As you say, じゃ can be the casual form of では. For instance, the forst じゃ in your sentence. So it comes to これではだめじゃん。 じゃ　is also one of the sentence ending particles. Often it is used when a old people speak. They put じゃ in the end of a sentence.

1) The じゃ in これじゃ is a colloquial, collapsed form of では. では, as you might know, consists of the case particle (格助詞) 「で」 and the adverbial particle (副助詞), 「contrastive は」. 2) じゃん in だめじゃん is a colloquial, shortened form of 「ではないか？」 So, here again, じゃ in だめじゃん！ is a collapsed form of では. (ではないか is shortened into →じゃないか？ditto→じゃない？ditto→じゃん？) 「だめではないか？」literally means "Isn't it useless/hopeless/impossible/no good?", practically meaning you think "it's no good", =「だめだよ」！ -------------- The sentence ending じゃ is a colloquial, collapsed form of だ. だ/じゃ are NOT sentence-ending particles(終助詞). they are "copulas"(断定の助動詞「だ」「です」), the casual versions of です. E.g.: The「だ」in 「君の絵はきれいだ」is a copula. You can also say it as 「君の絵はきれいです」. The 「ね」「よ」「な」etc. in「君の絵はきれいですね」「君の絵はきれいですよ」「君の絵はきれいだな」are the sentence-ending particles.

am referring to the first jya.

I can see two 「じゃ」in "これ「じゃ」だめ「じゃ」ん！", so which じゃ are you referring to?