Enums in TypeScript 0.9 are string+number based. You should not need type assertion for simple conversions:
enum Color{
Red, Green
}
// To String
var green: string = Color[Color.Green];
// To Enum / number
var color : Color = Color[green];
I have documention about this and other Enum patterns in my OSS book : https://basarat.gitbook.io/typescript/type-system/enums
As of Typescript 2.1 string keys in enums are strongly typed. keyof typeof is used to get info about available string keys (1):
enum Color{
Red, Green
}
let typedColor: Color = Color.Green;
let typedColorString: keyof typeof Color = "Green";
// Error "Black is not assignable ..." (indexing using Color["Black"] will return undefined runtime)
typedColorString = "Black";
// Error "Type 'string' is not assignable ..." (indexing works runtime)
let letColorString = "Red";
typedColorString = letColorString;
// Works fine
typedColorString = "Red";
// Works fine
const constColorString = "Red";
typedColorString = constColorString
// Works fine (thanks @SergeyT)
let letColorString = "Red";
typedColorString = letColorString as keyof typeof Color;
typedColor = Color[typedColorString];
https://www.typescriptlang.org/docs/handbook/advanced-types.html#index-types
enum Color{
Red, Green
}
// To String
var green: string = Color[Color.Green];
// To Enum / number
var color : Color = Color[green as keyof typeof Color]; //Works with --noImplicitAny
This example works with --noImplicitAny in TypeScript
Sources:
https://github.com/Microsoft/TypeScript/issues/13775#issuecomment-276381229 https://www.typescriptlang.org/docs/handbook/advanced-types.html#index-types
If you are sure that an input string has an exact match with Color enum then use:
const color: Color = (<any>Color)["Red"];
In the case where an input string may not match Enum, use:
const mayBeColor: Color | undefined = (<any>Color)["WrongInput"];
if (mayBeColor !== undefined){
// TypeScript will understand that mayBeColor is of type Color here
}
If we do not cast enum to <any> type then TypeScript will show the error:
Element implicitly has 'any' type because index expression is not of type 'number'.
It means that by default the TypeScript Enum type works with number indexes, i.e.
let c = Color[0], but not with string indexes like let c = Color["string"]. This is a known restriction by the Microsoft team for the more general issue Object string indexes.
Given you use typescript: Many of the solutions above might not work or are overly complex.
Situation: The strings are not the same as the enum values (casing differs)
enum Color {
Green = "green",
Red = "red"
}
Just use:
const color = "green" as Color
Please note that this does not guarantee a valid enum.
This note relates to basarat's answer, not the original question.
I had an odd issue in my own project where the compiler was giving an error roughly equivalent to "cannot convert string to Color" using the equivalent of this code:
var colorId = myOtherObject.colorId; // value "Green";
var color: Color = <Color>Color[colorId]; // TSC error here: Cannot convert string to Color.
I found that the compiler type inferencing was getting confused and it thought that colorId was an enum value and not an ID. To fix the problem I had to cast the ID as a string:
var colorId = <string>myOtherObject.colorId; // Force string value here
var color: Color = Color[colorId]; // Fixes lookup here.
I'm not sure what caused the issue but I'll leave this note here in case anyone runs into the same problem I did.
If the TypeScript compiler knows that the type of variable is string then this works:
let colorName : string = "Green";
let color : Color = Color[colorName];
Otherwise you should explicitly convert it to a string (to avoid compiler warnings):
let colorName : any = "Green";
let color : Color = Color["" + colorName];
At runtime both solutions will work.
enum Color { Red, Green }
const c1 = Color["Red"]
const redStr = "Red" // important: use `const`, not mutable `let`
const c2 = Color[redStr]
This works both for numeric and string enums. No need to use a type assertion.
const redStrWide: string = "Red" // wide, unspecific typed string
const c3 = Color[redStrWide as keyof typeof Color]
const isEnumName = <T>(str: string, _enum: T): str is Extract<keyof T, string> =>
str in _enum
const enumFromName = <T>(name: string, _enum: T) => {
if (!isEnumName(name, _enum)) throw Error() // here fail fast as an example
return _enum[name]
}
const c4 = enumFromName(redStrWide, Color)
String enums don't have a reverse mapping (in contrast to numeric ones). We can create a lookup helper to convert an enum value string to an enum type:
enum ColorStr { Red = "red", Green = "green" }
const c5_by_name = ColorStr["Red"] // ✅ this works
const c5_by_value_error = ColorStr["red"] // ❌ , but this not
const enumFromValue = <T extends Record<string, string>>(val: string, _enum: T) => {
const enumName = (Object.keys(_enum) as Array<keyof T>).find(k => _enum[k] === val)
if (!enumName) throw Error() // here fail fast as an example
return _enum[enumName]
}
const c5 = enumFromValue("red", ColorStr)
I was looking for an answer that can get an enum from a string, but in my case, the enums values had different string values counterpart. The OP had a simple enum for Color, but I had something different:
enum Gender {
Male = 'Male',
Female = 'Female',
Other = 'Other',
CantTell = "Can't tell"
}
When you try to resolve Gender.CantTell with a "Can't tell" string, it returns undefined with the original answer.
Basically, I came up with another answer, strongly inspired by this answer:
export const stringToEnumValue = <ET, T>(enumObj: ET, str: string): T =>
(enumObj as any)[Object.keys(enumObj).filter(k => (enumObj as any)[k] === str)[0]];
filter, assuming the client is passing a valid string from the enum. If it's not the case, undefined will be returned.enumObj to any, because with TypeScript 3.0+ (currently using TypeScript 3.5), the enumObj is resolved as unknown.const cantTellStr = "Can't tell";
const cantTellEnumValue = stringToEnumValue<typeof Gender, Gender>(Gender, cantTellStr);
console.log(cantTellEnumValue); // Can't tell
Note: And, as someone pointed out in a comment, I also wanted to use the noImplicitAny.
No cast to any and proper typings.
export const stringToEnumValue = <T, K extends keyof T>(enumObj: T, value: string): T[keyof T] | undefined =>
enumObj[Object.keys(enumObj).filter((k) => enumObj[k as K].toString() === value)[0] as keyof typeof enumObj];
Also, the updated version has a easier way to call it and is more readable:
stringToEnumValue(Gender, "Can't tell");
I needed to know how to loop over enum values (was testing lots of permutations of several enums) and I found this to work well:
export enum Environment {
Prod = "http://asdf.com",
Stage = "http://asdf1234.com",
Test = "http://asdfasdf.example.com"
}
Object.keys(Environment).forEach((environmentKeyValue) => {
const env = Environment[environmentKeyValue as keyof typeof Environment]
// env is now equivalent to Environment.Prod, Environment.Stage, or Environment.Test
}
Source: https://blog.mikeski.net/development/javascript/typescript-enums-to-from-string/
There's a lot of mixed information in this question, so let's cover the whole implementation for TypeScript 2.x+ in Nick's Guide to Using Enums in Models with TypeScript.
This guide is for: people who are creating client-side code that's ingesting a set of known strings from the server that would be conveniently modeled as an Enum on the client side.
Let's start with the enum. It should look something like this:
export enum IssueType {
REPS = 'REPS',
FETCH = 'FETCH',
ACTION = 'ACTION',
UNKNOWN = 'UNKNOWN',
}
Two thing of note here:
We're explicitly declaring these as string-backed enum cases which allows us to instantiate them with strings, not some other unrelated numbers.
We've added an option that may or may not exist on our server model: UNKNOWN. This can be handled as undefined if you prefer, but I like to avoid | undefined on types whenever possible to simplify handling.
The great thing about having an UNKNOWN case is that you can be really obvious about it in code and make styles for unknown enum cases bright red and blinky so you know you're not handling something correctly.
You might be using this enum embedded in another model, or all alone, but you're going to have to parse the string-y typed enum from JSON or XML (ha) into your strongly typed counterpart. When embedded in another model, this parser lives in the class constructor.
parseIssueType(typeString: string): IssueType {
const type = IssueType[typeString];
if (type === undefined) {
return IssueType.UNKNOWN;
}
return type;
}
If the enum is properly parsed, it'll end up as the proper type. Otherwise, it'll be undefined and you can intercept it and return your UNKNOWN case. If you prefer using undefined as your unknown case, you can just return any result from the attempted enum parsing.
From there, it's only a matter of using the parse function and using your newly strong typed variable.
const strongIssueType: IssueType = parseIssueType('ACTION');
// IssueType.ACTION
const wrongIssueType: IssueType = parseIssueType('UNEXPECTED');
// IssueType.UNKNOWN
Enum
enum MyEnum {
First,
Second,
Three
}
Sample usage
const parsed = Parser.parseEnum('FiRsT', MyEnum);
// parsed = MyEnum.First
const parsedInvalid= Parser.parseEnum('other', MyEnum);
// parsedInvalid = undefined
Ignore case sensitive parse
class Parser {
public static parseEnum<T>(value: string, enumType: T): T[keyof T] | undefined {
if (!value) {
return undefined;
}
for (const property in enumType) {
const enumMember = enumType[property];
if (typeof enumMember === 'string') {
if (enumMember.toUpperCase() === value.toUpperCase()) {
const key = enumMember as string as keyof typeof enumType;
return enumType[key];
}
}
}
return undefined;
}
}
Enums created in the way you did are compiled into an object that stores both forward (name -> value) and reverse (value -> name) mappings. As we can observe from this chrome devtools screenshot:
Here is an example of how dual mapping works and how to cast from one to another:
enum Color{
Red, Green
}
// To Number
var greenNr: number = Color['Green'];
console.log(greenNr); // logs 1
// To String
var greenString: string = Color[Color['Green']]; // or Color[Color[1]
console.log(greenString); // logs Green
// In your example
// recieve as Color.green instead of the string green
var green: string = Color[Color.Green];
// obtain the enum number value which corresponds to the Color.green property
var color: Color = (<any>Color)[green];
console.log(color); // logs 1
If you're interested in type guarding an what would otherwise be a string (which is how I came across this issue), this might work for you:
enum CurrencyCode {
cad = "cad",
eur = "eur",
gbp = "gbp",
jpy = "jpy",
usd = "usd",
}
const createEnumChecker = <T extends string, TEnumValue extends string>(
enumVariable: { [key in T]: TEnumValue }
) => {
const enumValues = Object.values(enumVariable);
return (value: string | number | boolean): value is TEnumValue =>
enumValues.includes(value);
};
const isCurrencyCode = createEnumChecker(CurrencyCode);
const input: string = 'gbp';
let verifiedCurrencyCode: CurrencyCode | null = null;
// verifiedCurrencyCode = input;
// ^ TypeError: Type 'string' is not assignable to type 'CurrencyCode | null'.
if (isCurrencyCode(input)) {
verifiedCurrencyCode = input; // No Type Error 🎉
}
Solution is taken from this github issue discussing generic Enums
If you're dealing with TypeScript 4.1+ and string enums, and you want a simple string-to-Enum converter with compile-time and run-time safety, the following works well:
export const asEnum = <
T extends { [key: string]: string },
K extends keyof T & string
>(
enumObject: T,
value: `${T[K]}`
): T[K] => {
if (Object.values(enumObject).includes(value)) {
return (value as unknown) as T[K];
} else {
throw new Error('Value provided was not found in Enum');
}
};
enum Test {
hey = 'HEY',
}
const test1 = asEnum(Test, 'HEY'); // no complaints here
const test2 = asEnum(Test, 'HE'); // compile-time error
const test3 = asEnum(Test, 'HE' as any); // run-time error
If you are using namespaces to extend the functionality of your enum then you can also do something like
enum Color {
Red, Green
}
export namespace Color {
export function getInstance(color: string) : Color {
if(color == 'Red') {
return Color.Red;
} else if (color == 'Green') {
return Color.Green;
}
}
}
and use it like this
Color.getInstance('Red');
