Is it possible to extend Array.prototype only for a specific type on typescript?

Question

I have the following prototype extension because I've been using a lot of reduce:

declare interface Array<T> {
    sum(): T;
}

Array.prototype.sum = function() {
    return this.reduce((acc, now) => acc + now, 0);
};

is it possible to force this extension to be typed only for number?

Since the function sum will be available for all arrays regardless, why is this a useful thing to do? Wouldn't it make more sense to derive from Array instead? — T.J. Crowder
– T.J. Crowder, Commented Jan 2, 2018 at 13:31

Badashi · Accepted Answer · 2020-12-29 19:26:01Z

8

As I wrote the question, I ended up finding out how to do it:

declare interface Array<T> {
    sum(this: Array<number>): number;
}

Object.defineProperty(Array.prototype, 'sum', {
    value: function(this: Array<number>): number {
        return this.reduce((acc, now) => acc + now, 0);
    },
});

It should also be noted that extending basic prototypes is usually not a good idea - in the event the base standard is changed to implement new functionality, there might be a conflict in your own code base. For this particular example on my particular codebase I feel it is fine, since sum is a fairly descriptive method name and very common along multiple languages, so a future standard will (probably) have a compatible implementation.

edited Dec 29, 2020 at 19:26

answered Jan 2, 2018 at 13:28

Badashi

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3 Comments

Badashi Over a year ago

I didn't know about enumeration on javascript types so I did a bit of research, thanks for pointing that out. I updated my own answer.

T.J. Crowder Over a year ago

FWIW, to match how other function-valued Array.prototype properties are configured, you'd add writable: true, configurable: true to that.

Sebastian Nielsen Over a year ago

@Badashi Will a warning or error be raised if someone attempts to redefine the sum prototype method?

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