JUSTUS

"justus" (latin, adj.): "proper" or "correct".

JUSTUS is a very simple library for validating JavaScript objects and properly annotating them with TypeScript types.

It focuses in providing an easy and terse syntax to define a simple schema, used to ensure that an object is correct and from which proper typing can be inferred.

Quick Start
Inferring Types
Validators
- Strings
- Numbers
- Booleans
- Constants
- Any
- Arrays
- Dates
- Tuples
- Objects (yes, this is the important one!!!)
- Optionals
- Any of, all of
Extra Validators
Validation Options
A (slightly more) complex example
Generating DTS files
Copyright Notice
License

Quick Start

You can use JUSTUS in your projects quite simply: import, write a schema and validate. For example:

import { number, object, string, validate } from 'justus'

// Create a validator, validating _objects_ with a specific schema
const validator = object({

  // The "foo" property in the objects to validate must be a "string"
  // with a minimum length of one character
  foo: string({ minLength: 1 }),

  // The "bar" property in the objects to validate must be a "number"
  bar: number,

// Always use `as const`: it correctly infers types for constants, tuples, ...
} as const)

// Use the validator to validate the object specified as the second argument
const validated = validate(validator, { foo: 'xyz', bar: 123 })

validated.foo // <-- its type will be "string"
validated.bar // <-- its type will be "number"

Easy, terse, ultimately very readable... And all types are inferred!

Shorthand syntax

The validate function (or anywhere a validation is needed) can accept a shorthand inline syntax. From our example above:

import { number, string, validate } from 'justus'

const validated = validate({
  foo: string({ minLength: 1 }),
  bar: number,
} as const, {
  foo: 'xyz',
  bar: 123,
})

... you get the drill! See below in each validator for their shorthand syntax.

Inferring Types

It is sometimes useful to have access to the inferred types produced by the validation function. The InferValidation type does just that:

const myTypeValidator = object({
  /* a required string, with minimum length of 1 character */
  foo: string({ minLength: 1 }),
  /* an optional number, defaulted to 12345 */
  bar: optional(number, 12345),
  /* an optional date, without default */
  baz: optional(date),
} as const)

type MyValidatedType = InferValidation<typeof myTypeValidator>
// here "MyValidatedType" will have the following shape:
// {
//   foo: string,             // plain "string"
//   bar: number,             // optional _with_ a default
//   baz?: Date | undefined , // optional _without_ a default
// }

Similarly, it might be useful to have access to a minimal type required as a validation input. This obviously differs from the input for several reasons, like:

optionals with default: on input we should represent optional(type, default) as type, but on input this should be type | undefined
type conversions: the date validator converts numbers or strings always into Date objects, but the input type should be Date | string | number

The InferInput helps in this case:

const myTypeValidator = object({
  /* a required string, with minimum length of 1 character */
  foo: string({ minLength: 1 }),
  /* an optional number, defaulted to 12345 */
  bar: optional(number, 12345),
  /* an optional date, without default */
  baz: optional(date),
} as const)

type MyInputType = InferInput<typeof myTypeValidator>
// here "MyInputType" will have the following shape:
// {
//   foo: string,                              // plain string
//   bar?: number | undefined,                 // optional, as it has a default
//   baz?: Date | string | number | undefined, // optional, also parsed from string, number
// }

String Validator

String validators are created using the string function:

import { string } from 'justus'

const s1 = string // validates any string
const s2 = string({ minLength: 1 }) // validate non empty strings

Options

minLength?: number: The minimum length of a valid string
maxLength?: number: The maximum length of a valid string
pattern?: RegExp: A RegExp enforcing a particular pattern for a valid string

Branded strings

Type branding can be used for string primitives. For example:

import { string, validate } from 'justus'

type UUID = string & { __brand_uuid: never }

const uuidValidator = string<UUID>({
  pattern: /^[a-fA-F0-9]{8}-[a-fA-F0-9]{4}-[a-fA-F0-9]{4}-[a-fA-F0-9]{4}-[a-fA-F0-9]{12}$/,
  minLength: 36,
  maxLength: 36,
})

const value = validate(uuidValidator, 'C274773D-1444-41E1-9D3A-9F9D584FE8B5')

value = 'foo' // <- will fail, as "foo" is a `string`, while "value" is a `UUID`

Implicit branding

Sometimes it might be useful to declare the branding of a string without recurring to an external type. We can easily do so by adding the brand property our string constraints. Following the example above:

import { string, validate } from 'justus'

const uuidValidator = string({
  pattern: /^[a-fA-F0-9]{8}-[a-fA-F0-9]{4}-[a-fA-F0-9]{4}-[a-fA-F0-9]{4}-[a-fA-F0-9]{12}$/,
  minLength: 36,
  maxLength: 36,
  brand: 'uuid',
})

const value = validate(uuidValidator, 'C274773D-1444-41E1-9D3A-9F9D584FE8B5')

value = 'foo' // <- fail! the type of "value" is "string & __brand_uuid: never"

Shorthand syntax

The shorthand syntax for string validators is simply string. For example:

import { object, string } from 'justus'

const validator = object({
  foo: string, // yep, no parenthesis, just "string"
})

Number Validator

Number validators are created using the number function:

import { number } from 'justus'

const n1 = number // validates any number
const n2 = number({ minimum: 123 }) // validate numbers 123 and greater
const n3 = number({ fromString: true }) // parse strings like "12.34" or "0x0CAFE"

Options

multipleOf?: number: The value for which a number must be multiple of for it to be valid
maximum?: number: The inclusive maximum value for a valid number
minimum?: number: The inclusive minimum value for a valid number
exclusiveMaximum?: number: The exclusive maximum value for a valid number
exclusiveMinimum?: number: The exclusive minimum value for a valid number
allowNaN?: boolean: Whether to allow NaN or not (default: false)
fromString?: boolean: Whether to parse numbers from strings or not (default: false)

Branded numbers

Type branding can be used for number primitives. For example:

import { number, validate } from 'justus'

type Price = number & { __brand_price: never }

const priceValidator = number<Price>({
  multipleOf: 0.01, // cents, anyone? :-)
  minimum: 0, // no negative prices, those are _discounts_
})

const value = validate(priceValidator, 123.45)

value = 432 // <- will fail, as 432 is a `number`, while "value" is a `Price`

Implicit branding

Sometimes it might be useful to declare the branding of a number without recurring to an external type. We can easily do so by adding the brand property our number constraints. Following the example above:

import { number, validate } from 'justus'

const priceValidator = number({
  multipleOf: 0.01, // cents, anyone? :-)
  minimum: 0, // no negative prices, those are _discounts_
  brand: 'price',
})

const value = validate(priceValidator, 123.45)

value = 432 // <- fail! the type of "value" is "number & __brand_price: never"

Shorthand syntax

The shorthand syntax for number validators is simply number. For example:

import { number, object } from 'justus'

const validator = object({
  foo: number, // yep, no parenthesis, just "number"
})

Boolean Validator

The boolean validator is represented by the boolean constant:

import { boolean, object } from 'justus'

const validator = object({
  foo: boolean, // it's a constant, no options!
})

To validate the string true or false as booleans, simply create the validator with the fromString option (defaults to false):

import { boolean, object, validate } from 'justus'

const validator = object({
  foo: boolean({ fromString: true }),
})

// Here myValue can be a boolean or the string "true" or "false" (case insensitive)
const bool = validate(validator, myValue)

Constant Validator

Cosntant validators are created using the constant function:

import { constant } from 'justus'

const c1 = constant('foo') // validates the `string` constant "foo"
const c2 = constant(12345) // validates the `number` constant 12345
const c3 = constant(false) // validates the `boolean` constant `false`
const c4 = constant(null) // validates the `null` constant

The constant validator requires a string, number, boolean or null constant.

Shorthand syntax

The shorthand syntax for constant validators is simply its value. For example:

import { object, validate } from 'justus'

const validator = object({
  foo: 'foo', // the string "foo"
  bar: 12345, // the number 12345
  baz: false, // the boolean false
  nil: null, // the null constant
} as const) // yep, don't forget "as const" to infer types correctly

const result = validate(validator, something)

result.foo // <- its type will be `"foo"` (or "string" if you didn't use "as const")
result.bar // <- its type will be `12345` (or "number" if you didn't use "as const")
result.baz // <- its type will be `false` (or "boolean" if you didn't use "as const")
result.nil // <- its type will be `null` (or "any" if you didn't use "as const")

Any Validator

The any validator is represented by the any constant:

import { any, object, validate } from 'justus'

const validator = object({
  foo: any // it's a constant, no options!
})

const result = validate(validator, something)

result.foo // <- its type will be `any`

Array Validator

Array validators are created using the array or arrayOf functions:

import { array, arrayOf, number, string } from 'justus'

const a1 = array // validates any array
const a2 = array({ maxItems: 10, items: string }) // array of strings
const a3 = arrayOf(number) // array of numbers

Options

maxItems?: number: The maximum number of elements a valid Array
minItems?: number: The minimum number of elements a valid Array
uniqueItems?: boolean: Indicates if the Array's elements must be unique
items?: V: The type of each individual item in the Array */

Shorthand syntax

The shorthand syntax for string validators is simply array. For example:

import { array, object } from 'justus'

const validator = object({
  foo: array, // validate any array, of any length, containing anything
})

The arrayOf function can also be considered a shorthand of the full array({ items: ... }). For example the two following declarations are equivalent:

import { array, arrayOf, string } from 'justus'

const a1 = array({ items: string })
const a2 = arrayOf(string) // same as above, just more readable

Date Validator

Date validators are created using the date function:

import { date } from 'justus'

const d1 = date // validates any date
const d2 = date({ format: 'iso' }) // validate ISO dates

NOTE: Date validators also convert dates (in string format), or timestamps (milliseconds from the epoch) into proper Date instances.

Options

format?: 'iso' | 'timestamp': The format for dates, iso for ISO Dates (as outlined in RFC 3339) or timestamp for the number of milliseconds since the epoch
from?: Date: The earliest value a date can have
until?: Date: The latest value a date can have

Shorthand syntax

The shorthand syntax for number validators is simply date. For example:

import { date, object } from 'justus'

const validator = object({
  foo: date, // anything that can be converted to `Date` will be!
})

Tuple Validator

A tuple is (by definition) a finite ordered list (sequence) of elements.

Tuple validators are created using the tuple function:

import { tuple, string, number, boolean } from 'justus'

// Validates 3 elements tuple: (in order) a string, a number and a boolean
const t1 = tuple([ string, number, boolean ])

// Validates a tuple whose first element is a string, followed by zero or more
// numbers, and wholse last element is a boolean
const t2 = tuple([ string, ...number, boolean ]) // yay! rest parameters!

As shown above, any Validator (or one of its shorthands) can be used as a rest parameter implying zero or more elements of the specified kind.

A more complex example:

import { number, object, string, tuple, validate } from 'justus'

const myObject = object({
  version: number,
  title: string,
})

// This is the silliest tuple ever written, but outlines our intentions:
const sillyTuple = tuple([ 'start', ...myObject, 'end' ] as const)

// Validate using our tuple:
validate(sillyTuple, [
  'start', // yep, a constant
  { version: 1, title: 'Hello world' },
  { version: 2, title: 'Foo, bar and baz' },
  'end', // the last
])

Object Validator

As seen in the examples above, object validators are created using the object function:

import { object, string, number } from 'justus'

const o1 = object // any object (excluding null - darn JavaScript)
const o2 = object({
  foo: string, // any string
  bar: number, // any number
  baz: 'Hello, world!', // the constant "Hello, world!"
} as const)

Shorthand syntax

The shorthand syntax for object validators is simply object. For example:

import { arrayOf, object } from 'justus'

const validator = arrayOf(object) // won't validate if the array has numbers, strings, ...

Allow additional properties

Sometimes it's necessary to allow additional properties in an object.

Destructuring ...allowAdditionalProperties in an objects does the trick!

import { allowAdditionalProperties, boolean, number, object, string, validate } from 'justus'

const o1 = object({
  foo: string, // any string
  bar: number, // any number
  ...allowAdditionalProperties, // any other key will be "any"
})

const result1 = validate(o1, something)

result1.foo // <-- this will be a "string"
result1.bar // <-- this will be a "number"
result1.baz // <-- additional property, this will be "any"

// additional properties with a type

const o2 = object({
  foo: string, // any string
  bar: number, // any number
  ...allowAdditionalProperties(boolean), // any other key will be "boolean"
})

const result2 = validate(o2, something)

result2.foo // <-- this will be a "string"
result2.bar // <-- this will be a "number"
result2.baz // <-- additional property, this will be "boolean"

Here allowAdditionalProperties is also a function, which can take some parameters to configure its behaviour:

...allowAdditionalProperties: default shorthand, allows additional properties and will infer the any type for them.
...allowAdditionalProperties(): as a function, and same as above, it allows additional properties and will infer the any type for them.
...allowAdditionalProperties(true): as a function, and same as above, it allows additional properties and will infer the any type for them.
...allowAdditionalProperties(false): as a function, it forbids any additional property in objects, useful when extending objects (see below)
...allowAdditionalProperties(... type ...): as a function, it allows additional properties in objects and ensures their type is correct

Extending objects

Simply destructure one into another. For example:

import { object, string, number, boolean } from 'justus'

const o1 = object({
  foo: string, // any string
  bar: number, // any number
})

const o2 = object({
  ...o1, // anything part of "o1" will be here as well!
  bar: boolean, // here "bar" is no longer a number, but a boolean
  baz: number, // add the "baz" property as a number
} as const)

A slightly more complex scenario arises when considering additional properties in the base object, but forcedly forbidding them in an extend one.

To do so, simply override in the extended object as follows:

Simply destructure one into another. For example:

import { allowAdditionalProperties, boolean, number, object, string } from 'justus'

const o1 = object({
  foo: string, // any string
  bar: number, // any number
  ...allowAdditionalProperties(boolean), // any other property is a boolean
})

const o2 = object({
  ...o1, // anything part of "o1" will be here as well!
  baz: boolean, // add "baz" to "o1", forcing it to be a "boolean"
  ...allowAdditionalProperties(false), // no more additional properties here!
} as const)

Ensure properties never exist

When allowing extra properties, or extending objects, we might want to validate the non-existance of a specific property. We can do this setting a property to never.

import { allowAdditionalProperties, never, number, object, string } from 'justus'

const o1 = object({
  foo: string, // any string
  bar: number, // any number
})

const o2 = object({
  ...o1, // anything part of "o1" will be here as well!
  bar: never, // remove "bar" from the properties inherited by "o1"
} as const)

const o3 = object({
  ...o1, // anything part of "o1" will be here as well!
  ...allowAdditionalProperties, // allow additional properties as "any"
  baz: never, // even with additional properties, "baz" must not exist
} as const)

Simple records

When attempting to validate a simple Record<string, ...anything...> the function objectOf can come handy:

import { objectOf, number } from 'justus'

const o1 = objectOf(number)
// here "o1" will have the shape "Record<string, number>""

const o2 = objectOf({ test: number })
// here "o2" will have the shape "Record<string, { test: number }>"

Optional Validator

Optional properties properties can also be declared in objects, arrays, ...:

import { object, arrayOf, optional, string, number, validate } from 'justus'

const o1 = object({
  foo: string, // any string, but must be a string
  bar: optional(number), // optional property as "number | undefined"
})

const r1 = validate(o1, something)
// here "r1" will have a shape like "{ foo: string, bar?: number | undefined }"

const o2 = arrayOf(optional(string)) // array members will be "string | undefined"
const r2 = validate(o2, something)
// here "r2" will have a shape like "(string | undefined)[]"

The optional validator can also be used to inject default values in case the source object doesn't have one. To do so, we can simply use the second parameter of our optional(...) function:

import { object, optional, number, validate } from 'justus'

const o1 = object({
  foo: optional(number, 123), // any number, default is 123
})

const r1 = validate(o1, {})
// here "r1" will be "{ foo: 123 }" (the default value)

const r2 = validate(o1, { foo: 321 })
// here "r2" will be "{ foo: 321 }" (overrides the default value)

Union Validators

Unions (either all or any) are defined using the allOf or oneOf functions.

To make sure all validations pass use allOf:

import { allOf, number, object, string, validate } from 'justus'

const o1 = object({ foo: string })
const o2 = object({ bar: number })

const result = validate(allOf(o1, o2), something)
// result here will have the type of what's inferred by o1 _and_ o2

result.foo // <-- this is a "string"
result.bar // <-- this is a "number"

// be careful about never!
const result2 = validate(allOf(number, string), something)
// obviously "result2" will be of type "never" as "number" and "string" do not match!

More useful, to make sure all validations pass use oneOf:

import { number, oneOf, string, validate } from 'justus'

const result = validate(oneOf(number, string), something)

result // <-- its type will be "number | string"

Extra Validators

A number of extra validators are also included, but not exported by default in order to reduce the bundle size when Justus is bundled in client apps.

Those are:

arn: Validate an ARN (Amazon Resource Name) as a string
- import { arn } from 'justus/extra/arn'
parseArn: Validate an ARN (Amazon Resource Name) and parse its components
- import { parseArn } from 'justus/extra/arn'
ean13: Validate a standard EAN-13 barcode number (as a string)
- import { ean13 } from 'justus/extra/ean13'
url: Validate a string URL and converts it into a proper URL object
- import { url } from 'justus/extra/url'
uuid: Validate a string UUID plain and simple
- import { uuid } from 'justus/extra/uuid'

Validation Options

The validate(...) function accepts (as a third parameter) some validation options. Those are:

stripAdditionalProperties: the validate(...) function will ignore any object property that was not declared in the schema, and will strip them out of the returned object (rather than failing).
stripForbiddenProperties: the validate(...) function will ignore any forbidden property that was declared in the schema (for more info see below), and will strip them out of the returned object (rather than failing).
stripOptionalNulls: the validate(...) function will ignore optional properties with null values and strip them out of the returned object. This is convenient when validating/stripping results coming from a database table, where columns are declared as nullable.

The strip(...) function is a convenience wrapper around validate(...) implying stripAdditionalProperties and stripOptionalNulls. Therefore:

import { validate, strip } from 'justus'

const result1 = strip(schema, object)

// is equivalent to

const result2 = validate(schema, object, {
  stripAdditionalProperties: true,
  stripForbiddenProperties: false,
  stripOptionalNulls: true,
})

A complex example

This example is not complicated at all, but it outlines the simplicity of the syntax intended for JUSTUS.

Let's assume we have some time series data, but we can expect this in a couple of different flavors, either V1 or V2 with some subtle differences:

import { arrayOf, date, number, object, oneOf, string, tuple, validate } from 'justus'

// Our V1 time-series tuple is simply a timestamp followed by a numeric value
const entryv1 = tuple([ date, number ] as const)

// Our V1 response from the time series database declares the version and data
const responsev1 = object({
  version: 1,
  entries: arrayOf(entryv1),
} as const)

// Our V2 time-series tuple is a timestamp, followed by a number and zero or
// more strings indicating some remarks on the measurements
const entryv2 = tuple([ date, number, ...string ] as const)

// Response for V2 is the same as V1, with some extra stuff
const responsev2 = object({
  version: 2,
  entries: arrayOf(entryv2),
  average: number, // this is extra!
} as const)

// Our combined response is either V1 or V2
const response = oneOf(responsev1, responsev2)

// GO! Validate!
const result = validate(response, {})

if (result.version === 1) {
  result.version // the type here will be the literal number 1
  result.entries.forEach((entry) => {
    entry[0] // this will be a `Date` instance
    entry[1] // this will be a "number"
    // entry[2] // will generate a typescript error
  })
  // response.average // will generate a typescript error
} else {
  result.version // the type here will be the literal number 2
  result.entries.forEach((entry) => {
    entry[0] // this will be a `Date` instance
    entry[1] // this will be a "number"
    entry[2] // this will be a "string"
    entry[999] // this too will be a "string"
  })
  result.average // this will be a "number""
}

Generating DTS files

Sometimes it might be necessary to generate .d.ts files for your schemas, rather than relying on the type inference provided by JUSTUS.

For example, if you were to use JUSTUS on a server application to validate HTTP requests and responses, and wanted to have strong typing when interacting with it from a client, you might not necessarily want to have JUSTUS (and your schemas) as a client dependency.

So, assuming your schemas might look something like this:

import { number, object, string } from 'justus'

// this will be exported as a type
const uuid = string({ brand: 'uuid ' })

// this will be embedded in product below
const price = number({ brand: 'price' })

// object mapping two validators above
const product = object({
  uuid,
  price,
  name: string({ minLength: 1 }),
})

We can generate the DTS for UUID and Product (we specifically not export Product in this example) using our dts-generator as follows:

import { generateTypes } from 'justus/dts-generator'

// Note how we rename the exports to "UUID" and "Product" (casing, ...)
const dts = generateTypes({
  UUID: uuid,
  Product: product,
})

The resulting dts will be a string containing the DTS as follows:

export type UUID = string & {
    __brand_uuid : never;
};
export type Product = {
    uuid: UUID;
    price: number & {
        __brand_price: never;
    };
    name: string;
};

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JUSTUS

Quick Start

Shorthand syntax

Inferring Types

String Validator

Options

Branded strings

Implicit branding

Shorthand syntax

Number Validator

Options

Branded numbers

Implicit branding

Shorthand syntax

Boolean Validator

Constant Validator

Shorthand syntax

Any Validator

Array Validator

Options

Shorthand syntax

Date Validator

Options

Shorthand syntax

Tuple Validator

Object Validator

Shorthand syntax

Allow additional properties

Extending objects

Ensure properties never exist

Simple records

Optional Validator

Union Validators

Extra Validators

Validation Options

A complex example

Generating DTS files