Enum as Parameter in TypeScript

Question

Isn\'t it possible to set the type of a parameter to an Enum? Like this:

private getRandomElementOfEnum(e : enum):string{
    var length:number = Object.         


        

Answer 1

Tested on TypeScript 3.9.7

Solution

type EnumTypeString =
    { [key in string]: TEnum | string; }

type EnumTypeNumber =
    { [key in string]: TEnum | number; }
    | { [key in number]: string; }

type EnumType =
    (TEnum extends string ? EnumTypeString : never)
    | (TEnum extends number ? EnumTypeNumber : never)

type EnumOf = TEnumType extends EnumType
    ? U
    : never

Usage

EnumType:

function forEachEnum(
    enumType: EnumType,
    callback: (value: TEnum, key: string) => boolean|void,
) {
    for (let key in enumType) {
        if (Object.prototype.hasOwnProperty.call(enumType, key) && isNaN(Number(key))) {
            const value = enumType[key] as any
            if (callback(value, key)) {
                return
            }
        }
    }
}

EnumOf:

function forEachEnum2(
    enumType: TEnumType,
    callback: (value: EnumOf, key: string) => boolean|void,
) {
    for (let key in enumType) {
        if (Object.prototype.hasOwnProperty.call(enumType, key) && isNaN(Number(key))) {
            const value = enumType[key] as any
            if (callback(value, key)) {
                return
            }
        }
    }
}

Tests

enum EnumAsString {
    Value1 = 'value 1',
    Value2 = 'value 2',
}

enum EnumAsNumber {
    Value1 = 1,
    Value2 = 2,
}

// Error
let sn: EnumType = EnumAsNumber

// Correct
let ns: EnumType = EnumAsString // I have not found a solution for the error here
let nn: EnumType = EnumAsNumber
let Nn: EnumType = EnumAsNumber
let ss: EnumType = EnumAsString
let Ss: EnumType = EnumAsString

forEachEnum(EnumAsString, value => {
    let e: EnumAsString = value
    let s: string = value
    let n: number = value // Error
})

forEachEnum(EnumAsNumber, value => {
    let e: EnumAsNumber = value
    let s: string = value // Error
    let n: number = value
})

forEachEnum2(EnumAsString, value => {
    let e: EnumAsString = value
    let s: string = value
    let n: number = value // Error
})

forEachEnum2(EnumAsNumber, value => {
    let e: EnumAsNumber = value
    let s: string = value // Error
    let n: number = value
})