に越したことはない vs に越したことないじゃない


I would appreciate help with the following sentence (which is spoken by a
rightfully angry girl).

> よしゅうふくしゅうをしておくにこしたことないじゃない!

I know that にこしたことはない is an expression that means "there's nothing better
than" and I roughly understand its construction. But I can't figure out how
ないじゃない instead of はない works.

So, first, I'd like to know what happens structurally. Does ないで get inserted
between こと and は, or does the は get dropped and じゃない added at the end?

Then, second, how does this affect the meaning? Is it emphasis? Is it
rhetorical? Does it show the speaker is upset? Or is it a form of feminine
speech?

Thank you in advance.




じゃない in this case functions as a confirmational rhetorical device, like,
'isn't that right?'. This can occur after verbs, nouns, and adjectives (in
positive and negative forms).

ことない isn't written as ことはない simply because it is more casual speech, and は
frequently gets dropped in casual speech （越したことはない = 越したことない).

[code]

     Does ないで get inserted between こと and は?  
    
[/code]

I assume you mean 「～こしたことはないではない」？  
じゃない is the casual form of ではない, and the first は has simply been omitted.
Again, trademarks of informal speech.

**_ない／（ん）じゃない rhetorical phrasings:_**

> 寒｛さむ｝い＋んじゃない？ Isn't it cold?  
>  寒｛さむ｝く＋ない？ Aren't you cold?  
>  寒｛さむ｝くない＋んじゃない。 It isn't cold, is it.  
>  田中｛たなか｝さん＋じゃない？ Isn't it/that Mr. Tanaka?  
>  あの人｛ひと｝はもう来｛こ｝ない＋んじゃない？ That guy won't come again, will he?  
>  来｛き｝てくれる＋んじゃない？ Aren't you coming?

You can add ですか? after じゃない to strengthen the positive presumption aspect
somewhat.

> 田中｛たなか｝さんじゃないですか I believe that is Mr. Tanaka.

Conversely, having a question mark at the end increases the unsure and
speculative nature of the rhetorical ('isn't it.' vs. 'isn't it?').

[code]

     Does it show the speaker is upset? Or is it a form of feminine speech?
    
[/code]

It is not a form of feminine speech. It is not an indication that the speaker
is upset, the exclamation point would be a likely indicator though.

Related post can be found
[here](https://japanese.stackexchange.com/questions/38428/double-negative-in-
sentence-%E4%B8%80%E4%BA%BA%E3%81%8F%E3%82%89%E3%81%84%E3%81%AF%E6%9D%A5%E3%81%A6%E3%81%8F%E3%82%8C%E3%82%8B%E3%82%93%E3%81%98%E3%82%83%E3%81%AA%E3%81%84%E3%81%8B%E3%81%AA).

