Function.prototype.apply 's handling of an object instead of an array. It forces the object through Array.prototype.slice function as 'this' to create an array, pretty much what we do to turn an arguments object into a real array.

Array.apply

Array.apply(null, [1, 2, 3])
> [1, 2, 3]

Array.apply(null, [1, 2, 3]).length
> 3

Array.apply(null, {length:3})
> [undefined, undefined, undefined]

Array.apply(null, {length:3}).length
> 3

Array.apply(null, Array.prototype.slice.call({length:3}))
> [undefined, undefined, undefined]

Array.apply(null, Array.protorype.slice.call({length:3})).length
> 3

Function.apply

Function.apply(null, ['a', 'b', 'return a + b'])
> function anonymous(a,b
/**/) {
return a + b
}

Function.apply(null, ['a', 'b', 'return a + b']).length
> 2

Function.apply(null, {length:3})
> function anonymous(undefined,undefined
/**/) {
undefined
}

Function.apply(null, {length:3}).length
> 2