You-dont-know-js: Up & Running Ch2 Transpiling - use of (void 0) and confusing paragraph

[darren-a]

In the example foo() function it's not clear why (void 0) was used when it appears "undefined" could be used with the same effect (at least that's what I've seen). "void" was very briefly mentioned earlier in the book but wasn't fully explained.

Re the paragraph:

"You may not have realized just from looking at the ES6 version that undefined is the only value that can't get explicitly passed in for a default-value parameter, but the transpiled code makes that much more clear."

This part: "can't get explicitly passed in for a default-value parameter" seems to be a contradiction, or at least ambiguous, is it:

1. undefined can't be passed into a function as an argumentr? (it can)
2. undefined can't be passed into a function that uses default parameter values? (it can)
3. undefined can't be written into a parameter as a default value? (it can)

Does it mean, in effect: "You can pass in "undefined" as an argument to a function, but if that function has a default parameter value that value will override undefined"?

[getify]

it's not clear why (void 0) was used when it appears "undefined" could be used with the same effect

That code is transpiled code -- it came from Babel. I wouldn't author it that way, I'd use undefined. I think Babel uses void 0 since it's 3 fewer characters.

Does it mean, in effect...

It means: "will be ignored as an argument to a parameter that has a default value, as that parameter will instead receive the default."

[darren-a]

Right, thanks.

May I suggest a note in the 2nd ed. about void 0 equivalency (in this case) to undefined,

and adding / re-write to include your sentence "will be ignored as an argument to a parameter that has a default value, as that parameter will instead receive the default."

[getify]

Reasonable suggestions for second edition. Thanks. :)