Enum as Parameter in TypeScript

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.

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);