Typescript function to convert string enum member to enum

Question

I'd like to write something like this in Typescript:

export function stringToEnum<T>(enumObj: T, str: string): keyof T {
    return enumObj[str];
}

and use it as follows:

enum MyEnum {
  Foo
}

stringToEnum<MyEnum>(MyEnum, 'Foo');

where it would return

MyEnum.Foo

The function above works as expected... but the typings are throwing errors. For the parameter MyEnum in stringToEnum<MyEnum>(MyEnum, 'Foo');, Typescript complains tha:

Argument of type 'typeof MyEnum' is not assignable to parameter of type 'MyEnum'

which makes sense... unfortunately. Any ideas on how I can get around this?

Why? Whats your usecase?

Jonas Wilms
– Jonas Wilms

2018-01-25 19:25:44 +00:00
Commented Jan 25, 2018 at 19:25 — Jonas Wilms
– Jonas Wilms, Commented Jan 25, 2018 at 19:25

Fenton · Accepted Answer · 2018-01-25 19:35:10Z

11

You can do it all natively without having to write a function:

enum Color {
    red,
    green,
    blue
}

// Enum to string
const redString: string = Color[Color.red];
alert(redString);

// String to enum
const str = 'red';
const redEnum: Color = Color[str];
alert(redEnum);

Or you can have some fun with it...

enum MyEnum {
  Foo,
  Bar
}

function stringToEnum<ET, T>(enumObj: ET, str: keyof ET): T{
    return enumObj[<string>str];
}

const val = stringToEnum<typeof MyEnum, MyEnum>(MyEnum, 'Foo');

// Detects that `foo` is a typo
const val2 = stringToEnum<typeof MyEnum, MyEnum>(MyEnum, 'foo');

edited Jan 25, 2018 at 19:35

answered Jan 25, 2018 at 19:24

Fenton

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4 Comments

sir_thursday Over a year ago

Right. Is there any way to do it with a method, though?

Fenton Over a year ago

I've added an example for fun... the call to the function is significantly trickier than just using MyEnum['Foo'].

BoomZilla Over a year ago

Hey @Fenton. TS compiler errors out with 7053 in current TS versions, also within your function. This explains it. So the easiest solution would be: const str: string = 'red'; const redEnum: Color = (<any>Color)[str];

BoomZilla Over a year ago

In addition to my last comment: If you got an enum like enum Color { Red = "red"} then const redEnum: Color = (<any>Color)["red"] won't match as the indexer matches the enum-member-name. So only const redEnum: Color = (<any>Color)["Red"] would work.

Jeff Lau · Accepted Answer · 2018-03-04 06:06:43Z

Your signature is a bit mixed up. The return type should be T[keyof T] if you intend for the method to return an enum value. The type of the str param should also be keyof T to prevent you from passing invalid strings in, but this will limit you to passing string literals in (or well-typed variables of type keyof T, but not string):

function stringToEnum<T>(enumObj: T, str: keyof T): T[keyof T]

Then either don't specify the type param and let the compiler infer the type correctly:

// type: Foo
// value: 0  
const result = stringToEnum(MyEnum, 'Foo');

Or you need to provide typeof MyEnum as the type param:

// type: Foo
// value: 0  
const result = stringToEnum<typeof MyEnum>(MyEnum, 'Foo');

If you really want to be able to pass in any arbitrary string enum name, then the return type is a lie: should be T[keyof T] | undefined. You'll also run into trouble when attempting enumObj[str] if the type of str is string and you have noImplicitAny compiler option enabled.

There's a bit more to making generic functions that work with enum types properly, especially numeric enums that have reverse lookup entries at run-time. Take a look at the source code for ts-enum-util (github, npm) for inspiration

Jonas Wilms · Accepted Answer · 2018-01-25 19:34:41Z

1

 stringToEnum(MyEnum, 'Foo');

Just leave away the generic and let typescript do that. Thats because the type stored under MyEnum does not match the Enum itself but is a union type of its values:

 enum Test { A, B };

 const value: Test /* "A" | "B" */ = Test.A;

edited Jan 25, 2018 at 19:34

answered Jan 25, 2018 at 19:27

Jonas Wilms

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