$.append() earned $.add() — is still there
What does not work? Let us examine your line:
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$(this).add("div").css("class", "div");
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1) Select in the object element this (paragraph).
2) Add this object with all paragraphs found on the page of the diva. As DIVS on the page, probably not, does not change the result, $(this).
3) Applied to these objects .css("class", "div").
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By the way, what is this new css-property, class? Never even heard of.
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With regard to paragraph 1) Syntax error at line 5 while loading:
The proposed download the archive code looks like this:
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$("p #add").click(function() {...}
$("p #append").click(function() {...}
Without closing braces and semicolons.