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?

Answer 1

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');

Answer 2

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

Answer 3

 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;