Object index key type in Typescript

You can achieve that just by using a
IDictionary<TValue> { [key: string]: TValue } since numeric values will be automatically converted to string.

Here is an example of usage:

interface IDictionary<TValue> {
    [id: string]: TValue;
}

class Test {
    private dictionary: IDictionary<string>;

    constructor() {
       this.dictionary = {}
       this.dictionary[9] = "numeric-index";
       this.dictionary["10"] = "string-index"

       console.log(this.dictionary["9"], this.dictionary[10]);
    }
}
// result => "numeric-index string-index"

As you can see string and numeric indices are interchangeable.