My personal hammerspoon config.

init.lua

-- global config --
-- disable all window animation
hs.window.animationDuration = 0

-- custom alert style
hs.alert.defaultStyle.radius = 4
hs.alert.defaultStyle.fillColor = {white = 0, alpha = 0.9}
hs.alert.defaultStyle.strokeColor = {white = 0, alpha = 0}
hs.alert.defaultStyle.fadeInDuration = 0
hs.alert.defaultStyle.fadeOutDuration = 0
hs.alert.defaultStyle.textSize = 14
hs.alert.defaultStyle.textStyle = {paragraphStyle = {alignment = "center"}}

-- display names
local dellU2414H = "DELL U2414H"
local builtInDisplay = "Color LCD"

-- display layouts --
-- layout if *only* Dell Monitor is running
local dellU2414HLayout = {
  {"Tweetbot", hs.appfinder.windowFromWindowTitlePattern("Main Window - "), dellU2414H, hs.geometry.unitrect(0,0,0.175,1), nil, nil},
  {"TaskPaper", nil, builtInDisplay, hs.geometry.unitrect(0,0,0.175,1), nil, nil},
  {"Google Chrome", nil, dellU2414H, hs.geometry.unitrect(0.175,0,0.825,1), nil, nil},
  {"MacVim", nil, dellU2414H, hs.geometry.unitrect(0.175,0,0.825,1), nil, nil}
}

-- layout if no external displays are connected
local macbookProLayout = {
  {"Tweetbot", hs.appfinder.windowFromWindowTitlePattern("Main Window - "), builtInDisplay, hs.geometry.unitrect(0,0,0.275,1), nil, nil},
  {"TaskPaper", nil, builtInDisplay, hs.geometry.unitrect(0,0,0.275,1), nil, nil},
  {"Google Chrome", nil, builtInDisplay, hs.geometry.unitrect(0.275,0,0.725,1), nil, nil},
  {"MacVim", nil, builtInDisplay, hs.geometry.unitrect(0.275,0,0.725,1), nil, nil}
}

-- layout if Dell Monitor and Macbook Pro displays are running
local dualMonitorLayout = {
  {"Tweetbot", hs.appfinder.windowFromWindowTitlePattern("Main Window - "), dellU2414H, hs.geometry.unitrect(0,0,0.2,1), nil, nil},
  {"Google Chrome", nil, dellU2414H, hs.geometry.unitrect(0.2,0,0.8,1), nil, nil},
}

-- function to check if external display is connected
function hasExternalDisplay()
  for _, screen in pairs(hs.screen.allScreens()) do
    if screen:name() == dellU2414H then
      return true
    end
  end
  return false
end

-- function to watch applications
function applicationWatcher(appName, eventType, appObject)
  if (eventType == hs.application.watcher.activated) then
    if (appName == "Tweetbot" or appName == "MacVim" or appName == "TaskPaper") then
      if hasExternalDisplay() then
        hs.layout.apply(dellU2414HLayout)
      else
        hs.layout.apply(macbookProLayout)
      end
    end
  end
end

usbWatcher = nil

-- usb device uuids
local targusHub = 0x0610
local pokerII = 0x0611
local mxMaster2S = 0xc52b
local audioEngineA2 = 0x2704

-- custom hs.alert.show function
function showAlert(message, style, screen, duration)
  -- show only one alert at a time
  hs.alert.closeAll()
  hs.alert.show(message, style, screen, duration)
end

-- function to check if USB devices are connected/removed
function usbDeviceCallback(data)
  if (data["productID"] == targusHub) then
    if (data["eventType"] == "added") then
      showAlert('🔌USB: Targus Hub is connected.')
    elseif (data["eventType"] == "removed") then
      showAlert('🔌USB: Targus Hub is removed.')
    end
  end

  if (data["productID"] == pokerII) then
    if (data["eventType"] == "added") then
      showAlert('⌨️USB: Poker II is connected.')
    elseif (data["eventType"] == "removed") then
      showAlert('⌨️USB: Poker II is removed.')
    end
  end

  if (data["productID"] == mxMaster2S) then
    if (data["eventType"] == "added") then
      showAlert('🖱USB: MX Master 2S is connected.')
    elseif (data["eventType"] == "removed") then
      showAlert('🖱USB: MX Master 2S is removed.')
    end
  end

  if (data["productID"] == audioEngineA2) then
    if (data["eventType"] == "added") then
      showAlert('🔊USB: AudioEngine A2+ is connected.')
    elseif (data["eventType"] == "removed") then
      showAlert('🔊USB: AudioEngine A2+ is removed.')
    end
  end
end

-- Spotify bindings --
-- toggle play/pause
hs.hotkey.bind({"cmd", "alt", "shift"}, "8", function() hs.spotify.playpause() end)

-- volume down
hs.hotkey.bind({"cmd", "alt", "shift"}, "-", function()
  hs.spotify.volumeDown()
  showAlert(
    "🔈Spotify volume: " .. hs.spotify.getVolume() .. "%"
  )
end)

-- volume up
hs.hotkey.bind({"cmd", "alt", "shift"}, "=", function()
  hs.spotify.volumeUp()
  showAlert(
    "🔊Spotify volume: " .. hs.spotify.getVolume() .. "%"
  )
end)

-- next track
hs.hotkey.bind({"cmd", "alt", "shift"}, "9", function()
  hs.spotify.next()
  showAlert(
    "⏭Spotify: " .. hs.spotify.getCurrentTrack() .. " by " .. hs.spotify.getCurrentArtist()
  )
end)

-- previous track
hs.hotkey.bind({"cmd", "alt", "shift"}, "7", function()
  hs.spotify.previous()
  showAlert(
    "⏮Spotify: " .. hs.spotify.getCurrentTrack() .. " by " .. hs.spotify.getCurrentArtist()
  )
end)

-- rewind 5 seconds
hs.hotkey.bind({"cmd", "alt", "shift"}, ",", function()
  hs.spotify.rw()
  showAlert(
    "⏪Spotify: Rewind track by 5 seconds"
  )
end)

-- forward 5 seconds
hs.hotkey.bind({"cmd", "alt", "shift"}, ".", function()
  hs.spotify.ff()
  showAlert(
    "⏩Spotify: Forward track by 5 seconds"
  )
end)

-- display current track info as alert
hs.hotkey.bind({"cmd", "alt", "shift"}, "0", function()
  showAlert(
    "💿Spotify: " .. hs.spotify.getCurrentTrack() .. " by " .. hs.spotify.getCurrentArtist()
  )
end)

-- reload hammerspoon config
hs.hotkey.bind({"cmd", "alt", "ctrl"}, "r", function()
  hs.reload()
  showAlert('✨Config reloaded.')
end)

-- show current time in an alert
hs.hotkey.bind({"cmd", "alt", "ctrl"}, "t", function()
  showAlert(
    "⏰Current time: " .. os.date("%I:%M%p")
  )
end)

-- show current battery in an alert
hs.hotkey.bind({"cmd", "alt", "ctrl"}, "b", function()
  showAlert(
    "⚡️Current battery: " .. hs.battery.percentage() .. "%"
  )
end)

-- function to set muted(boolean)
function micCheck()
  if hs.audiodevice.defaultInputDevice():muted() then
    hs.audiodevice.defaultInputDevice():setMuted(false)

    showAlert(
      "🎙Microphone: Live and recording... (Menu + M to mute)"
    )
  else
    hs.audiodevice.defaultInputDevice():setMuted(true)
    showAlert(
      "🎙Microphone: Muted"
    )
  end
end

-- mute / unmute microphone
hs.hotkey.bind({"cmd", "alt", "shift"}, "m", function()
  micCheck()
end)

appWatcher = hs.application.watcher.new(applicationWatcher)
appWatcher:start()

layoutWatcher = hs.screen.watcher.new(hasExternalDisplay)
layoutWatcher:start()

usbWatcher = hs.usb.watcher.new(usbDeviceCallback)
usbWatcher:start()