Reverse-Mapping for String Enums

Question

I wanted to use string enums in typescript but I can't see a support for reversed mapping in it. I have an enum like this:

enum Mode {
    Silent = "Silent",
    Normal = "Normal",
    Deleted = "Deleted"
}

and I need to use it like this:

let modeStr: string;
let mode: Mode = Mode[modeStr];

and yes I don't know what is it there in modeStr string and I need it parsed to the enum or a fail at parsing in runtime if the string is not presented in the enum definition. How can I do that as neat as it can be? thanks in advance

Answer 1

We can make the Mode to be a type and a value at the same type.

type Mode = string;
let Mode = {
    Silent: "Silent",
    Normal: "Normal",
    Deleted: "Deleted"
}

let modeStr: string = "Silent";
let mode: Mode;

mode = Mode[modeStr]; // Silent
mode = Mode.Normal; // Normal
mode = "Deleted"; // Deleted
mode = Mode["unknown"]; // undefined
mode = "invalid"; // "invalid"

A more strict version:

type Mode = "Silent" | "Normal" | "Deleted";
const Mode = {
    get Silent(): Mode { return "Silent"; },
    get Normal(): Mode { return "Normal"; },
    get Deleted(): Mode { return "Deleted"; }
}

let modeStr: string = "Silent";
let mode: Mode;

mode = Mode[modeStr]; // Silent
mode = Mode.Normal; // Normal
mode = "Deleted"; // Deleted
mode = Mode["unknown"]; // undefined
//mode = "invalid"; // Error

String Enum as this answer:

enum Mode {
    Silent = <any>"Silent",
    Normal = <any>"Normal",
    Deleted = <any>"Deleted"
}

let modeStr: string = "Silent";
let mode: Mode;

mode = Mode[modeStr]; // Silent
mode = Mode.Normal; // Normal
//mode = "Deleted"; // Error
mode = Mode["unknown"]; // undefined

Answer 2

The cleanest way I found so far is to make a secondary map:

let reverseMode = new Map<string, Mode>();
Object.keys(Mode).forEach((mode: Mode) => {
    const modeValue: string = Mode[<any>mode];
    reverseMode.set(modeValue, mode);
});

Thus you can do let mode: Mode = reverseMode.get('Silent');

Advantages: no need to repeat the values, provides a way to enumerate the enum, keeps TSLint happy...

Edit: I originally write Mode[mode] but then TS might throw the error TS7015 on this line, so I added the cast.

Answer 3

If you're ok with using Proxies, I made a more generic & compact version:

stringEnum.ts

export type StringEnum<T extends string> = {[K in T]: K}
const proxy = new Proxy({}, {
  get(target, property) {
    return property;
  }
})
export default function stringEnum<T extends string>(): StringEnum<T> {
  return proxy as StringEnum<T>;
}

Usage:

import stringEnum from './stringEnum';
type Mode = "Silent" | "Normal" | "Deleted";
const Mode = stringEnum<Mode>();