Array subclass ES5

extend-array.js

// No need to sub class Array if what you need is just an extended
// array. Example below illustrates the way to extend Array.  

function SubArray() {
  return Object.defineProperties(Array.prototype.slice.call(arguments), SubArrayDescriptor)
}
SubArray.prototype = Array.prototype
var SubArrayDescriptor =
{ constructor: { value: SubArray }
, last: { value: function last() {
    return this[this.length - 1]
  }}
}

sub-class-array.js

// Sub classing array works as expected. Many people have false expectation that
// special behavior of number properties (sub[10]) is supposed to be inherited by a subclass.

function SubArray() {
  var subArray = Object.create(SubArray.prototype)
  Array.prototype.push.apply(subArray, arguments)
  return subArray
}
SubArray.prototype = Object.create(Array.prototype,
{ constructor: { value: SubArray }
, last: { value: function last() {
    return this[this.length - 1]
  }}
})

I'd like to note that the above examples re-write Array.from, which can break newer engines. For example, after the above patch, the following breaks in ES6:

Array.from( new Set([1,2,3,1,2,3]) )

The answers that use the patched Array.from are for ES5, and the newer example should be used for ES6.