Is output=false by default for cfscript function in CFC?

Short answer: It doesn't matter.

cfscript does not output anything unless you explicitly call writeOutput() from it. This includes functions in cfscript as well as any cfscript code outside of a function.

This is different from CF tags' syntax, which, by default, output at least whitespace between the tags. In cfscript, any text you write will be compiled by the CFML engine. in CF tags, any text you write will be written to the output buffer and sent to browser.

Long answer: It's the same as not specifying an output attribute.

cfscript blocks don't output anything. Any tag blocks, unless wrapped in cfsilent, do output whitespace if nothing else. Yes, even cffunctions do, but the output is discarded if the output attribute is set to false.

The essence of Peter Boughton's answer is correct. It's neither wrapped in cfsilent nor cfoutput. Output is not forbidden, but it doesn't happen unless you do it explicitly.

You can always combine a tag-based cffunction with scripting to get the best of both worlds. Something like...

<cffunction name="x" output="false" access="package">
    <cfargument name="y" type="string" required="false" default="YY" />
    <cfscript>
        var someValue = y & "something";
        writeOutput("test"); // this will not be written
        return someValue;
    </cfscript>
</cffunction>

This lets you specify an output and access on the cffunction tag as well as allow arguments to be optional (which you can't do through cfscript functions), then fill the body with cfscript, including var statements and the function return.

Of course, for that function, if you remove the output attribute or change it to true, it will