typescript using keyof enum with generics

Question

i want to create an error code counter, but having issues with typescript using enums and generics.

this is the working version (without generics)

enum ErrorCode  {
    MX1 = 'MX1',
    MX2 = 'MX2'
}

type Codes = keyof typeof ErrorCode;
type ErrorCodeCounter = {
    [code in Codes]: number
}

const counter = <ErrorCodeCounter>{}
counter[ErrorCode.MX1] = 3
counter['randomCode'] = 3 // Valid TS error: 'randomCode' does not exist on type 'ErrorCodeCounter

How can we make we make a generic Counter interface, to be used as:

const counter = <Counter<ErrorCode>>{}
counter[ErrorCode.MX1] = 3

one way that comes to mind is

type Counter<T> = {
    [code in keyof T] : number
}

but that does not work. Any ideas how to make a generic version?

Note that replacing enum by interface works but i would prefer Enums over interface

interface ErrorCodeI {
    MS1: string;
    MS2: string;
}

type Counter<T> = {
    [code in keyof T] : number
}
const counter = <Counter<ErrorCodeI>>{}
counter['MS1'] = 3
counter['random'] = 3 // Valid TS error.

Titian Cernicova-Dragomir · Accepted Answer · 2020-05-20 11:30:10Z

6

You don't need keyof, the enum type itself is already the union of the enum elements you want to map over:

type Counter<T extends PropertyKey> = {
    [code in T] : number
}


enum ErrorCode  {
    MX1 = 'MX1',
    MX2 = 'MX2'
}

const counter = <Counter<ErrorCode>>{}
counter[ErrorCode.MX1] = 3
counter['randomCode'] = 3 //ERR

Playground Link

answered May 20, 2020 at 11:30

Titian Cernicova-Dragomir

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1 Comment

sbr Over a year ago

this is great. didn't know about PropertyKey and couldnt' find it in docs. please share any resource with more context around this if you are aware. thanks.

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