Can you write a generic copy object function in typescript?

Question

So far I've tried:

function copyObject<K, V> (object: { [k: K]: V; }) {
    var objectCopy = {};

    for (var key in object)
    {
        if (object.hasOwnProperty(key))
        {
            objectCopy[key] = object[key];
        }
    }

    return objectCopy;
}

But this gives a compiler warning: "Index signature parameter type must be 'string' or 'number'".

Maybe it's possible to constrain the key type to number or string? Or just overload it with both types as keys?

Answer 1

You can simply do the following :

function copyObject<T> (object:T): T {
    var objectCopy = <T>{};

    for (var key in object)
    {
        if (object.hasOwnProperty(key))
        {
            objectCopy[key] = object[key];
        }
    }

    return objectCopy;
}

And usage:

// usage: 
var foo = {
    bar: 123
};

var baz = copyObject(foo);
baz.bar = 456;

Answer 2

Typescript doesn't have typeclasses and I'm not aware of any interface that only string and number types satisfy so overloading seems to be the only option:

export function copyObject<V> (object: { [s: string]: V; }) : { [s: string]: V; }
export function copyObject<V> (object: { [n: number]: V; }): { [n: number]: V; }
export function copyObject (object: {}): {}
export function copyObject (object: {}) {
    var objectCopy = <any>{};

    for (var key in object)
    {
        if (object.hasOwnProperty(key))
        {
            objectCopy[key] = (<any>object)[key];
        }
    }

    return objectCopy;
}

(The string and number overloads allow you to copy objects with homogeneous types)