Getting the enum key with the value string (reverse mapping) in TypeScript

I have an enum:

export enum ApiMessages {
    logged_ok = 'Logged OK',
    register_ok = 'Register OK'
}

I have a function with the enum as a parameter:

export function responseOK(message: ApiMessages, result ?: any): ApiResponse {
    return {
        "status": "ok",
        "code": 200,
        "messageId": ApiMessages[message], <-- KO TS7015
        "message": message,
        "result": result
    };
}

I am calling the function like that:

responseOK(ApiMessages.logged_ok, {user: userRes})

I am trying to return the enum key and the enum string value to the response but I get the TS error:

TS7015: Element implicitly has an 'any' type because index expression is not of type 'number'.

I have strict TypeScript config. Adding suppressImplicitAnyIndexErrors is not an option.

TypeScript version: 2.9.2

As described in the handbook:

Keep in mind that string enum members do not get a reverse mapping generated at all.

That means there is no simple reverse mapping in your case.

Workaround: Getting a reverse mapping for string enum members

To get the key of an enum member by its value, you have to iterate through the enum keys and compare the associated value with your target value.

function getEnumKeyByEnumValue(myEnum, enumValue) {
    let keys = Object.keys(myEnum).filter(x => myEnum[x] == enumValue);
    return keys.length > 0 ? keys[0] : null;
}

You can type this more strictly as follows (note that we can interpret our enum as an indexable type with key and value both being strings here):

function getEnumKeyByEnumValue<T extends {[index:string]:string}>(myEnum:T, enumValue:string):keyof T|null {
    let keys = Object.keys(myEnum).filter(x => myEnum[x] == enumValue);
    return keys.length > 0 ? keys[0] : null;
}

Some demo code follows. You can also see it in action on the TypeScript Playground

enum ApiMessages {
    logged_ok = 'Logged OK',
    register_ok = 'Register OK'
}

let exampleValue = ApiMessages.logged_ok;
let exampleKey = getEnumKeyByEnumValue(ApiMessages, exampleValue);

alert(`The value '${exampleValue}' has the key '${exampleKey}'`)

function getEnumKeyByEnumValue<T extends {[index:string]:string}>(myEnum:T, enumValue:string):keyof T|null {
    let keys = Object.keys(myEnum).filter(x => myEnum[x] == enumValue);
    return keys.length > 0 ? keys[0] : null;
}

Adding this into your responseOK() you end up with:

function responseOK(message: ApiMessages, result ?: any) {
    return {
        "status": "ok",
        "code": 200,
        "messageId": getEnumKeyByEnumValue(ApiMessages, message),
        "message": message,
        "result": result
    };
}