How to escape JavaScript in JSP?

The forward slash is not an escape character. That's the backslash.

${fn:replace(Desc, "'", "\\'")}

(yes, it's been presented twice, because that's also an escape character in Java!)

However, you don't only need to repace ' by \', you also need to replace \n (newlines) by \\n. The string is been printed over multiple lines, which makes it also an invalid JS string variable. Your final result must basically look like this:

var itemNameList = ''
    + '\nWeyland Estate Santa Barbara Pinot Noir'
    + '\nRaymond \'Prodigal\' North Coast Cabernet Sauvignon'
    + '\nChateau Haute Tuque'; 

(please note that the syntax highlighter agrees on me here but not on yours)

There are however much more possible special characters which needs to be escaped. They are all covered by Apache Commons Lang StringEscapeUtils#escapeEcmaScript(). Much easier is to create a custom EL function which calls exactly that method. If not done yet, download and drop commons-lang.jar in /WEB-INF/lib. Then create a /WEB-INF/functions.tld file like follows:

<?xml version="1.0" encoding="UTF-8" ?>
<taglib 
    xmlns="http://java.sun.com/xml/ns/javaee"
    xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
    xsi:schemaLocation="http://java.sun.com/xml/ns/javaee http://java.sun.com/xml/ns/javaee/web-jsptaglibrary_2_1.xsd"
    version="2.1">

    <display-name>Custom Functions</display-name>    
    <tlib-version>1.0</tlib-version>
    <uri>http://example.com/functions</uri>

    <function>
        <name>escapeJS</name>
        <function-class>org.apache.commons.lang3.StringEscapeUtils</function-class>
        <function-signature>java.lang.String escapeEcmaScript(java.lang.String)</function-signature>
    </function>
</taglib>

So that you can use it as follows:

<%@taglib prefix="util" uri="http://example.com/functions" %>
...
${util:escapeJS(Desc)}