Debugging javascript in wkhtmltopdf

Question

Where I can see the output of javascript debug when wkhtmltopdf runs in debug mode (--debug-javascript)

Answer 1

Although Daffy Punks answer is correct, I had an additional idea some weeks ago, that helped me a lot. This I want to share: Show it inside PDF

When rendering the layout for PDF I put an additional (hidden) DIV #pdf_errors

And very early in source - if #pdf_errors is here - I let point console output to fill this div, and on error - I show it. Its not really debugging, but at least I see now what was going wrong.

Source in coffeescript, my plain javascript times are long gone ...

if ($pdf=$("#pdf_is_here")).length
    for type in ["log","warn","error"]
        do (type) =>
            console[type]= (a...) =>
                $pdf.append("
#{s}
") for s in a

    window.onerror= (messageOrEvent, source, lineno, colno, error) =>
        $pdf.append("
#{messageOrEvent}
")
        $pdf.show()

    $pdf.hide() # just in case css is not here