`null` and default arguments

[Isiah Meadows]

I know it's a little late for this, but what was the rationale of using
only `undefined` instead of both that and `null` to denote omitted values
for optional arguments in ES6? Before this change, it was a frequent idiom
to check optional arguments via `== null` instead of `=== undefined` and
pass `null` as the argument to denote the absence of a value.

Just a curious question.
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