type-plus

More than 200 type utilities for TypeScript for applications, library, and type-level programming.

Table of Contents

1. Table of Contents
2. Installation
3. What's in the package?
   1. Update organization
   2. Update documentation
4. Assertion Function
   1. assertType
5. Type Guard
6. Type Utilities
7. Type Specific Utilities
   1. any
   2. Array
   3. Union
   4. bigint
   5. Boolean
   6. Function
   7. Never
   8. Null
   9. Number
   10. Numeric
   11. Object
   12. Promise
   13. String
   14. Symbol
   15. Tuple
   16. Undefined
   17. unknown
   18. void
8. Testing Utilities
9. Constant Types
10. JSON Support
11. Type manipulation
12. Type Predicates
    1. Logical
13. Math
14. Utility Functions
15. Nominal Types
16. Functional Types
17. Attribution
18. Useful Tips
19. Similar projects

Installation

npm install type-plus

yarn add type-plus

pnpm add type-plus
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What's in the package?

With over 200 types in type-plus, it can become difficult to find the types you need.

Also, some types need to be updated as TypeScript continue to evolve.

Currently, we are updating type-plus with the following objective:

• Update organization
• Update documentation
• Clean up and deprecate types
• Upgrade TypeScript from 5.0.4 to 5.1 (potential breaking changes)

Update organization

Top-level exports of type-plus will contain types and functions that do not expect the input to be a specific type. For example,

• assertType(), isType(), and testType()
• Type filters and predicates such as AnyType or IsArray

It can also have types and functions for specific types if it is a common convention, or the is no ambiguity, or for backwards compatibility purpose.

Other type specific utilities will be added under their respective *Plus namespaces such as ArrayPlus.At or NumberPlus.Positive.

Update documentation

Each type and utility function in type-plus will be updated to include examples and tags to indicate its category and behavior.

Each tag has an associated icon:

• 👽 alias: Alias of another type
• 🚦 assertion: assertion function
• 🔢 customizable: the behavior of the type is customizable.
• 💀 deprecated: deprecated and will be removed soon
• 🌪️ filter: a.k.a. parse These types perform some kind of test. If the input passes the test, the input is returned. Otherwise, it returns never
• 🛡️ guard: type guard function
• 💥 immediate: the effect of the type can be observed immediately during development
• ㊙️ internal: the type is internal and should not be used directly
• 🏃 runtime: the function has runtime effect
• 🩳 shortcut: Shortcut or convenient types
• 🧪 testing: the type or function are designed for test
• ⚗️ transform: these types transforms the input to another category
• 🧰 type util: types for building types
• 🦴 utilities: provide various functionalities
• 🎭 predicate: a.k.a. predicate or logical. These types perform some kind of test. If the input passes the test, it returns true or false

Assertion Function

Assertion Functions are special functions that asserts certain conditions of your program.

It is introduced in TypeScript 3.7.

They throw an error if the condition is not met, and return nothing otherwise.

These assertion functions are typically used in runtime, so that that type of the value can be narrowed down.

assertType

assertType provides a generic assertion function, as well as many assertion functions for built-in types.

assertType<T>(subject)

💀 deprecated. Use assertType.as() instead.

assertType<T>(subject, validator):

🚦assertion: assert the subject is type T with the specified validator.

If subject fails the assertion, a standard TypeError will be thrown and provide better error info. For example:

const s: unknown = 1

// TypeError: subject fails to satisfy s => typeof s === 'boolean'
assertType<boolean>(s, s => typeof s === 'boolean')
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The message beautification is provided by tersify.

assertType.isUndefined()

🚦assertion: assert the subject is undefined.

assertType.noUndefined()

🚦assertion: assert the subject is not undefined.

assertType.isNull()

🚦assertion: assert the subject is null.

assertType.noNull()

🚦assertion: assert the subject is not null.

assertType.isNumber()

🚦assertion: assert the subject is number.

assertType.noNumber()

🚦assertion: assert the subject is not number.

assertType.isBoolean()

🚦assertion: assert the subject is boolean.

assertType.noBoolean()

🚦assertion: assert the subject is not boolean.

assertType.isTrue()

🚦assertion: assert the subject is true.

assertType.noTrue()

🚦assertion: assert the subject is not true.

assertType.isFalse()

🚦assertion: assert the subject is false.

assertType.noFalse()

🚦assertion: assert the subject is not false.

assertType.isString()

🚦assertion: assert the subject is string.

assertType.noString()

🚦assertion: assert the subject is not string.

assertType.isFunction()

🚦assertion: assert the subject is function.

assertType.noFunction()

🚦assertion: assert the subject is not function.

assertType.isError()

🚦assertion: assert the subject is an Error.

assertType.noError()

🚦assertion: assert the subject is not an Error.

assertType.isConstructor()

💀 deprecated. It does not work in all cases.

It passes for function that can be called with new. If the subject is an arrow function, it can still return true after compilation.

assertType.isNever()

🚦assertion: assert the subject is never.

assertType.custom()

🚦assertion: creates a custom assertion function.

Using it to create a custom assertion function that provides better error messages.

The message beautification is provided by tersify.

assertType.as<T>()

🚦assertion: assert the subject as T without validator.

This works similar to manual assertion ;(subject as T)

Type Guard

User-defined type guard functions is a function which its return type is specified as x is T.

isType()

🛡️ guard: a generic type guard function

isType.t()

💀 deprecated: use testType.true() instead.

isType.f()

💀 deprecated: use testType.false() instead.

isType.never()

💀 deprecated: use testType.never() instead.

isType.equal()

💀 deprecated: use testType.equal() instead.

Type Utilities

Equal<A, B, Then = true, Else = false>

💀 deprecated. Use IsEqual instead. This will be converted to a ↪️ parse.

IsEqual<A, B, Then = true, Else = false>

⭕ predicate: if A and B are the same.

NotEqual<A, B, Then = true, Else = false>

💀 deprecated. Use IsNotEqual instead. This will be converted to a ↪️ parse.

IsNotEqual<A, B, Then = true, Else = false>:

⭕ predicate: check if A and B are not the same.

Extendable<A, B, Then = A, Else = never>

↪️ parse: check if A extends B.

IsExtend<A, B, Then = true, Else = false>

⭕ predicate: check if A extends B.

NotExtendable<A, B, Then = A, Else = never>:

↪️ parse: check if A not extends B.

IsNotExtend<A, B, Then = true, Else = false>:

⭕ predicate: check if A not extends B.

IsAssign<A, B, Then = true, Else = false>

💀 deprecated: use CanAssign instead.

CanAssign<A, B, Then = true, Else = false>:

⭕ predicate: check can A assign to B.

A typical usage is using it with assertType:

assertType.isFalse(false as CanAssign<boolean, { a: string }>)
assertType.isTrue(true as CanAssign<{ a:string, b:number }, { a: string }>)
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StrictCanAssign<A, B, Then = true, Else = false>

⭕ predicate: can A strictly assign to B

When A is a union, all branches must be assignable to B.

StrictCanAssign<number | string, number> // false
StrictCanAssign<number | string, number | string> // true
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canAssign<T>(): (subject) => true

⭕💻 predicate, compile-time

Returns a compile-time validating function to ensure subject is assignable to T.

const isConfig = canAssign<{ a: string }>()
assertType.isTrue(isConfig({ a: 'a' }))
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canAssign<T>(false): (subject) => false:

⭕💻 predicate, compile-time

Returns a compile-time validating function to ensure subject is not assignable to T.

const notA = canAssign<{ a: string }>(false)
assertType.isTrue(notA({ a: 1 }))

notA({ a: '' }) // TypeScript complains
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Type Specific Utilities

type-plus provides type checking utilities for every type.

Each type has at least 4 type checks. Using string as an example, there are StringType<T>, IsString<T>, NotStringType<T>, and IsNotString<T>.

Some types will have more checks, such as boolean has StrictBooleanType<T>, TrueType<T>, FalseType<T>.

You can learn more in their respective sections:

• any
• array
• bigint
• boolean
• function
• never
• null
• number
• object
• string
• symbol
• tuple
• undefined
• unknown
• void
• mix types

any

• AnyType
• IsAny
• NotAnyType
• IsNotAny
• AnyOrNeverType
• IsAnyOrNever

Array

• 🌪️ArrayType: Filter T to ensure it is an array, excluding tuple.
• 🎭IsArray: Validate that T is an array, excluding tuple.
• 🌪️NotArrayType: Filter T to ensure it is not an array, excluding tuple.
• 🎭IsNotArrayType: Validate that T is not an array, excluding tuple.
• 🦴At: Gets the type of the array or tuple at positive or negative index N.
• 🦴💀Concat: Concatenates two arrays or tuples.
• 🦴🔢FindFirst: Find the first type in the array or tuple A that matches Criteria.
• FindLast
• Some
• ⚗️Filter: Filter the array or tuple A, keeping entries satisfying Criteria.
• ⚗️👽KeepMatch: Keeps entries satisfying Criteria in array or tuple A.
• 🦴🔢Head: Gets the first entry in the tuple or the type of array T.
• IntersectOfProps
• MapToProp
• 🦴🔢Last: Gets the last entry in the tuple or the type of array T.
• literalArray
• PadStart
• reduceWhile
• Reverse
• PropUnion
• UnionOfProps
• UnionOfValues
• ArrayPlus.At
• ArrayPlus.Concat
• ArrayPlus.Entries
• 🦴🔢ArrayPlus.Find: Finds the type in array A that matches Criteria.
• ArrayPlus.FindLast
• ArrayPlus.Reverse
• ArrayPlus.SplitAt
• ArrayPlus.Some

Union

• IsUnion
• UnionType

bigint

BigintType<T, Then = T, Else = never>

↪️ parse: if T is bigint or bigint literal.

IsBigint<T, Then = true, Else = false>

⭕ predicate: if T is bigint or bigint literal.

NotBigintType<T, Then = T, Else = never>

↪️ parse: if T is not bigint or bigint literal.

IsNotBigInt<T, Then = true, Else = false>

⭕ predicate: if T is not bigint or bigint literal.

StrictBigintType<T, Then = T, Else = never>

↪️ parse: if T is exactly bigint.

IsStrictBigint<T, Then = true, Else = false>

⭕ predicate: if T is exactly bigint.

NotStrictBigintType<T, Then = T, Else = never>

↪️ parse: if T is not exactly bigint.

IsNotStrictBigint<T, Then = true, Else = false>

⭕ predicate: if T is not exactly bigint.

Boolean

BooleanType<T>

↪️ parse: T === boolean.

IsBoolean<T>

⭕ predicate: T === boolean

NotBooleanType<T>

↪️ parse: T !== boolean.

IsNotBoolean<T>

⭕ predicate: T !== boolean

Function

FunctionType<T>

↪️ parse: T === function.

IsFunction<T>

⭕ predicate: T === function

NotFunctionType<T>

↪️ parse: T !== function.

IsNotFunction<T>

⭕ predicate: T !== function

AnyFunction<P, R>

🔨 utilities: a generic type for any function

ExtractFunction<F>

🔨 utilities: extract the function signature from a type F.

extractFunction(fn: F)

🔨 utilities: adjust type of fn to its function signature only.

inspect<T>(value: T, inspector?: (v: T) => void)

🔨 utilities: inspect a value and return it. Inspector defaults to console.dir()

Never

NeverType<T>

↪️ parse: T === never.

IsNever<T>

⭕ predicate: T === never

NotNeverType<T>

↪️ parse: T !== never.

IsNotNever<T>

⭕ predicate: T !== never

Null

NullType<T>

↪️ parse: T === null.

IsNull<T>

⭕ predicate: T === null

NotNullType<T>

↪️ parse: T !== null.

IsNotNull<T>

⭕ predicate: T !== null

Number

NumberType<T, Then = N, Else = never>

↪️ parse: is the type T number.

IsNumber<T, Then = true, Else = false>

⭕ predicate: is the type T number.

NotNumberType<T, Then = T, Else = never>

↪️ parse: is the type T not number.

IsNotNumber<T, Then = true, Else = false>

⭕ predicate: is the type T not number.

StrictNumberType<T, Then = N, Else = never>

↪️ parse: is the type T exactly number.

IsStrictNumber<T, Then = true, Else = false>

⭕ predicate: is the type T exactly number.

NotStrictNumberType<T, Then = T, Else = never>

↪️ parse: is the type T not exactly number.

IsNotStrictNumber<T, Then = true, Else = false>

⭕ predicate: is the type T not exactly number.

Numeric

Numeric

📘 definition: number | bigint.

Zero

📘 definition: 0 | 0n

Integer<N, Then = N, Else = never>

↪️ parse: is integer.

IsInteger<N, Then = true, Else = false>

⭕ predicate: is integer.

NotInteger<N, Then = N, Else = never>

↪️ parse: is not integer.

IsNotInteger<N, Then = true, Else = false>

⭕ predicate: is not integer.

IsWhole<N, Then = true, Else = false>

💀⭕ deprecated, predicate: is integer. Use IsInteger instead.

Negative<N, Then = N, Else = never>

↪️ parse: is negative.

IsNegative<N, Then = true, Else = false>

⭕ predicate: is negative.

NonNegative<N, Then = N, Else = never>

↪️ parse: is not negative.

IsNonNegative<N, Then = N, Else = never>

⭕ predicate: is not negative.

Positive<N, Then = N, Else = never>

↪️ parse: is positive.

IsPositive<N, Then = true, Else = false>

⭕ predicate: is positive.

NotPositive<N, Then = N, Else = never>

↪️ parse: is not positive.

IsNotPositive<N, Then = true, Else = false>

⭕ predicate: is not positive.

Object

filterKey()

🔨 utilities: type adjusted filter by key.

findKey()

🔨 utilities: type adjusted find by key.

forEachKey()

🔨 utilities: type adjusted for each by key.

HasKey<T, K>

🔨 utilities: predicate type checking T has key K.

hasKey()

🔨 utilities: function of HasKey.

IsRecord<T>

🔨 utilities: logical predicate for Record.

KeysWithDiffTypes<A, B>

🔨 utilities: gets the keys common in A and B but with different value type.

mapKey()

🔨 utilities: type adjusted map by key.

RecordValue<R>

🔨 utilities: gets the value type Tfrom Record<any, T>

Video.

reduceByKey()

🔨 utilities: type adjusted reduce by key.

someKey()

🔨 utilities: type adjusted some by key.

SpreadRecord<A, B>

🔨 utilities: type for {...a, ...b} when both a and b are Record
for array, just do [...A, ...B].

Promise

AwaitedProp<T, V>

🔨 utilities: Awaited on specified props P in T.

isPromise<R>(subject: any)

🔨 utilities: isPromise() type guard.

MaybePromise<T>

🔨 utilities: Alias of T | Promise<T>.

PromiseValue<P>

🔨 utilities: Gets the type within the Promise.

PromiseValueMerge<P1, P2, ...P9>

🔨 utilities: Merge the values of multiple promises.

mapSeries()

🔨 utilities: Similar to bluebird.mapSeries() but works with async/await.

transformMaybePromise(value, transformer)

🔨 utilities: Apply the transformer to the value.
It is also exported under MaybePromise.transform().

String

StringType<T>

↪️ parse: is string.

IsString<T>

⭕ predicate: is string.

NotStringType<T>

↪️ parse: is not string.

IsNotString<T>

⭕ predicate: is not string.

Symbol

SymbolType<T>

↪️ parse: is symbol.

IsSymbol<T>

⭕ predicate: is symbol.

NotSymbolType<T>

↪️ parse: is not symbol.

IsNotSymbol<T>

⭕ predicate: is not symbol.

Tuple

• 🌪️ TupleType: Filter T to ensure it is a tuple, excluding array.
• 🎭 IsTuple: Validate that T is a tuple, excluding array.
• 🌪️ NotTupleType: Filter T to ensure it is not a tuple, excluding array.
• 🎭 IsNotTuple: Validate that T is not a tuple, excluding array.
• ⚗️🔢CommonPropKeys: Gets the common property keys of the elements in tuple T.
• ⚗️💀CommonKeys: Deprecated. Please use CommonPropKeys instead.
• ⚗️🔢DropFirst: Drops the first entry in the tupleT.
• ⚗️🔢DropLast: Drops the last entry in the tupleT.

CreateTuple<L, T>

🔨 utilities: creates tuple<T> with L number of elements.

drop(array, value)

🔨 utilities: drop a particular value from an array.

DropMatch<A, Criteria>

🔨 utilities: drops entries matching Criteria in array or tuple A.

DropUndefined<A>

🔨 utilities: drop undefined entries from array of tuple A.

Undefined

UndefinedType<T>

↪️ parse: T === undefined.

IsUndefined<T>

⭕ predicate: T === undefined

NotUndefinedType<T>

↪️ parse: T !== undefined.

IsNotUndefined<T>

⭕ predicate: T !== undefined

unknown

UnknownType<T>

↪️ parse: T === unknown.

IsUnknown<T>

⭕ predicate: T === unknown

NotUnknownType<T>

↪️ parse: T !== unknown.

IsNotUnknown<T>

⭕ predicate: T !== unknown

void

VoidType<T>

↪️ parse: T === void.

IsVoid<T>

⭕ predicate: T === void

NotVoidType<T>

↪️ parse: T !== void.

IsNotVoid<T>

⭕ predicate: T !== void

Testing Utilities

type-plus provides some testing utilities to help you test your types.

One of the key utilities is testType.

import { testType } from 'type-plus'

testType.any<T>(true) // T is `any`
testType.equal<A, B>(true) // A is equal to B
testType.never<T>(false) // T is not `never`
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You can learn more about them in the docs.

Constant Types

KeyTypes

📘 definition: type of all keys.

PrimitiveTypes

📘 definition: all primitive types, including Function, symbol, and bigint.

ComposableTypes

📘 definition: Types that can contain custom properties. i.e. object, array, function.

NonComposableTypes

📘 definition: Types that cannot contain custom properties. i.e. not composable.

JSON Support

JSONPrimitive

📘 definition: primitive types valid in JSON

JSONObject

📘 definition: JSON object

JSONArray

📘 definition: JSON array

JSONTypes

📘 definition: all JSON compatible types.

JSONTypes.get<T>(obj, ...props)

🔨 utilities: get a cast value in JSON

import { JSONTypes } from 'type-plus'

const someJson: JSONTypes = { a: { b: ['z', { c: 'miku' }]}}

JSONTypes.get<string>(someJson, 'a', 'b', 1, 'c') // miku
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Type manipulation

ANotB<A, B>

🔨 utilities: get object with properties in A and not in B, including properties with a different value type.

BNotA<A, B>

🔨 utilities: flip of ANotB

as<T>(subject)

🔨 utilities: assert subject as T. Avoid ASI issues such as ;(x as T).abc

asAny(subject)

🔨 utilities: assert subject as any. Avoid ASI issue such as ;(x as any).abc

EitherAnd<A, B, [C, D]>

💀🔨 deprecated,utilities: Renamed to EitherOrBoth. Combines 2 to 4 types as A | B | (A & B).

This is useful for combining options.

EitherOrBoth<A, B, [C, D]>

🔨 utilities: combines 2 to 4 types as A | B | (A & B).

This is useful for combining options video.

Except<T, K>

💀🔨 deprecated,utilities: same as Omit<T, K>.

ExcludePropType<T, U>

🔨 utilities: excludes type U from properties in T.

KeyofOptional<T>

🔨 utilities: keyof that works with Record<any, any> | undefined.

KnownKeys<T>

🔨 utilities: extract known (defined) keys from type T.

LeftJoin<A, B>

🔨 utilities: left join A with B

NonNull<T>

🔨 utilities: remove null

NonNullable<T> (built-in)

🔨 utilities: adjust the type not to nullable

NonUndefined<T>

🔨 utilities: remove undefined

Omit<T, K>

🔨 utilities: From T, pick a set of properties whose keys are not in the union K. This is the opposite of Pick<T, K>.

OptionalKeys<T>

🔨 utilities: gets keys of optional properties in T.

PartialExcept<T, U>

💀🔨 deprecated,utilities: same as PartialOmit<T, U>.

PartialOmit<T, U>

🔨 utilities: makes the properties not specified in U becomes optional.

PartialPick<T, U>

🔨 utilities: makes the properties specified in U becomes optional.

Pick<T, K>

🔨 utilities: pick properties K from T. Works with unions.

RecursivePartial<T>

🔨 utilities: make type T optional recursively.

RecursiveRequired<T>

🔨 utilities: make type T required recursively.

ReplaceProperty<T, K, V>

🔨 utilities: replace property K in T with V.

RequiredKeys<T>

🔨 utilities: gets keys of required properties in T.

RequiredPick<T, U>

🔨 utilities: makes the properties specified in U become required.

RequiredExcept<T, U>

🔨 utilities: makes the properties not specified in U become required.

RecursiveIntersect<T, U>

🔨 utilities: intersect type U onto T recursively.

ValueOf<T>

🔨 utilities: type of the value of the properties of T.

Widen<T>

🔨 utilities: widen literal types.

PropType

💀 ...no helper type for this. Just do YourType['propName'].

Type Predicates

Type predicates are type alias that returns true or false. They can be used to compose complex types.

HasKey<T, K>

🔨 utilities: predicate type checking T has key K.

IsAny<T>

🔨 utilities: T === any.

IsBoolean<T>

🔨 utilities: check for boolean, but not for true nor false.

IsDisjoint<A, B>

🔨 utilities: is A and B is a disjoint set.

IsEmptyObject<T>

🔨 utilities: is T === {}.

IsLiteral<T>

🔨 utilities: is T a literal type (literal string or number).

Logical

If<Condition, Then = true, Else = false>

🔨 utilities: if statement

And<A, B, Then = true, Else = false>

🔨 utilities: logical AND

Or<A, B, Then = true, Else = false>

🔨 utilities: logical OR

Xor<A, B, Then = true, Else = false>

🔨 utilities: logical XOR

Not<X, Then = true, Else = false>

🔨 utilities: logical NOT

Note that these types work correctly with the boolean type. e.g.:

type R = And<boolean, true> // boolean
type R = Not<boolean>       // boolean`
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There is a problem with generic distribution: https://github.com/microsoft/TypeScript/issues/41053 So you may encounter some weird behavior if your logic is complex.

Math

The math types in type-plus works with most numeric types.

It works with number and bigint, positive and negative number, including floating point numbers.

It will cast the type between number and bigint if needed.

Abs<N, Fail = never>

🔨 utilities: Abs(N).

Max<A, B, Fail = never>

🔨 utilities: max(A, B)

GreaterThan<A, B>

🔨 utilities: A > B.

Add<A, B>

🔨 utilities: A + B.

Subtract<A, B>

🔨 utilities: A > B.

Increment<A>

🔨 utilities: alias of Add<A, 1>.

Decrement<A>

🔨 utilities: alias of Subtract<A, 1>.

Multiply<A, B

🔨 utilities: A * B.

Utility Functions

amend(subject)...

🔨 utilities: amend subject as union or intersect of T.

facade(subject, ...props)

🔨 utilities: create a facade of subject.

getField(subject, key, defaultValue)

🔨 utilities: get a field from a subject. Works against nullable and optional subject.

hasKey()

🔨 utilities: function of HasKey.

hasProperty(value, prop)

🔨 utilities: assert value has property prop. This will pick the correct union type.

isConstructor(subject)

🔨 utilities: type guard subject is a constructor.

isSystemError(code, err)

🔨 utilities: type guard err with NodeJS error code.

omit(obj, ...props)

🔨 utilities: omit properties from obj.

pick(obj, ...props)

🔨 utilities: pick properties from obj.

record<K, V>(value?)

🔨 utilities: create a Record<K, V> without extra object prototype.

record<R>(value?)

🔨 utilities: create a record R (e.g. { a: number }) without extra object prototype.

required(...)

🔨 utilities: merge options and remove Partial<T>. From unpartial

requiredDeep(...)

🔨 utilities: merge options deeply and remove Partial<T>. From unpartial

split(target, ...splitters)

🔨 utilities: split one object into multiple objects.

stub<T>(value)

🔨 utilities: stub a particular type T.

stub.build<T>(init?)

🔨 utilities: build a stub for particular type T.

typeOverrideIncompatible<T>()

🔨 utilities: override only the incompatible portion between two types.

type A =  {
  foo: boolean,
  bar: string,
  baz: string
}

const overrider = typeOverrideIncompatible<A>()
const source = {
  foo: 1,
  bar: 'bar',
  baz: 'baz'
}

// only the `foo` property is available to override.
overrider(source, { foo: !!source.foo })
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unpartial()

🔨 utilities: merge options and remove Partial<T> values. From unpartial

context()

🔨 utilities: a context builder.

This is useful to build context for functional programming. It is a sync version of the AsyncContext from async-fp.

import { context } from 'type-plus'

// { a: 1, b: 2 }
const ctx = context({ a: 1 })
  .extend(c => ({ b: c.a + 1 }))
  .build()
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Nominal Types

The TypeScript type system is structural.

In some cases, we want to express a type with nominal behavior. type-plus provides two kinds of nominal types: Brand and Flavor.

Brand<B, T>:

brand(type, subject?):

Branded nominal type is the stronger nominal type of the two. It disallows unbranded type assigned to it:

const a = brand('a', { a: 1 })
const b = { a: 1 }
a = b // error
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subject can be any type, from primitive to strings to objects.

brand(type):

If you do not provide subject, brand(type) will return a brand creator, so that you can use it to create multiple branded values:

const nike = brand('nike')
const shirt = nike('shirt')
const socks = nike('socks')
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Flavor<F, T>:

flavor(type, subject?):

The key difference between Flavor and Brand is that unflavored type can be assigned to Flavor:

let f = flavor('orange', 'soda')
f = 'mist' // ok
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Also, Brand of the same name can be assigned to Flavor, but Flavor of the same name cannot be assigned to Brand.

nominalMatch(a, b):

🔨 utilities: compare if the two values are nominally equal.

Works with both Brand and Flavor.

const b1 = brand('x', 1)
const b2 = brand('y', 1)

nominalMatch(b1, b2) // false
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Functional Types

ChainFn<T>: T

🔨 utilities: chain function that returns the input type.

compose(...fns): F

🔨 utilities: compose functions

Attribution

Some code in this library is created by other people in the TypeScript community. I'm merely adding them in and maybe making some adjustments. Whenever possible, I add attribution to the person who created those codes in the file.

Useful Tips

https://github.com/microsoft/TypeScript/wiki/Performance

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