reporter.js

const mocha       = require ('mocha')
    , ansi        = require ('ansicolor')
    , ololog      = require ('ololog')
    , StackTracey = require ('stacktracey')
    , log         = ololog.configure ({ locate: false })
    , { isBlank } = require ('printable-characters')

/*  ------------------------------------------------------------------------ */

process.on ('uncaughtException',  e => log.red.error (e))
process.on ('unhandledRejection', e => log.red.error (e))

/*  ------------------------------------------------------------------------ */

const sleep = ms => new Promise (resolve => setTimeout (resolve, ms))

/*  ------------------------------------------------------------------------ */

module.exports = function (runner) {

    // NB: Do this before calling the Base constructor, because in the recent Mocha version, the `err` object
    //     comes with the `actual` and `expected` fields already stringified, and we cannot access the original
    //     objects no more, to perform a custom formatting.
    // 
    //     But if we add this event listener before the default ones, we intercept the `err` object before it
    //     gets irreversibly "corrupted" by the Mocha's internals.
    //
    runner.on ('fail', (test, err) => { log.red.error (err) })

    mocha.reporters.Base.call (this, runner)

    const originalImpl = Object.assign ({}, ololog.impl)
    const cursorUp     = '\u001b[1A'

    StackTracey.resetCache () // when running mocha under --watch, this is needed to re-load the changed sources

    runner.on ('suite', ({ title }) => {

        if (title) {
            log.bright (title + ':', '\n')
        }
    })

    runner.on ('suite end', ({ title, tests }) => {

        if (tests.length) {
            console.log ('')
        }
    })

    runner.on ('test', test => {
            
        test.logBuffer = ''

        let prevLocation

        ololog.impl.render = text => {

            const lines = text.split ('\n')
            const multiline = lines.length > 1

            const location = new StackTracey ().clean.at (2)
            const locationChanged = prevLocation && !StackTracey.locationsEqual (location, prevLocation)
            prevLocation = location

            test.logBuffer += ((locationChanged || multiline) ? '\n' : '') + text + '\n'
        }

        ;(async () => {
            
            const clock = ['◐', '◓', '◑', '◒']
            
            console.log ('')
            
            for (let i = 0, n = clock.length; !test.state; i++) {

                console.log (ansi.darkGray (cursorUp + clock[i % n] + '  ' + test.title + ' ' + '.'.repeat (i/2 % 5).padEnd (5)))
                await sleep (100)
            }

        }) ()
    })

    runner.on ('test end', ({ state = undefined, title, logBuffer, only, verbose = false, parent, ...other }) => {

        ololog.impl.render = originalImpl.render

        if (state) {
            
            log.darkGray (cursorUp + { 'passed': '😎', 'failed': '👹' }[state] + ' ',  title, '    ')
            
            const onlyEnabled = parent._onlyTests.length

            while (!verbose && parent) {
                verbose = parent.verbose
                parent = parent.parent
            }

            let show = (onlyEnabled || verbose || (state === 'failed')) && logBuffer
            if (show) {

                const sanitized = logBuffer
                                    .split ('\n')
                                    .map (line => isBlank (line) ? '' : line)
                                    .join ('\n')

                log ('  ', '\n' + sanitized.replace (/\n\n\n+/g, '\n\n').trim () + '\n')
            }
        }
    })
}