Map Typescript Enum

Question

How would I map a typescript enum? For example, with strings you can do this:

let arr = [ 'Hello', 'Goodbye' ];  arr.map(v => {   if (v === 'Hello') {     return ':)';   } else if (v === 'Goodbye') {     return ':(';   } ); // [ ':)', ':(' ] 

This, of course, doesn't work with enums:

enum MyEnum { Hello, Goodbye };  MyEnum.map(v => {   if (v === MyEnum.Hello) {     return ':)';   } else if (v === MyEnum.Goodbye) {     return ':(';   } }); // does not work 

Ideally, I'd like to do this in a generalized way so I can simply take any enum I have and put it through a map function while preserving type information. Usage might look something like this:

map(MyEnum, v => {   if (v === MyEnum.Hello) {     return ':)';   } else if (v === MyEnum.Goodbye) {     return ':(';   } }); // [ ':)', ':(' ] 

I've been fiddling around with getting a function that does this for me but keep having issues getting the generics just right.

Answer 1

To map an enum do this:

(Object.keys(MyEnum) as Array<keyof typeof MyEnum>).map((key) => {}) 

Answer 2

The function to solve this is quite simple.

// you can't use "enum" as a type, so use this. type EnumType = { [s: number]: string };  function mapEnum (enumerable: EnumType, fn: Function): any[] {     // get all the members of the enum     let enumMembers: any[] = Object.keys(enumerable).map(key => enumerable[key]);      // we are only interested in the numeric identifiers as these represent the values     let enumValues: number[] = enumMembers.filter(v => typeof v === "number");      // now map through the enum values     return enumValues.map(m => fn(m)); } 

As you can see, we first need to get all of the keys for the enum (MyEnum.Hello is actually 1 at runtime) and then just map through those, passing the function on.

Using it is also simple (identical to your example, although I changed the name):

enum MyEnum { Hello, Goodbye };  let results = mapEnum(MyEnum, v => {   if (v === MyEnum.Hello) {     return ':)';   } else if (v === MyEnum.Goodbye) {     return ':(';   } });  console.log(results); // [ ':)', ':(' ] 

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The reason we need to filter the enum to be numbers only is because of the way enums are compiled.

Your enum is actually compiled to this:

var MyEnum; (function (MyEnum) {     MyEnum[MyEnum["Hello"] = 0] = "Hello";     MyEnum[MyEnum["Goodbye"] = 1] = "Goodbye"; })(MyEnum || (MyEnum = {})); ; 

However we are not interested in "Hello" or "Goodbye" as we can't use those at runtime.

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You will also notice a funny type statement right before the function. This is because you can't type a parameter as someParameter: enum, you need to explicitly state it as a number -> string map.