I have some emun like this
export enum Languages {
nl = 1,
fr = 2,
en = 3,
de = 4
}
and some const language ='de';
I only need to check does enum constraint const language, i know with array I can do
includes but how to check ENUM?
Also I know i can check like this
if (type in Languages) {
}
but this work is it number and not string
You can use Object.keys() in order to access the enum keys followed by a simple Array.prototype.includes():
enum Languages {
nl = 1,
fr = 2,
en = 3,
de = 4,
}
const language = "de";
console.log(Object.keys(Languages).includes(language)); //true
This works because an Enum, when transpiled into JavaScript, becomes nothing more than a simple object:
var Languages;
(function(Languages) {
Languages[(Languages["nl"] = 1)] = "nl";
Languages[(Languages["fr"] = 2)] = "fr";
Languages[(Languages["en"] = 3)] = "en";
Languages[(Languages["de"] = 4)] = "de";
})(Languages || (Languages = {}));
console.log(Languages);.as-console-wrapper {
min-height: 100%;
}
Let's do it in O(1) ...
Given generic invariant function -
function invariant(condition: unknown, message?: string): asserts condition {
if (condition) return
throw Error(`Invariant violation: ${message ?? "truthy value expected"}`)
}
Write invariantLanguage using keyof typeof Language -
function invariantLanguage(lang: string): asserts lang is keyof typeof Language {
invariant(lang in Language, `"${lang}" is not a valid language`)
}
Now you can use invariantLanguage on your string input -
// ...
enum Language {
nl = 1,
fr = 2,
en = 3,
de = 4,
}
const x = "en"
invariantLanguage(x)
console.log(x) // : "en"
const y = "foo"
invariantLanguage(y) // ERROR Invariant violation: "foo" is not a valid language
console.log(y) // : never
Verify on typescript playground.
It is also demonstrated working properly with string enums -
// ...
enum Language {
nl = 1,
fr = 2,
en = 3,
de = 4,
ja = "japanese", // also works if you have strings
}
const x = "ja"
invariantLanguage(x)
console.log(x) // : "ja"
Notice is does not mistake a value for a key -
const y = "japanese"
invariantLanguage(y) // ERROR Invariant violation: "japanese" is not a valid language
console.log(y) // : never
Verify on typescript playground.