feat: add extractFunction()

README.md

@@ -301,7 +301,7 @@ type Yes = IsAny<any, 'yes', 'no'> // 'yes'
 type No = IsAny<1, 'yes', 'no'> // 'no'
 ```
 
-### Array function
+### Array Utilities
 
 - `CommonPropKeys<A>`: gets common keys inside the records in the array `A` (deprecate `CommonKeys`).
 - `Concat<A, B>`: `[...A, ...B]`.
@@ -552,10 +552,11 @@ const b2 = brand('y', 1)
 nominalMatch(b1, b2) // false
 ```
 
-## Function Types
+## Function Utilties
 
 - `AnyFunction<P, R>`: a generic type for any function
 - `ExtractFunction<F>`: extract the function signature from a type `F`.
+- `extractFunction(fn: F)`: adjust type of `fn` to its function signature only.
 
 ## Functional Types
 

ts/function/ExtractFunction.spec.ts

@@ -1,4 +1,4 @@
-import { ExtractFunction, isType } from '../index.js'
+import { ExtractFunction, extractFunction, isType } from '../index.js'
 
 it('gets the function itself if it is not composite', () => {
   type F = () => void
@@ -13,3 +13,13 @@ it('omits other props from the type', () => {
 
   isType.equal<true, () => void, R>()
 })
+
+describe(`${extractFunction.name}()`, () => {
+  it('adjust type to its function type', () => {
+    const fn = Object.assign(() => { }, { a: 1 })
+
+    const r = extractFunction(fn)
+    type R = typeof r
+    isType.equal<true, () => void, R>()
+  })
+})

ts/function/ExtractFunction.ts

@@ -5,3 +5,7 @@ import { AnyFunction } from './AnyFunction.js'
  */
 export type ExtractFunction<T extends AnyFunction> = T extends AnyFunction<infer P, infer R>
   ? (...args: P) => R : never
+
+export function extractFunction<T extends AnyFunction>(fn: T) {
+  return fn as any as ExtractFunction<T>
+}

ts/function/index.ts

@@ -1,2 +1,2 @@
 export type { AnyFunction } from './AnyFunction.js'
-export type { ExtractFunction } from './ExtractFunction.js'
+export * from './ExtractFunction.js'

ts/index.ts

@@ -4,7 +4,7 @@ export * from './array/index.js'
 export * from './assertion/index.js'
 export * from './class/index.js'
 export type { ComposableTypes, NonComposableTypes } from './ComposableTypes.js'
-export type { AnyFunction, ExtractFunction } from './function/index.js'
+export * from './function/index.js'
 export * from './functional/index.js'
 export type { JSONArray, JSONObject, JSONPrimitive, JSONTypes } from './JSONTypes.js'
 export type { Abs, Add, Decrement, Digit, DigitArray, GreaterThan, Increment, IsPositive, IsWhole, Max, Subtract } from './math/index.js'