I wasn't confused by the spec text as is, but I'm not surprised that
others are. The language is battling against two different "standards"
for optional arguments. In ES5 typically "not present" is used; in ES6
for consistency with the new language optional arguments, "undefined"
is treated as "not present" and `arguments.length` is ignored.

This issue arose in a discussion with the `bluebird` maintainer about
the proper semantics for `Promise.reduce`; he wanted to use "ES6
style" for the final optional argument (which meant that you can't use
`undefined` as an `initialValue`), while I wanted "ES5 style" to be
consistent with the ES5 method `Array#reduce`.

It might be worth coming up with good terms for "ES5-style optional
arguments" and "ES6-style optional arguments" which can be used
consistently in the spec.  They can already be distinguished by
signature, ie:

```js
Array.prototype.reduce ( callbackfn [ , initialValue ] )
Array.prototype.splice (start, deleteCount [ , item1 [ , item2 [ , … ] ] ] )
```

versus

```js
Array.prototype.fill (value, start = 0, end = this.length)
Array.prototype.copyWithin (target, start, end = this.length)
```

Another alternative might be to define a helper function used wherever
"ES5-style" optional arguments were used.

On the gripping hand, ES5-style optional arguments are actually more
similar semantically to ES6 'rest' arguments; perhaps the spec can be
written to use a rest argument.  For example:

```js
Array.prototype.splice (start, deleteCount, ...items) // like in the
signature for Array.of()
```
  --scott

ps. http://esdiscuss.org/topic/changing-behavior-of-array-copywithin-when-used-on-array-like-with-negative-length

I wasn't confused by the spec text as is, but I'm not surprised that
others are. The language is battling against two different "standards"
for optional arguments. In ES5 typically "not present" is used; in ES6
for consistency with the new language optional arguments, "undefined"
is treated as "not present" and `arguments.length` is ignored.

This issue arose in a discussion with the `bluebird` maintainer about
the proper semantics for `Promise.reduce`; he wanted to use "ES6
style" for the final optional argument (which meant that you can't use
`undefined` as an `initialValue`), while I wanted "ES5 style" to be
consistent with the ES5 method `Array#reduce`.

It might be worth coming up with good terms for "ES5-style optional
arguments" and "ES6-style optional arguments" which can be used
consistently in the spec.  They can already be distinguished by
signature, ie:

```js
Array.prototype.reduce ( callbackfn [ , initialValue ] )
Array.prototype.splice (start, deleteCount [ , item1 [ , item2 [ , … ] ] ] )
```

versus

```js
Array.prototype.fill (value, start = 0, end = this.length)
Array.prototype.copyWithin (target, start, end = this.length)
```

Another alternative might be to define a helper function used wherever
"ES5-style" optional arguments were used.

On the gripping hand, ES5-style optional arguments are actually more
similar semantically to ES6 'rest' arguments; perhaps the spec can be
written to use a rest argument.  For example:

```js
Array.prototype.splice (start, deleteCount, ...items) // like in the
signature for Array.of()
```

ps. http://esdiscuss.org/topic/changing-behavior-of-array-copywithin-when-used-on-array-like-with-negative-length