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 = acc + now, 0); }; is it possible to force this extension to be typed only for number?
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 = 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.