type-plus

type-plus

Provides additional types and type adjusted utilities for TypeScript.

Feature Highlights

• Runtime type checker
• Type assertion
• Nominal Types
• Type Utilities

Installation

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

Runtime type checker

Bringing the power of TypeScript to JavaScript runtime. This version of type checker has some limitation due to recursion. For the time being, please use another excellent type checking library such as zod.

const eslintConfig = T.object.create({
  env: O.object.create({
    es6: O.boolean
  }),
  parseOptions: O.object.create({
    ecmaVersion: O.number.list(3, 5, 6, 7, 8, 9, 10, 11, 12),
    sourceType: O.string.list('script', 'module'),
    ecmaFeatures: O.object.create({
      globalReturn: O.boolean,
      impliedStrict: O.boolean,
      jsx: O.boolean
    })
  }),
  ...
})

const config: unknown = require('.eslintrc.json')
if (T.satisfy(eslintConfig, config)) {
  // `config` is typed here
  config.parseOptions?.ecmaVersion // 3 | 5 | 6 | 7 | 8 | 9 | 10 | 11 | 12
}
else {
  console.error(T.satisfy.getReport())
}

All type checker functionalities are exposed as types (alias to T). In addition, O and R are exposed to make it easier to access optional types and required types respectively.

Types supported: any, array, boolean, null, number, object, record, string, symbol, tuple, undefined, union, unknown.

i.e., most of the basic types are supported except bigint. It is left out for backward compatibility reasons.

You can use one of the three functions to perform type check:

satisfy(type, subject): A loose type check that permits extra elements in Tuple and properties in Object.

conform(type, subject): A strick type check that does not allow extra elements in Tuple and properties in Object.

check(options, type, subject): A general form of satisfy() and conform().

Type Assertion

Besides the runtime type checker, type-plus also provides a few other ways to do type assertions.

There are actually at least 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 types 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 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 allows 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: any = 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 accepts 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 type.

This is useful for variable. 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 accepts subject with type any or unknown, assertion will happens in runtime to ensure subject is the specific type.

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):

✔️ type guard, runtime

These overloads of isType allows 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

Nominal Type

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 return the input type.

Type Utilities

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

Array function

• ArrayValue<A>: gets the value type of the array A.
• CommonKeys<A>: gets common keys inside the records in the array A.
• First<A, Criteria>: gets first entry satisfying Criteria.
• Head<A>: gets the first entry in the array.
• IsArray<T>: logical predicate for Array.
• literalArray(...entries): return an array those items are restricted to the provided literals.
• MapToProp<A, K>: Map A: Array<E> to Array<E[K]>.
• reduceWhile(): reduce() with predicate for early termination.
  A simple version of the same function in the ramda package.
• Tail<A>: gets the remaining entries in the array except the first.

Constant Types

• JSONTypes: all JSON compatible types.
• KeyTypes: type of all keys.
• PrimitiveTypes: all primitive types, including Function, symbol, and bigint.

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.
• reduceKey(): 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 different value type.
• BNotA<A, B>: flip of ANotB
• 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
• 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 union.
• 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 becomes required.
• RequiredExcept<T, U>: makes the properties not specified in U becomes 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.
• IsDisjoint<A, B>: is A and B is a disjoint set.

Logical

• 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 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.

Utility Functions

• 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.
• pick(obj, ...props): pick properties from obj.
• omit(obj, ...props): omit properties from obj.
• required(...): merge options and removing Partial<T>. From unpartial
• requiredDeep(...): merge options deeply and removing Partial<T>. From unpartial
• 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 })

Attribution

Some of the code in this library are created by other people in the TypeScript community. I merely adding them in and may be making some adjustments. When ever possible, I add attribution to the person who created those code in the file.

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