Utility type to introspect an interface

InterfaceIntrospection.ts

// Author: Flavio

type __PairList<T> = {
    [Key in keyof T]: {
        readonly key: Key
        readonly value: T[Key]
    }
}

type __Pair<T> = __PairList<T>[keyof T]

type AnyPair = __Pair<any>

// --------------------------
// utils - interface Parser
// --------------------------

type TKey = string | number | symbol

/** Final operations over a Pair */
type __GetKeys<T extends {readonly key: any}> = T['key']
type __GetValue<T extends {readonly value: any}> = T['value']

/** Result is a Pair */
type __FilterKeysByValue<T, Value extends T[keyof T]> = Extract<__Pair<T>, {readonly value: Value}>

type GetKeys<T> = __GetKeys<__Pair<T>>
type GetValues<T> = __GetValue<__Pair<T>>


type FilterKeysByValue<T, Value extends T[keyof T]> = __GetKeys<__FilterKeysByValue<T, Value>>

// ========================================
//  informal test
// ========================================


const Test01 = {
    // space
    'milimeter': 'Space',
    'meter': 'Space',

    // time
    'second': 'Time',
    'minute': 'Time',

} as const


type Test01 = typeof Test01

// put cursor over types bellow to see type inference result (use vscode with Typescript Plugin)

type Get_Keys = GetKeys<Test01> // OK "milimeter" | "meter" | "second" | "minute"
type Get_Values = GetValues<Test01> // OK  "Space" | "Time"
type Get_Only_Space = FilterKeysByValue<Test01,'Space'> // OK "milimeter" | "meter"



// ---------
// Test 02
// ---------


const Test02 = {
    // space
    'milimeter': {name: 'Oi', number: 10},
    'meter': 'Space',

    // time
    'second': 'Time',
    'minute': {name: 'Oi', number: 20},

} as const


type Test02 = typeof Test02


type _PairList = __PairList<Test02> // OK
type _Get_Keys = GetKeys<Test02> // OK "milimeter" | "meter" | "second" | "minute"
type _Get_Values = GetValues<Test02> // OK
type _Get_Only_Space = FilterKeysByValue<Test02, {
    readonly name: "Oi";
    readonly number: 10;
}> // // OK "milimeter"