diff --git a/calculator/test.js b/calculator/test.js
index e69de29..4d31fbf 100644
--- a/calculator/test.js
+++ b/calculator/test.js
@@ -0,0 +1,284 @@
+const fs = require('fs');
+const path = require('path');
+const assert = require('assert');
+
+// --- Mock Bangle.js environment ---
+let mock_display_str = "";
+let mock_ui_callbacks = {};
+
+// Mock 'g' (graphics)
+global.g = {
+  _col: 0,
+  _bg: 0,
+  _font: "6x8",
+  _font_size: 1,
+  clear: () => {},
+  clearRect: () => {},
+  setColor: (c) => { global.g._col = c; return global.g; },
+  setBgColor: (c) => { global.g._bg = c; return global.g; },
+  fillRect: () => {},
+  setFont: function(font, size) {
+    if (font && font.includes(":")) {
+      const parts = font.split(":");
+      this._font = parts[0];
+      this._font_size = parseInt(parts[1], 10);
+    } else {
+      this._font = font;
+      if (size) this._font_size = size;
+    }
+    return this;
+  },
+  setFontAlign: () => {},
+  stringWidth: (s) => s.length * 8 * (global.g._font_size || 1), // simple approximation
+  getWidth: () => 176,
+  getHeight: () => 176,
+  drawString: (s) => { mock_display_str = String(s); },
+  getFontHeight: () => 8 * (global.g._font_size || 1)
+};
+
+global.Graphics = {};
+
+// Mock 'process'
+global.process = {
+  env: { HWVERSION: 2 } // Simulate Bangle.js 2
+};
+
+// Mock 'Bangle'
+global.Bangle = {
+  setUI: (callbacks) => { mock_ui_callbacks = callbacks; },
+  http: () => new Promise(resolve => resolve({resp: "{}"})),
+  buzz: () => {},
+};
+
+// Mock 'load' (to exit app)
+global.load = () => {};
+
+// Mock 'require' for fonts
+const originalRequire = require;
+global.require = (name) => {
+  if (name === "FontDylex7x13") {
+    return { add: () => {} };
+  }
+  return originalRequire(name);
+};
+// --- End Mock ---
+
+
+// --- Load calculator app ---
+const calculatorCode = fs.readFileSync(path.join(__dirname, 'calculator.app.js'), 'utf8');
+eval(calculatorCode);
+// --- End Load ---
+
+
+// --- Test Framework ---
+const test_suite = [];
+let tests_passed = 0;
+let tests_failed = 0;
+
+function test(name, fn) {
+  test_suite.push({ name, fn });
+}
+
+function runTests() {
+  test_suite.forEach(t => {
+    // Reset state before each test
+    buttonPress('R'); // C
+    buttonPress('R'); // AC
+    if (mock_display_str !== '0') {
+      console.error(`[FAIL] ${t.name} - Failed to reset state. Display is: "${mock_display_str}"`);
+      tests_failed++;
+      return;
+    }
+
+    try {
+      t.fn();
+      console.log(`[PASS] ${t.name}`);
+      tests_passed++;
+    } catch (e) {
+      console.error(`[FAIL] ${t.name}`);
+      console.error(e);
+      tests_failed++;
+    }
+  });
+
+  console.log(`\nTests finished. Passed: ${tests_passed}, Failed: ${tests_failed}.`);
+  if (tests_failed > 0) {
+    process.exit(1);
+  }
+}
+
+function press(buttons) {
+  for (const button of buttons) {
+    buttonPress(button);
+  }
+}
+
+function checkDisplay(expected, message) {
+  assert.strictEqual(mock_display_str, String(expected), message);
+}
+// --- End Test Framework ---
+
+
+// --- Test Cases ---
+
+test('Addition', () => {
+  press('1+2=');
+  checkDisplay('3');
+});
+
+test('Subtraction', () => {
+  press('9-5=');
+  checkDisplay('4');
+});
+
+test('Multiplication', () => {
+  press('3*4=');
+  checkDisplay('12');
+});
+
+test('Division', () => {
+  press('10/2=');
+  checkDisplay('5');
+});
+
+test('Chained operations (no precedence)', () => {
+  press('2+3*4='); // (2+3)*4 = 20
+  checkDisplay('20');
+});
+
+test('Decimal numbers', () => {
+  press('1.5+2.5=');
+  checkDisplay('4');
+});
+
+test('Clear and All Clear', () => {
+  press('123+');
+  checkDisplay('123');
+  press('R'); // Clear
+  checkDisplay('0');
+  press('456');
+  checkDisplay('456');
+  press('=');
+  checkDisplay('579'); // 123 + 456
+  press('R'); // Clear
+  checkDisplay('0');
+  buttonPress('R'); // All Clear
+  press('+1=');
+  checkDisplay('1');
+});
+
+
+test('Negative result', () => {
+  press('5-10=');
+  checkDisplay('-5');
+});
+
+test('Negative input', () => {
+  press('5*');
+  press('2');
+  press('N'); // make it -2
+  press('=');
+  checkDisplay('-10');
+});
+
+test('Division by zero', () => {
+  press('5/0=');
+  checkDisplay('INF');
+});
+
+test('Percentage', () => {
+  press('200*5%=');
+  checkDisplay('10');
+});
+
+test('Square root', () => {
+  press('16');
+  press('r');
+  checkDisplay('4');
+  press('='); // pressing equals after unary op
+  checkDisplay('4');
+});
+
+test('Square', () => {
+  press('5');
+  press('s');
+  checkDisplay('25');
+  press('=');
+  checkDisplay('25');
+});
+
+test('Inverse', () => {
+  press('4');
+  press('i');
+  checkDisplay('0.25');
+});
+
+test('Backspace', () => {
+  press('123B');
+  checkDisplay('12');
+  press('B');
+  checkDisplay('1');
+  press('B');
+  checkDisplay('0');
+  press('B');
+  checkDisplay('0');
+});
+
+test('Repeated equals', () => {
+  press('2+3=');
+  checkDisplay('5');
+  press('=');
+  checkDisplay('8');
+  press('=');
+  checkDisplay('11');
+});
+
+test('Operator change', () => {
+  press('10+-*/2=');
+  checkDisplay('5'); // 10/2=5
+});
+
+test('Long number with separators', () => {
+  press('1234567');
+  checkDisplay('1,234,567');
+});
+
+test('Sine (deg)', () => {
+  press('30');
+  press('sin');
+  checkDisplay('0.5');
+  press('R'); press('R');
+  press('90');
+  press('sin');
+  checkDisplay('1');
+  press('R'); press('R');
+  press('180');
+  press('sin');
+  checkDisplay('0');
+});
+
+test('Cosine (deg)', () => {
+  press('60');
+  press('cos');
+  checkDisplay('0.5');
+  press('R'); press('R');
+  press('0');
+  press('cos');
+  checkDisplay('1');
+  press('R'); press('R');
+  press('90');
+  press('cos');
+  checkDisplay('0');
+});
+
+test('Angle mode switch to radians and back', () => {
+    press('angleMode'); // now rad
+    // The buttonPress function doesn't return the state, but we can check the object it modifies
+    assert.strictEqual(scientificOperators.angleMode.val, 'rad');
+    press('angleMode'); // back to deg
+    assert.strictEqual(scientificOperators.angleMode.val, 'deg');
+});
+
+
+// Run all the defined tests
+runTests();