Specify files and add testing

src/calculator/calculator.test.ts

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+import Calculator from './calculator'
+
+test('adds 2 + 3 to equal 5', () => {
+  const calc = new Calculator()
+  expect(calc.add(2, 3)).toBe(5);
+});
+
+test('subtracts 2 - 3 to equal -1', () => {
+  const calc = new Calculator()
+  expect(calc.subtract(2, 3)).toBe(-1);
+});

src/calculator/calculator.ts

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+export default class Calculator {
+
+  add(x : number, y : number) : number {
+    return x + y;
+  }
+
+  subtract(x: number, y: number) : number {
+    return x - y;
+  }
+}