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?

basarat · Accepted Answer · 2014-03-07 11:25:13Z

8

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;

answered Mar 7, 2014 at 11:25

basarat

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1 Comment

AGD Over a year ago

But that doesn't look like it restricts type T to be of type 'object' which means that you get no type errors even if you did, say: copyObject(5). The overload solution I added in my comment above, does give you a warning in that case so seems better.

AGD · Accepted Answer · 2014-03-10 12:04:21Z

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)

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