type-plus

Provides additional types and type adjusted utilities for TypeScript.

Feature Highlights

• Type assertions
• Type Utilities
• Nominal Types
• Functional Types

Installation

npm install type-plus
// or
yarn add type-plus

Type Assertions

Type assertion is one of the main features of type-plus.

There are 5 kinds of type assertions:

• runtime: validates during runtime.
• immediate: validates at compile time.
• type guard: User-defined type guard functions (if (isBool(s))) introduced in TypeScript 1.6.
• assertion function: assertion functions (assertIsBool(a)) introduced in TypeScript 3.7.
• logical: functions or generic types that returns true or false type to be used in type level programming.
• filter: generic type that returns never if the test fails.

Here are the type assertions provided in type-plus. Use the one that fits your specific needs.

assertType<T>(subject):

✔️ immediate

It ensures subject satisfies T. It is similar to const x: T = subject without introducing an unused variable. You need to specify T for it to work.

assertType<T>(subject, validator):

assertType<T>(subject, Class):

✔️ assertion function, runtime

These overloads of assertType allow you to specify a validator. With these overloads, subject can be unknown or any.

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')

The message beautification is provided by tersify.

assertType.isUndefined(subject):

assertType.isNull(subject):

assertType.isNumber(subject):

assertType.isBoolean(subject):

assertType.isTrue(subject):

assertType.isFalse(subject):

assertType.isString(subject):

assertType.isFunction(subject):

assertType.isConstructor(subject):

assertType.isError(subject):

✔️ immediate, assertion function, runtime

Compiler and runtime assertion with type narrowing from any. They assert the type of subject is that specific type. i.e. union type will fail at type level:

const s: number | undefined = undefined
assertType.isUndefined(s) // TypeScript complains

They accept any and will be narrowed to the specific type.

const s: any = undefined
assertType.isUndefined(s)
s // type is undefined

assertType.isNever(subject):

✔️ immediate

Check if the subject type is never. This function is not very useful in actual code as TypeScript will indicate the error. But it can be useful when writing tests for types.

This is useful for variables. For type level only check, do the following:

assertType.isTrue(true as Equal<YourType, never>)

assertType.noUndefined(subject):

assertType.noNull(subject):

assertType.noNumber(subject):

assertType.noBoolean(subject):

assertType.noTrue(subject):

assertType.noFalse(subject):

assertType.noString(subject):

assertType.noFunction(subject):

assertType.noError(subject):

✔️ immediate, runtime

Compiler and runtime assertion. Assert subject type does not contain the specific type. Work against unions.

const s: number | undefined = 1
assertType.noUndefined(s) // TypeScript complains

They accept subject with type any or unknown, the assertion will happen in runtime to ensure subject is the specific type.

assertType.as<T>(subject):

✔️ immediate

Assert subject as T inline. This is useful to help TypeScript to adjust the type on the fly.

let s: number | undefined = 1
assertType.as<1>(s) // `s` type is now `1`

isType<T>(subject: T):

✔️ immediate

It ensures subject satisfies T. It is identical to assertType<T>(subject: T). You need to specify T.

isType<T>(subject, validator):

isType<T>(subject, Class):

isType.t<T>(subject?: T):

✔️ immediate, runtime

It can be used as type check: isType.t<Equal<A, B>>(), or value type check: isType.t(valueTypeIsTrue). It returns true when passes (which is the only case when used in TypeScript).

isType.f<T>(subject?: T):

✔️ immediate, runtime

It can be used as type check: isType.f<Equal<A, B>>(), or value type check: isType.f(valueTypeIsFalse). It returns true when passes (which is the only case when used in TypeScript).

isType.equal<true|false, A, B>():

✔️ immediate

Slightly easier to use then isType.t<>() and isType.f<>(), when doing type-level only equality comparison as you don't have to import Equal<>.

✔️ type guard, runtime

These overloads of isType allow you to specify a validator. With these overloads, subject can be unknown or any.

Equal<A, B>: IsEqual<A, B>:

✔️ logical

Check if A and B are the same.

NotEqual<A, B>: IsNotEqual<A, B>:

✔️ logical

Check if A and B are not the same.

IsExtend<A, B>: IsNotExtend<A, B>:

✔️ logical

Check if A extends or not extends B.

Extendable<A, B>: NotExtendable<A, B>:

✔️ filter

Check if A extends or not extends B.

IsAssign<A, B>: CanAssign<A, B>:

✔️ logical

Check if A can be assigned 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 }>)

canAssign<T>(): (subject) => true:

✔️ immediate, logical

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

const isConfig = canAssign<{ a: string }>()
assertType.isTrue(isConfig({ a: 'a' }))

canAssign<T>(false): (subject) => false:

✔️ immediate, logical

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

Type Utilities

type-plus also provides additional type utilities. These utilities include utility types and type-adjusted functions.

Note that most predicate types (such as IsAny<>) have a Then and Else that you can override.

e.g.:

type Yes = IsAny<any, 'yes', 'no'> // 'yes'
type No = IsAny<1, 'yes', 'no'> // 'no'

Array function

• CommonPropKeys<A>: gets common keys inside the records in the array A (deprecate CommonKeys).
• Concat<A, B>: [...A, ...B].
• CreateTuple<L, T>: Creates Tuple<T> with L number of elements.
• drop(array, value): drop a particular value from an array.
• DropFirst<A>: drops the first value type of A.
• DropLast<A>: drops the last value type of A.
• DropMatch<A, Criteria>: drops entries matching Criteria in array or tuple A.
• DropUndefined<A>: drop undefined entries from array of tuple A.
• Filter<A, Criteria>: filter the array or tuple A, keeping entries satisfying Criteria. Deprecated. Renaming to KeepMatch
• FindFirst<A, Criteria>: gets the first type satisfying Criteria.
• FindLast<A, Criteria>: gets the last type satisfying Criteria.
• Head<A>: gets the first entry in the array.
• IntersectOfProps<A, K>: gets the intersect of A[K] types (deprecate MapToProp)
• IsArray<T>: logical predicate for Array.
• KeepMatch<A, Criteria>: keeps entries satisfying Criteria in array or tuple A.
• Last<A>: gets the last type of array or tuple.
• literalArray(...entries): return an array whose items are restricted to the provided literals.
• PadLeft<A, Total, PadWith>: pads A with PadWith if the length of A is less than L.
• reduceWhile(): reduce() with predicate for early termination.
  A simple version of the same function in the ramda package.
• Reverse<A>: reverses the order of A.
• Some<A, Criteria>: true if some elements in A matches Criteria.
• Tail<A>: Gets the types of a tuple except the first entry.
• UnionOfProps<A, K>: gets the union of A[K] types (deprecate PropUnion).
• UnionOfValues<A>: gets the union of value types in A (deprecate ArrayValue).

Constant Types

• KeyTypes: type of all keys.
• PrimitiveTypes: all primitive types, including Function, symbol, and bigint.
• ComposableTypes: Types that can contain custom properties. i.e. object, array, function.
• NonComposableTypes: Types that cannot contain custom properties. i.e. not composable.

JSON Support

• JSONPrimitive: primitive types valid in JSON
• JSONObject: JSON object
• JSONArray: JSON array
• JSONTypes: all JSON compatible types.
• JSONTypes.get<T>(obj, ...props): 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

Object Key functions

• filterKey(): type adjusted filter by key.
• findKey(): type adjusted find by key.
• forEachKey(): type adjusted for each by key.
• HasKey<T, K>: predicate type checking T has key K.
• hasKey(): function of HasKey.
• IsRecord<T>: logical predicate for Record.
• KeysWithDiffTypes<A, B>: gets the keys common in A and B but with different value type.
• mapKey(): type adjusted map by key.
• reduceByKey(): type adjusted reduce by key.
• someKey(): type adjusted some by key.
• SpreadRecord<A, B>: type for {...a, ...b} when both a and b are Record
  for array, just do [...A, ...B].

Promise function

• isPromise<R>(subject: any): isPromise() type guard.
• PromiseValue<P>: Gets the type within the Promise.
• PromiseValueMerge<P1, P2, ...P9>: Merge the values of multiple promises.
• mapSeries(): Similar to bluebird.mapSeries() but works with async/await.

Type manipulation

• ANotB<A, B>: get object with properties in A and not in B, including properties with a different value type.
• BNotA<A, B>: flip of ANotB
• as<T>(subject): assert subject as T. Avoid ASI issues such as ;(x as T).abc
• asAny(subject): assert subject as any. Avoid ASI issue such as ;(x as any).abc
• Except<T, K>: Deprecated. Same as Omit<T, K>.
• ExcludePropType<T, U>: excludes type U from properties in T.
• KeyofOptional<T>: keyof that works with Record<any, any> | undefined.
• KnownKeys<T>: extract known (defined) keys from type T.
• LeftJoin<A, B>: left join A with B
• NonNull<T>: remove null
• NonUndefined<T>: remove undefined
• Omit<T, K>: 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>: gets keys of optional properties in T.
• PartialExcept<T, U>: Deprecated. Same as PartialOmit<T, U>.
• PartialOmit<T, U>: makes the properties not specified in U becomes optional.
• PartialPick<T, U>: makes the properties specified in U becomes optional.
• Pick<T, K>: pick properties K from T. Works with unions.
• RecursivePartial<T>: make type T optional recursively.
• RecursiveRequired<T>: make type T required recursively.
• ReplaceProperty<T, K, V>: replace property K in T with V.
• RequiredKeys<T>: gets keys of required properties in T.
• RequiredPick<T, U>: makes the properties specified in U become required.
• RequiredExcept<T, U>: makes the properties not specified in U become required.
• RecursiveIntersect<T, U>: intersect type U onto T recursively.
• ValueOf<T>: type of the value of the properties of T.
• Widen<T>: 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>: predicate type checking T has key K.
• IsAny<T>: T === any.
• IsBoolean<T>: check for boolean, but not for true nor false.
• IsDisjoint<A, B>: is A and B is a disjoint set.
• IsEmptyObject<T>: is T === {}.
• IsLiteral<T>: is T a literal type (literal string or number).

Logical

• If<Condition, Then = true, Else = false>: if statement.
• And<A, B>: logical AND.
• Or<A, B>: logical OR.
• Xor<A, B>: logical XOR.
• Not<X>: logical NOT.

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

• And<boolean, true> -> boolean
• Not<boolean> -> boolean

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

• Abs<N, Fail=never>: Abs(N), Abs<number> returns Fail.
• IsPositive<N>: is N a positive number literal. IsPositive<number> returns false.
• IsWhole<N>: is N a whole number literal. IsWhole<number> returns false.
• Max<A, B, Fail=never>: max(A, B), for whole number, Fail otherwise.
• GreaterThan<A, B, Fail=never>: A > B for whole numbers, Fail otherwise.

Arithmetic

• Add<A, B, Fail=never>: A + B for positive and whole numbers, Fail otherwise.
• Subtract<A, B, Fail=never>: A - B for positive and whole numbers, Fail otherwise.
• Increment<A, Fail=never>: alias of Add<A, 1, Fail>.
• Decrement<A, Fail=never>: alias of Subtract<A, 1, Fail>.

Utility Functions

• amend(subject)...: amend subject as union or intersect of T.
• facade(subject, ...props): create a facade of subject.
• getField(subject, key, defaultValue): get a field from a subject. Works against nullable and optional subject.
• hasKey(): function of HasKey.
• hasProperty(value, prop): assert value has property prop. This will pick the correct union type.
• isConstructor(subject): type guard subject is a constructor.
• isSystemError(code, err): type guard err with NodeJS error code.
• omit(obj, ...props): omit properties from obj.
• pick(obj, ...props): pick properties from obj.
• record<K, V>(value?): create a Record<K, V> without extra object prototype.
• required(...): merge options and removing Partial<T>. From unpartial
• requiredDeep(...): merge options deeply and removing Partial<T>. From unpartial
• split(target, ...splitters): split one object into multiple objects.
• stub<T>(value): stub a particular type T.
• stub.build<T>(init?): build a stub for particular type T.
• typeOverrideIncompatible<T>(): 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 })

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

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')

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

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):

nominalMatch() can be used to compare Brand or Flavor.

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

nominalMatch(b1, b2) // false

Functional Types

• ChainFn<T>: T: chain function that returns the input type.
• compose(...fns): F: 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.

Similar projects

• ts-essentials, all essential TypeScript types in one place.
• ts-toolbelt, a more mature type lib.
• type-fest, a collection of essential TypeScript types.
• type-zoo, a modest type lib usable today.
• typepark, a new type collection offering tuple manipulation and Pipe.
• typelevel-ts, a type lib by @gcanti, author of several FP libraries in TS.
• typical, a playground of type-level operations for TypeScript.

Contribute

# after fork and clone
npm install

# begin making changes
git checkout -b <branch>
npm run watch

# after making change(s)
git commit -m "<commit message>"
git push

# create PR