Enum as Parameter in TypeScript

Question

Isn't it possible to set the type of a parameter to an Enum? Like this:

private getRandomElementOfEnum(e : enum):string{     var length:number = Object.keys(e).length;     return e[Math.floor((Math.random() * length)+1)]; } 

Following error is thrown by compilation:

Argument expression expected.(1135)

With any obviously everyhting is alright:

private getRandomElementOfEnum(e : any):string{     var length:number = Object.keys(e).length;     return e[Math.floor((Math.random() * length)+1)]; } 

This Code works fine. but isn't as elegant and typesafe.

Is there a possibility or a little workaround to define an enum as a parameter?

Answer 1

It's not possible to ensure the parameter is an enum, because enumerations in TS don't inherit from a common ancestor or interface.

TypeScript brings static analysis. Your code uses dynamic programming with Object.keys and e[dynamicKey]. For dynamic codes, the type any is convenient.

Your code is buggy: length() doesn't exists, e[Math.floor((Math.random() * length)+1)] returns a string or an integer, and the enumeration values can be manually set…

Here is a suggestion:

function getRandomElementOfEnum<E>(e: any): E {     var keys = Object.keys(e),         index = Math.floor(Math.random() * keys.length),         k = keys[index];     if (typeof e[k] === 'number')         return <any>e[k];     return <any>parseInt(k, 10); }  function display(a: Color) {     console.log(a); }  enum Color { Blue, Green }; display(getRandomElementOfEnum<Color>(Color)); 

Ideally, the parameter type any should be replaced by typeof E but the compiler (TS 1.5) can't understand this syntax.

Answer 2

You can do better than any:

enum E1 {     A, B, C } enum E2 {     X, Y, Z }  function getRandomElementOfEnum(e: { [s: number]: string }): number {     /* insert working implementation here */     return undefined; }  // OK var x: E1 = getRandomElementOfEnum(E1); // Error var y: E2 = getRandomElementOfEnum(window); // Error var z: string = getRandomElementOfEnum(E2);