I write the sample nginxscript.
I wrote the line
req.log("hello from foo() handler");

But I can't find the log file, do you know the path of the log file or any document to guide to enable log message, thanks.

js_include http.js;
server {
location /hello {
js_set $foo foo;
add_header X-Foo $foo;
js_content baz;
}
}

https.js
function foo(req, res) {
req.log("hello from foo() handler");
return "foo";
}

function baz(req, res) {
res.headers.foo = 1234;
res.status = 200;
res.contentType = "text/plain; charset=utf-8";
res.contentLength = 15;
res.sendHeader();
res.send("nginx");
res.send("java");
res.send("script");

res.finish();
}

I found the solution to enable a debugging log by http://nginx.org/en/docs/debugging_log.html
./configure --with-debug