detach methods as functions

detach.js

function detach(fn, cls) {
  if (typeof cls.prototype[fn] !== 'function') {
    throw new Error(fn + ' is not an instance function');
  }
  return cls.prototype[fn].call.bind(cls.prototype[fn]);
}

var slice = detach('slice', Array);
var map = detach('map', Array);

slice([1, 2, 3], 1); // => [2, 3]
map([1, 2, 3], function (x) { return x*2 }); // => [2, 4, 6]