In contrast, an object's actual prototype (obtained with Object.getPrototypeOf) is the real and only definition of an object's nature -- even in the possible case that its prototype has been changed after creation (either though the spec compliant Object.setPrototypeOf or by changing the commonly implemented __proto__ property).

But I'll definitely take underscore mixins over extending String.prototype or other clunky implementations any day.

Try to avoid mucking with native prototypes, including Array.prototype, if you don't know who will be consuming your code (3rd parties, coworkers, yourself at a later date, etc.). There are ways to safely extend prototypes (but not in all browsers) and there are ways to safely consume objects created from extended prototypes, but a better rule of thumb is to follow the Principle of Least Surprise and avoid these practices altogether.

He's not messing with them he's extending them for Arrays.