Example of getting enum values by a string key in TypeScript

ts-enum-from-string.ts

interface AxiosResponse {
    data?: any;
}

enum ErrorKey {
    ServerError = 1,
    WeirdException,
    UnknownError,
}


type ErrorKeyString = keyof typeof ErrorKey;

function errorKeyFromString(response: AxiosResponse): ErrorKey | null {
    if (!("data" in response && "errorKey" in response.data)) {
        return null;
    }
    // this works because data has any type
    const keyString: ErrorKeyString = response.data.errorKey;
    return ErrorKey[keyString] || null;
}

function isErrorKeyString(maybeKey: string): maybeKey is ErrorKeyString {
    return !!maybeKey && ErrorKey[maybeKey as ErrorKeyString] !== undefined;
}

function errorKeyFromString2(response: AxiosResponse): ErrorKey | null {
    const hasErrorKey = "data" in response && "errorKey" in response.data;
    if (!hasErrorKey) {
        return null;
    }
    const keyString = response.data.errorKey;
    // isErrorKeyString acts as a type predicate here, so we don't need to define the type
    // ref: https://www.typescriptlang.org/docs/handbook/advanced-types.html#using-type-predicates
    return isErrorKeyString(keyString) ? ErrorKey[keyString] : null;
}

function demo(keyString: string | undefined) {
    const response: AxiosResponse = {
        data: {
            errorKey: keyString,
            parameters: [],
        }
    }
    // this is actually the enum ordinal at runtime
    const keyOrdinal = errorKeyFromString(response);
    const keyOrdinal2 = errorKeyFromString2(response);
    keyOrdinal == keyOrdinal2 || console.error('impl mismatch!');
    if (!keyOrdinal) {
        console.log(" ->", keyString, "is an unknown error key.");
    } else {
        // note that we could just use keyString here as well,
        // this is mainly for demonstrating how to get the key string
        // from the enum object/ordinal
        const keyName = ErrorKey[keyOrdinal];
        console.log(" ->", keyString, "was mapped to ordinal", keyOrdinal, "which is", keyName);
    }
}

demo("WeirdException");
demo("whatever");
demo(undefined);

// output:
//  -> WeirdException was mapped to ordinal 2 which is WeirdException
//  -> whatever is an unknown error key.
//  -> undefined is an unknown error key.