diff --git a/.docker/ci.Dockerfile b/.docker/ci.Dockerfile
index 3b483e1..a8bad23 100644
--- a/.docker/ci.Dockerfile
+++ b/.docker/ci.Dockerfile
@@ -1,4 +1,8 @@
 FROM akorn/luarocks:lua5.4-alpine
 
-RUN apk add gcc musl-dev
-RUN luarocks install luacheck
+RUN apk add gcc musl-dev make
+
+RUN luarocks install luacheck \
+    && luarocks install luaunit
+
+WORKDIR shiftit
diff --git a/.docker/docker-compose.ci.yaml b/.docker/docker-compose.ci.yaml
index 9c77b53..15d6728 100644
--- a/.docker/docker-compose.ci.yaml
+++ b/.docker/docker-compose.ci.yaml
@@ -5,4 +5,4 @@ services:
       context: ..
       dockerfile: ./.docker/ci.Dockerfile
     volumes:
-      - ..:/shifit
+      - ..:/shiftit
diff --git a/.github/workflows/ci.yaml b/.github/workflows/ci.yaml
index 45ceaaa..ffcceea 100644
--- a/.github/workflows/ci.yaml
+++ b/.github/workflows/ci.yaml
@@ -16,3 +16,6 @@ jobs:
 
       - name: Run linter
         run: make ci-lint
+
+      - name: Run tests
+        run: make ci-test
diff --git a/Makefile b/Makefile
index 4fc9d1f..74efea2 100644
--- a/Makefile
+++ b/Makefile
@@ -1,7 +1,13 @@
+test:
+	find . -name "*_test.lua" | xargs sh -c 'lua $$0 -v'
+
 ci-init:
 	docker-compose -f .docker/docker-compose.ci.yaml build
 
 ci-lint:
-	docker-compose -f .docker/docker-compose.ci.yaml run hammerspoon-shiftit-ci luacheck --globals hs -- shifit/
+	docker-compose -f .docker/docker-compose.ci.yaml run hammerspoon-shiftit-ci luacheck --globals hs -- .
+
+ci-test:
+	docker-compose -f .docker/docker-compose.ci.yaml run hammerspoon-shiftit-ci make test
 
 .PHONY: ci-init ci-lint
diff --git a/hammerspoon_mocks.lua b/hammerspoon_mocks.lua
new file mode 100644
index 0000000..f6a527d
--- /dev/null
+++ b/hammerspoon_mocks.lua
@@ -0,0 +1,61 @@
+local lu = require('luaunit')
+
+local hotkey = {
+    bindings = {},
+}
+
+local defaultScreenRect = { x = 0, y = 0, w = 1200, h = 800 }
+local defaultWindowRect = { x = 100, y = 100, w = 1000, h = 600 }
+
+function hotkey.bind(mods, key, fn)
+    lu.assertNotNil(mods)
+    lu.assertNotNil(key)
+    lu.assertIsFunction(fn)
+    table.insert(hotkey.bindings, { mods, key, fn })
+end
+
+local screen = {
+    rect = defaultScreenRect,
+}
+
+function screen:frame()
+    return self.rect
+end
+
+local window = {
+    rect = defaultWindowRect,
+    _screen = screen,
+}
+
+function window.focusedWindow()
+    return window
+end
+
+function window:frame()
+    return self.rect
+end
+
+function window:screen()
+    return self._screen
+end
+
+function window:move(rect, _, _, _)
+    lu.assertIsNumber(rect.x)
+    lu.assertIsNumber(rect.y)
+    lu.assertIsNumber(rect.w)
+    lu.assertIsNumber(rect.h)
+    self.rect = rect
+end
+
+local mocks = {
+    hotkey = hotkey,
+    window = window,
+}
+
+function mocks:reset()
+    self.hotkey.bindings = {}
+    self.window.rect = defaultWindowRect
+    self.window._screen.rect = defaultScreenRect
+end
+
+return mocks
diff --git a/init.lua b/init.lua
index 34042c7..8f4ec36 100644
--- a/init.lua
+++ b/init.lua
@@ -4,7 +4,9 @@
 ---
 --- Download: https://github.com/peterklijn/hammerspoon-shiftit/raw/master/Spoons/ShiftIt.spoon.zip
 
-local obj = {}
+local obj = {
+  hs = hs
+}
 obj.__index = obj
 
 -- Metadata
@@ -48,11 +50,11 @@ local units = {
   maximum = { x = 0.00, y = 0.00, w = 1.00, h = 1.00 },
 }
 
-local function move(unit) hs.window.focusedWindow():move(unit, nil, true, 0) end
+function obj:move(unit) self.hs.window.focusedWindow():move(unit, nil, true, 0) end
 
-local function resizeWindowInSteps(increment)
-  local screen = hs.window.focusedWindow():screen():frame()
-  local window = hs.window.focusedWindow():frame()
+function obj:resizeWindowInSteps(increment)
+  local screen = self.hs.window.focusedWindow():screen():frame()
+  local window = self.hs.window.focusedWindow():frame()
   local wStep = math.floor(screen.w / 12)
   local hStep = math.floor(screen.h / 12)
   local x, y, w, h = window.x, window.y, window.w, window.h
@@ -67,7 +69,7 @@ local function resizeWindowInSteps(increment)
     w = math.min(screen.w - x + screen.x, w + wStep)
     h = math.min(screen.h - y + screen.y, h + hStep)
   else
-local     noChange = true
+    local noChange = true
     local notMinWidth = w > wStep * 3
     local notMinHeight = h > hStep * 3
 
@@ -105,44 +107,44 @@ local     noChange = true
       h = notMinHeight and h - hStep * 2 or h
     end
   end
-  hs.window.focusedWindow():move({ x = x, y = y, w = w, h = h }, nil, true, 0)
+  self:move({ x = x, y = y, w = w, h = h })
 end
 
-function obj.left() move(units.left50, nil, true, 0) end
+function obj:left() self:move(units.left50) end
 
-function obj.right() move(units.right50, nil, true, 0) end
+function obj:right() self:move(units.right50) end
 
-function obj.up() move(units.top50, nil, true, 0) end
+function obj:up() self:move(units.top50) end
 
-function obj.down() move(units.bot50, nil, true, 0) end
+function obj:down() self:move(units.bot50) end
 
-function obj.upleft() move(units.upleft50, nil, true, 0) end
+function obj:upleft() self:move(units.upleft50) end
 
-function obj.upright() move(units.upright50, nil, true, 0) end
+function obj:upright() self:move(units.upright50) end
 
-function obj.botleft() move(units.botleft50, nil, true, 0) end
+function obj:botleft() self:move(units.botleft50) end
 
-function obj.botright() move(units.botright50, nil, true, 0) end
+function obj:botright() self:move(units.botright50) end
 
-function obj.maximum() move(units.maximum, nil, true, 0) end
+function obj:maximum() self:move(units.maximum) end
 
-function obj.toggleFullScreen() hs.window.focusedWindow():toggleFullScreen() end
+function obj:toggleFullScreen() self.hs.window.focusedWindow():toggleFullScreen() end
 
-function obj.toggleZoom() hs.window.focusedWindow():toggleZoom() end
+function obj:toggleZoom() self.hs.window.focusedWindow():toggleZoom() end
 
-function obj.center() hs.window.focusedWindow():centerOnScreen(nil, true, 0) end
+function obj:center() self.hs.window.focusedWindow():centerOnScreen(nil, true, 0) end
 
-function obj.nextScreen()
-  hs.window.focusedWindow():moveToScreen(hs.window.focusedWindow():screen():next(), false, true, 0)
+function obj:nextScreen()
+  self.hs.window.focusedWindow():moveToScreen(self.hs.window.focusedWindow():screen():next(), false, true, 0)
 end
 
-function obj.previousScreen()
-  hs.window.focusedWindow():moveToScreen(hs.window.focusedWindow():screen():previous(), false, true, 0)
+function obj:prevScreen()
+  self.hs.window.focusedWindow():moveToScreen(self.hs.window.focusedWindow():screen():previous(), false, true, 0)
 end
 
-function obj.resizeOut() resizeWindowInSteps(true) end
+function obj:resizeOut() self:resizeWindowInSteps(true) end
 
-function obj.resizeIn() resizeWindowInSteps(false) end
+function obj:resizeIn() self:resizeWindowInSteps(false) end
 
 --- HammerspoonShiftIt:bindHotkeys(mapping)
 --- Method
@@ -172,24 +174,24 @@ function obj:bindHotkeys(mapping)
     for k, v in pairs(mapping) do self.mapping[k] = v end
   end
 
-  hs.hotkey.bind(self.mapping.left[1], self.mapping.left[2], function() self:left() end)
-  hs.hotkey.bind(self.mapping.right[1], self.mapping.right[2], function() self:right() end)
-  hs.hotkey.bind(self.mapping.up[1], self.mapping.up[2], function() self:up() end)
-  hs.hotkey.bind(self.mapping.down[1], self.mapping.down[2], function() self:down() end)
-  hs.hotkey.bind(self.mapping.upleft[1], self.mapping.upleft[2], function() self:upleft() end)
-  hs.hotkey.bind(self.mapping.upright[1], self.mapping.upright[2], function() self:upright() end)
-  hs.hotkey.bind(self.mapping.botleft[1], self.mapping.botleft[2], function() self:botleft() end)
-  hs.hotkey.bind(self.mapping.botright[1], self.mapping.botright[2], function() self:botright() end)
-  hs.hotkey.bind(self.mapping.maximum[1], self.mapping.maximum[2], function() self:maximum() end)
-  hs.hotkey.bind(self.mapping.toggleFullScreen[1], self.mapping.toggleFullScreen[2], function()
+  self.hs.hotkey.bind(self.mapping.left[1], self.mapping.left[2], function() self:left() end)
+  self.hs.hotkey.bind(self.mapping.right[1], self.mapping.right[2], function() self:right() end)
+  self.hs.hotkey.bind(self.mapping.up[1], self.mapping.up[2], function() self:up() end)
+  self.hs.hotkey.bind(self.mapping.down[1], self.mapping.down[2], function() self:down() end)
+  self.hs.hotkey.bind(self.mapping.upleft[1], self.mapping.upleft[2], function() self:upleft() end)
+  self.hs.hotkey.bind(self.mapping.upright[1], self.mapping.upright[2], function() self:upright() end)
+  self.hs.hotkey.bind(self.mapping.botleft[1], self.mapping.botleft[2], function() self:botleft() end)
+  self.hs.hotkey.bind(self.mapping.botright[1], self.mapping.botright[2], function() self:botright() end)
+  self.hs.hotkey.bind(self.mapping.maximum[1], self.mapping.maximum[2], function() self:maximum() end)
+  self.hs.hotkey.bind(self.mapping.toggleFullScreen[1], self.mapping.toggleFullScreen[2], function()
     self:toggleFullScreen()
   end)
-  hs.hotkey.bind(self.mapping.toggleZoom[1], self.mapping.toggleZoom[2], function() self:toggleZoom() end)
-  hs.hotkey.bind(self.mapping.center[1], self.mapping.center[2], function() self:center() end)
-  hs.hotkey.bind(self.mapping.nextScreen[1], self.mapping.nextScreen[2], function() self:nextScreen() end)
-  hs.hotkey.bind(self.mapping.previousScreen[1], self.mapping.previousScreen[2], function() self:previousScreen() end)
-  hs.hotkey.bind(self.mapping.resizeOut[1], self.mapping.resizeOut[2], function() self:resizeOut() end)
-  hs.hotkey.bind(self.mapping.resizeIn[1], self.mapping.resizeIn[2], function() self:resizeIn() end)
+  self.hs.hotkey.bind(self.mapping.toggleZoom[1], self.mapping.toggleZoom[2], function() self:toggleZoom() end)
+  self.hs.hotkey.bind(self.mapping.center[1], self.mapping.center[2], function() self:center() end)
+  self.hs.hotkey.bind(self.mapping.nextScreen[1], self.mapping.nextScreen[2], function() self:nextScreen() end)
+  self.hs.hotkey.bind(self.mapping.previousScreen[1], self.mapping.previousScreen[2], function() self:prevScreen() end)
+  self.hs.hotkey.bind(self.mapping.resizeOut[1], self.mapping.resizeOut[2], function() self:resizeOut() end)
+  self.hs.hotkey.bind(self.mapping.resizeIn[1], self.mapping.resizeIn[2], function() self:resizeIn() end)
 
   return self
 end
diff --git a/init_test.lua b/init_test.lua
new file mode 100644
index 0000000..1605e22
--- /dev/null
+++ b/init_test.lua
@@ -0,0 +1,214 @@
+local lu = require('luaunit')
+local shiftit = require('init')
+local hsmocks = require('hammerspoon_mocks')
+
+-- disable lint errors for mutating global variable TestShiftIt:
+-- luacheck: ignore 112
+
+TestShiftIt = {} -- luacheck: ignore 111
+
+function TestShiftIt.setUp()
+    shiftit.hs = hsmocks
+    hsmocks:reset()
+end
+
+function TestShiftIt.testBindDefault()
+    shiftit:bindHotkeys({})
+    lu.assertEquals(#hsmocks.hotkey.bindings, 16)
+
+    local expected = {
+        'left', 'right', 'up', 'down',
+        '1', '2', '3', '4', 'm',
+        'f', 'z', 'c', 'n', 'p',
+        '=', '-',
+    }
+    for i, item in pairs(expected) do
+        lu.assertEquals(hsmocks.hotkey.bindings[i][1], shiftit.mash)
+        lu.assertEquals(hsmocks.hotkey.bindings[i][2], item)
+    end
+end
+
+function TestShiftIt.testBindOverrideVimKeys()
+    shiftit:bindHotkeys({
+        left = { { 'ctrl', 'alt', 'cmd' }, 'h' },
+        down = { { 'ctrl', 'alt', 'cmd' }, 'j' },
+        up = { { 'ctrl', 'alt', 'cmd' }, 'k' },
+        right = { { 'ctrl', 'alt', 'cmd' }, 'l' },
+    })
+    lu.assertEquals(#hsmocks.hotkey.bindings, 16)
+
+    local expected = {
+        'h', 'l', 'k', 'j',
+        '1', '2', '3', '4', 'm',
+        'f', 'z', 'c', 'n', 'p',
+        '=', '-',
+    }
+    for i, item in pairs(expected) do
+        lu.assertEquals(hsmocks.hotkey.bindings[i][1], shiftit.mash)
+        lu.assertEquals(hsmocks.hotkey.bindings[i][2], item)
+    end
+end
+
+function TestShiftIt.testResizeWindowInStepsStickingToSides()
+    local tests = {
+        {
+            desc = 'increase window sticking to left',
+            before = { x = 0, y = 0, w = 600, h = 800 },
+            expect = { x = 0, y = 0, w = 700, h = 800 },
+            increase = true,
+        },
+        {
+            desc = 'decrease window sticking to left',
+            before = { x = 0, y = 0, w = 600, h = 800 },
+            expect = { x = 0, y = 0, w = 500, h = 800 },
+            increase = false,
+        },
+        {
+            desc = 'increase window sticking to right',
+            before = { x = 600, y = 0, w = 600, h = 800 },
+            expect = { x = 500, y = 0, w = 700, h = 800 },
+            increase = true,
+        },
+        {
+            desc = 'decrease window sticking to right',
+            before = { x = 600, y = 0, w = 600, h = 800 },
+            expect = { x = 700, y = 0, w = 500, h = 800 },
+            increase = false,
+        },
+        {
+            desc = 'increase window sticking to top',
+            before = { x = 0, y = 0, w = 1200, h = 400 },
+            expect = { x = 0, y = 0, w = 1200, h = 466 },
+            increase = true,
+        },
+        {
+            desc = 'decrease window sticking to top',
+            before = { x = 0, y = 0, w = 1200, h = 400 },
+            expect = { x = 0, y = 0, w = 1200, h = 334 },
+            increase = false,
+        },
+        {
+            desc = 'increase window sticking to bottom',
+            before = { x = 0, y = 400, w = 1200, h = 400 },
+            expect = { x = 0, y = 334, w = 1200, h = 466 },
+            increase = true,
+        },
+        {
+            desc = 'decrease window sticking to bottom',
+            before = { x = 0, y = 400, w = 1200, h = 400 },
+            expect = { x = 0, y = 466, w = 1200, h = 334 },
+            increase = false,
+        },
+    }
+    for _, test in pairs(tests) do
+        hsmocks.window.rect = test.before
+        shiftit:resizeWindowInSteps(test.increase)
+        lu.assertEquals(hsmocks.window.rect, test.expect, test.desc)
+    end
+end
+
+function TestShiftIt.testResizeWindowInStepsStickingToCorners()
+    local tests = {
+        {
+            desc = 'increase window sticking to left top',
+            before = { x = 0, y = 0, w = 600, h = 400 },
+            expect = { x = 0, y = 0, w = 700, h = 466 },
+            increase = true,
+        },
+        {
+            desc = 'decrease window sticking to left top',
+            before = { x = 0, y = 0, w = 600, h = 400 },
+            expect = { x = 0, y = 0, w = 500, h = 334 },
+            increase = false,
+        },
+        {
+            desc = 'increase window sticking to right top',
+            before = { x = 600, y = 0, w = 600, h = 400 },
+            expect = { x = 500, y = 0, w = 700, h = 466 },
+            increase = true,
+        },
+        {
+            desc = 'decrease window sticking to right top',
+            before = { x = 600, y = 0, w = 600, h = 400 },
+            expect = { x = 700, y = 0, w = 500, h = 334 },
+            increase = false,
+        },
+        {
+            desc = 'increase window sticking to left bottom',
+            before = { x = 0, y = 400, w = 600, h = 400 },
+            expect = { x = 0, y = 334, w = 700, h = 466 },
+            increase = true,
+        },
+        {
+            desc = 'decrease window sticking to left bottom',
+            before = { x = 0, y = 400, w = 600, h = 400 },
+            expect = { x = 0, y = 466, w = 500, h = 334 },
+            increase = false,
+        },
+        {
+            desc = 'increase window sticking to right bottom',
+            before = { x = 0, y = 400, w = 600, h = 400 },
+            expect = { x = 0, y = 334, w = 700, h = 466 },
+            increase = true,
+        },
+        {
+            desc = 'decrease window sticking to right bottom',
+            before = { x = 600, y = 400, w = 600, h = 400 },
+            expect = { x = 700, y = 466, w = 500, h = 334 },
+            increase = false,
+        },
+    }
+    for _, test in pairs(tests) do
+        hsmocks.window.rect = test.before
+        shiftit:resizeWindowInSteps(test.increase)
+        lu.assertEquals(hsmocks.window.rect, test.expect, test.desc)
+    end
+end
+
+function TestShiftIt.testResizeWindowInStepsEdgeCases()
+    local tests = {
+        {
+            desc = 'does not exceed screen width',
+            before = { x = 5, y = 200, w = 1190, h = 400 },
+            expect = { x = 0, y = 134, w = 1200, h = 532 },
+            increase = true,
+        },
+        {
+            desc = 'does not exceed screen height',
+            before = { x = 200, y = 5, w = 800, h = 790 },
+            expect = { x = 100, y = 0, w = 1000, h = 800 },
+            increase = true,
+        },
+        {
+            desc = 'does not exceed screen size',
+            before = { x = 5, y = 5, w = 1190, h = 790 },
+            expect = { x = 0, y = 0, w = 1200, h = 800 },
+            increase = true,
+        },
+        {
+            desc = 'does not become too narrow',
+            before = { x = 0, y = 0, w = 300, h = 800 },
+            expect = { x = 0, y = 66, w = 300, h = 668 },
+            increase = false,
+        },
+        {
+            desc = 'does not become too short',
+            before = { x = 0, y = 0, w = 1200, h = 198 },
+            expect = { x = 100, y = 0, w = 1000, h = 198 },
+            increase = false,
+        },
+        {
+            desc = 'does not become too tiny',
+            before = { x = 100, y = 100, w = 300, h = 198 },
+            expect = { x = 100, y = 100, w = 300, h = 198 },
+            increase = false,
+        },
+    }
+    for _, test in pairs(tests) do
+        hsmocks.window.rect = test.before
+        shiftit:resizeWindowInSteps(test.increase)
+        lu.assertEquals(hsmocks.window.rect, test.expect, test.desc)
+    end
+end
+
+os.exit(lu.LuaUnit.run())