccd)
Test ability to easily generate documentation etc. based on a nested complex type (good navigability and meta data)
More test cases: Enum, nested objects/arrays, AnyOf, AllOf, custom types, optional arrays (ArrayOrScalar)
Integration with React when used as PropTypes. Ability to turn off in production. PropTypes compatibility layer?
Apply to the assertValidOptions use case, maybe in versioned-api?
Tuple type

Resources

superstruct - Data Validation. It seems superstruct only reports on the first key having an error, not all keys. This may be an issue for form validation.
joi - Object Schema Validation
facebook/prop-types - Type Checking React Props
type_spec - Runtime Type Checks in Python
Understanding JSON Schema
kind-of - Check Type of Value in JavaScript
Clojure Spec
Active Record Validations