timer.specs.js

import { expect } from 'chai'
import { useFakeTimers } from 'sinon'

const createStopWatch = () => {
    const createDisplays = (main, laps = []) => ({main, laps})
    let startTime
    const sw = {
        displays: createDisplays(null),
        toggle: () => {
            sw.displays = createDisplays(0)
            startTime = new Date()
            setInterval(() => {
                sw.displays.main = new Date() - startTime
            }, 10)
        },
    }
    return sw
}

// ...

        const elapses = (ms, inner) => {
            describe(`${toSec(ms)} elapses`, () => {
                beforeEach(() => clock.tick(ms))
                inner()
            })
        }

        main_and_laps_should_display(null, [])

        elapses(1000, () => {
            main_and_laps_should_display(null, [])
        })

        describe(`started`, () => {
            beforeEach(() => sw.toggle())
            main_and_laps_should_display(0, [])

            elapses(10000, () => {
                main_and_laps_should_display(10000, [])

                elapses(1000, () => {
                    main_and_laps_should_display(11000, [])
                })
            })
        })