Runtime Type System for TypeScript

Description

RunType is a runtime type system for TypeScript.

It was inspired by IO-TS, but I made some opinionated changes in the concept. IO-TS is mathematically correct and follows JavaScript and TypeScript specifications to the letter. With RunType I wanted to create something more practical.

Some of the changes:

• I am not too familiar with functional programming concepts, so I don't use them in RunType.
• The struct combinator handles optional fields easier (without the partial + intersection things in IO-TS)
• number decoder does not accept NaN.
• Decoder accepts a config argument and supports type coercion and some other modifiers
• Validators
• Runtime type description generation (print() method)

Installation

npm install @symbion/runtype

Usage

Basic Usage

First create a type:

import T from '@symbion/runtype'

const tMyType = T.struct({
    s: T.string,
    n: T.optional(T.number)
})

You can extract a TypeScript type from it:

type MyType = T.TypeOf<typeof tMyType>

You can decode an unknown value:

const u: unknown = { s: 'string', n: 42 }

const decoded = T.decode(tMyType, u)
T.isOk(decoded)
// = true

const value: MyType = decoded.ok
// = { s: 'string', n: 42 }

Type Constructors

Primitive Types

TypeScript	RunType
string	T.string
number	T.number
number (integer only)	T.integer
number (integer alias)	T.id
boolean	T.boolean
bigint	T.bigint
symbol	T.symbol
Date	T.date

Special Types

TypeScript	RunType
undefined	T.undefinedValue
null	T.nullValue
true	T.trueValue
false	T.falseValue
any	T.any
unknown	T.unknown
{} (non-null object)	T.unknownObject
void	T.voidType
never	T.never

Literal Types

T.literal('a', 'b', 3)
// TypeScript: 'a' | 'b' | 3

Compound Types

Type	TypeScript	RunType
Array	Array<Type>	T.array(tType)
Record	Record<string, Type>	T.record(tType)
Struct	{ s: string, n: number }	T.struct({ s: T.string, n: T.number })
Tuple	[string, number, Type]	T.tuple(T.string, T.number, tType)
Union	string | number | Type	T.union(T.string, T.number, tType)
Intersection	{ s: string } & { n: number }	T.intersection(T.struct({ s: T.string }), T.struct({ n: T.number }))
Tagged union	{ tag: 's', s: string } | { tag: 'n', n: number }	T.taggedUnion('tag')({ str: T.struct({ tag: T.literal('str'), s: T.string }), num: T.struct({ tag: T.literal('num'), n: T.number }) })
Key of	keyof { s: string, n: number }	T.keyof(T.struct({ s: T.string, n: T.number }))

Wrapper Types

Type	TypeScript	RunType
Optional	Type | undefined	T.optional(tType) or tType.optional()
Nullable	Type | null | undefined	T.nullable(tType) or tType.nullable()
Default	Type (with fallback)	tType.default(value) or T.withDefault(tType, value)

Chainable Methods

The optional(), nullable(), and default() modifiers can be chained directly on types:

// Chainable syntax
const tOptional = T.string.optional()
const tNullable = T.number.nullable()
const tWithDefault = T.string.default('fallback')

// Factory functions (also available)
const tOptional2 = T.optional(T.string)
const tNullable2 = T.nullable(T.number)
const tWithDefault2 = T.withDefault(T.string, 'fallback')

The default() modifier accepts either a value or a factory function:

T.string.default('static value')
T.array(T.string).default(() => [])  // Factory function for mutable defaults

Recursive Types

Recursive types can be created with T.lazy() and manual TypeScript types (because TypeScript can't infer recursive types):

interface Recursive {
    name: string
    children: Recursive[]
}

const tRecursive: T.Type<Recursive> = T.lazy(() => T.struct({
    name: T.string,
    children: T.array(tRecursive)
}))

Type Modifiers

partial

The T.partial() type modifier takes a Struct type and converts all fields to optional:

const tStruct = T.struct({
    s: T.string,
    n: T.optional(T.number)
})
// = { s: string, n?: number }

const tPartialType = T.partial(tStruct)
// = { s?: string, n?: number }

patch

The T.patch() type modifier takes a Struct type and converts all optional fields to nullable and all required fields to optional. It is useful for update APIs, where undefined or missing fields mean not to update and null value means to clear that field.

const tStruct = T.struct({
    s: T.string,
    n: T.optional(T.number)
})
// = { s: string, n?: number }

const tPatchType = T.patch(tStruct)
// = { s?: string, n?: number | null }

pick

The T.pick() type modifier takes a Struct type and picks the specified fields.

const tStruct = T.struct({
    s: T.string,
    n: T.optional(T.number),
    b: T.boolean
})
// = { s: string, n?: number, b: boolean }

const tPickType = T.pick(tStruct, ['s', 'n'])
// = { s: string, n?: number }

omit

The T.omit() type modifier takes a Struct type and omits the specified fields.

const tStruct = T.struct({
    s: T.string,
    n: T.optional(T.number),
    b: T.boolean
})
// = { s: string, n?: number, b: boolean }

const tOmitType = T.omit(tStruct, ['b'])
// = { s: string, n?: number }

deepPartial

The T.deepPartial() type modifier recursively makes all nested struct fields optional:

const tStruct = T.struct({
    name: T.string,
    address: T.struct({
        city: T.string,
        zip: T.string
    })
})
// = { name: string, address: { city: string, zip: string } }

const tDeepPartial = T.deepPartial(tStruct)
// = { name?: string, address?: { city?: string, zip?: string } }

Arrays and Date types are preserved as-is (not recursed into).

deepPatch

The T.deepPatch() type modifier is the deep version of patch(), applying patch semantics recursively:

const tStruct = T.struct({
    name: T.string,
    profile: T.struct({
        bio: T.optional(T.string),
        age: T.number
    })
})

const tDeepPatch = T.deepPatch(tStruct)
// Required fields become optional, optional fields become nullable, recursively

Decoder Options

The decode() function accepts an optional config argument. It can be used for type coercion:

T.decode(T.number, '42')
// = { _tag: 'Err', err: [ { path: [], error: 'expected number' } ] }

T.decode(T.number, '42', { coerceStringToNumber: true })
// = { _tag: 'Ok', ok: 42 }

Scalar Coercion

Option	Description
coerceNumberToString	Coerce numbers to string
coerceNumberToBoolean	Coerce numbers to boolean
coerceStringToNumber	Coerce string to number
coerceScalar	Enable all scalar coercions above

Date Coercion

Option	Description
coerceStringToDate	Coerce string to Date
coerceNumberToDate	Coerce number to Date (timestamp)
coerceDate	Enable all date coercions above

BigInt Coercion

Option	Description
coerceStringToBigInt	Coerce string to bigint
coerceNumberToBigInt	Coerce integer number to bigint
coerceBigInt	Enable all bigint coercions above

Array Coercion

Option	Description
coerceToArray	Custom function (value: unknown) => unknown to convert values to arrays

Other Options

Option	Type	Description
coerceAll	boolean	Enable all coercion options
acceptNaN	boolean	Make T.number accept NaN as a valid number
unknownFields	'reject' | 'drop' | 'discard'	How to handle unknown fields in structs: reject (error, default), drop (remove from output), discard (keep in output)

Validation

The decode() function does type decoding, which is a synchronous function. RunType also handles data validation, which is defined as an asynchronous function. The type constructors define some validator methods and user defined functions can also be used.

const tMyType = T.struct({
    s: T.string.minLength(2)
})

Validation works like decoding:

await T.validate(T.string.minLength(2), 'abc')
// = { _tag: 'Ok', ok: 'abc' }

T.decode(T.string.minLength(2), 'a')
// = { _tag: 'Ok', ok: 'a' }

await T.validate(T.string.minLength(2), 'a')
// = { _tag: 'Err', err: [ { path: [], error: 'length must be at least 2' } ] }

String Validators

Validator	Description
in(value1, value2, ...)	Value is one of the specified values
length(len)	Length equals len
length(min, max)	Length is between min and max
minLength(len)	Length is at least len
maxLength(len)	Length is at most len
matches(pattern)	Value matches the RegExp pattern
email()	Value is a valid email address

Number Validators

Validator	Description
in(value1, value2, ...)	Value is one of the specified values
integer()	Value is an integer
min(minValue)	Value is at least minValue
max(maxValue)	Value is at most maxValue
between(min, max)	Value is between min and max

BigInt Validators

Validator	Description
min(minValue)	Value is at least minValue
max(maxValue)	Value is at most maxValue
between(min, max)	Value is between min and max
positive()	Value is greater than 0
negative()	Value is less than 0
nonNegative()	Value is 0 or greater

Boolean Validators

Validator	Description
true()	Value is true
false()	Value is false

Array Validators

Validator	Description
length(len)	Length equals len
length(min, max)	Length is between min and max
minLength(len)	Length is at least len
maxLength(len)	Length is at most len

Literal Validators

Validator	Description
in(value1, value2, ...)	Value is one of the specified values

Custom Validators

function max42(v: number | undefined) {
    return (v || 0) <= 42 ? T.ok(v) : T.error("Max 42 is allowed!")
}

await T.validate(T.number.addValidator(max42), 43)
// = { _tag: 'Err', err: [ { path: [], error: "Max 42 is allowed!" } ] }

Internals

Missing Properties vs Undefined

TypeScript (because of JavaScript) differentiates missing properties and properties with undefined value. This is sometimes useful, however it makes it more difficult to handle this in runtime type systems. Take the following simple TypeScript type:

interface Person {
    name: string
    age?: number
}

In IO-TS you can create it like this:

const tPerson = T.intersection([
    T.type({
        name: T.string
    }),
    T.partial({
        age: T.number
    })
])
type Person = T.TypeOf<typeof tPerson>

RunType uses complex TypeScript mechanisms to achieve a simpler and readable syntax:

const tPerson = T.struct({
    name: T.string,
    age: T.optional(T.number)
})
type Person = T.TypeOf<typeof tPerson>

Under the hood RunType generates the same intersection type because of limitations in TypeScript, but it works the same as the original type:

type Person = { name: string } & { age?: number }

Closing Thoughts

If you want to boost your TypeScript knowledge to the next level I highly recommend to write a runtime type system. I guarantee it will be fun! :)