Enum as Parameter in TypeScript

Issue

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?

Solution

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.

Answered By - Paleo