Reverse-Mapping for String Enums
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
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
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.