Log to a file from Javascript code executed with no console window

Question

Working on a Cinnamon applet (Example). These use Javascript interpreted by the cjs interpreter and run without a console window.

How can I log something to a file or to a console from within the code I am working on?

I can not put test code into a separate Javascript file and run it with cjs <the file>, because there I do not have access to modules provided to applets by Cinnamon.

Answer 1

Logging

To log from the javascript:

global.log('my message')

To inspect things, including objects with possibly circular references, define this function:

const inspect = (function() {
    function replace_circular_references(show_private_keys) {
        let fullpaths = new Map(), cleanpaths= new Map(), rootobject = null
        return function(key, value) {
            let decycled; {
                if (!show_private_keys && key.startsWith('_')) return undefined
                let path = fullpaths.get(this) + (
                    Array.isArray(this) ? `[${key}]` : `.${key}` )
                let is_complex = value === Object(value)
                if (is_complex) fullpaths.set(value, path)
                let cleanpath = cleanpaths.get(value) || ''
                if (!cleanpath && is_complex)
                    cleanpaths.set(value, path.replace(/undefined\.\.?/, ''))
                decycled = cleanpath ?
                    `${cleanpath[0] == '[' ? '(this)' : '(this).'}${cleanpath}`
                    : value
                if (rootobject === null) rootobject = value
                else if (decycled === rootobject) { decycled = "(this)" }
            }
            return decycled
        }
    }
    return function (nameofthing, thing, show_private_keys=true, fold=null) {
        try {
            let thingrepr; {
                thingrepr = JSON.stringify(
                    thing, replace_circular_references(show_private_keys), '\t'
                ).replace(
                    /"([^\n"]+)"[:] /g, "$1: "  // prettify keys
                ).replace(
                    /"(\(this\)[^\n"]*)"/g, "$1"  // prettify circular refs
                )
                if (fold !== null) {  // maybe fold away some things
                    for (const key of fold) {
                        thingrepr = thingrepr.replace(
                            new RegExp(`^([ ]*)${key}: [{][^]*?\\n\\1[}]`, 'gm'),
                            "$1" + key + ": {...}"
                        ).replace(
                            new RegExp(`^([ ]*)${key}: \\[[^]*?\\n\\1\\]`, 'gm'),
                            "$1" + key + ": [...]"
                        )
                    }
                }
            }
            global.log(`\n${nameofthing} = ${thingrepr}`)
        } catch (error) {
            global.log(`${nameofthing} = (oops, can not do that)`)
            global.log(error)
            global.log(error.message)
        }
    }
})()

and use it like so:

inspect('myobject', myobject)

or like so, to hide keys starting with underscores:

inspect('myobject', myobject, false)

or like so, to fold specific keys you are not interested in:

inspect('myobject', myobject, true, ['key1', 'key2', ...])

Viewing the log

To view the logged messages, either use the console or a GUI (or both).

Using the console

tail -f ~/.xsession-errors

Or, prior to Cinnamon 3.8.8:

tail -f ~/.cinnamon/glass.log 

Press ctrl + c to abort the logging.

To reload the applet:

dbus-send --session --dest=org.Cinnamon.LookingGlass -- \
type=method_call /org/Cinnamon/LookingGlass \
org.Cinnamon.LookingGlass.ReloadExtension \
string:'your-applet-uuid' \
string:'APPLET'

where your-applet-uuid is the value of the uuid key in metadata.json.

It is simplest to run this command from another console, so you do not have to abort the logging.

Using a GUI

Cinnamon also has the Melange-Cinnamon debugger. To open it:

• Right click on the task bar and select Troubleshoot → Looking Glass.
• Alternatively, press Win key + L.
• Alternatively, press Alt + F2 → type "lg" → Enter.

The logging happens under the log tab.

To reload the code of the applet, find it under the extensions tab, right click → reload code.

To reload Cinnamon and do other things, use the Actions button.

The debugger has a small bug, sometimes the scroll bar disappears when long stuff gets logged. To get it back, press ctrl + Home and ctrl + End.

To be able to have the debugger window in the background, right click its title bar.

Kudos

• refReplacer function
• Prettyprinting JSON
• Cinnamon log location
• Reload applet from console
• Melange-Cinnamon Debugger
• tail command
• Alt + F2 shortcut