Update first_rule.md

testing/first_rule.md

@@ -11,4 +11,64 @@ Clean tests should follow the rules:
 -   **Self-Validating** a test should answer with either _Passed_ or _Failed_. You don't need to compare log files to answer if a test passed.
 
 -   **Timely** unit tests should be written before the production code. If you write tests after the production code, you might find writing tests too hard.
-
+
+** Good
+
+```typescript
+// add.ts
+
+export const add = (a: number, b: number): number => {
+  return a + b;
+};
+
+// add.test.ts
+
+import { add } from './add';
+
+describe('add function', () => {
+  // Fast: This test is very quick to run.
+  // Independent: This test does not depend on any other tests.
+  // Repeatable: This test is repeatable, as it will always produce the same result given the same inputs.
+  // Self-Validating: The test itself will report if it passes or fails.
+  // Timely: The test is written before the actual function (in a TDD manner).
+
+  it('correctly adds two numbers', () => {
+    const result = add(1, 2);
+    expect(result).toBe(3);
+  });
+
+  // We can also add more tests to cover more cases
+  it('correctly adds two negative numbers', () => {
+    const result = add(-1, -2);
+    expect(result).toBe(-3);
+  });
+
+  it('correctly adds a positive and a negative number', () => {
+    const result = add(-1, 2);
+    expect(result).toBe(1);
+  });
+});
+```
+
+** Bad
+
+```typescript
+// add.test.ts
+
+import { add } from './add';
+
+let previousResult = 0;
+
+describe('add function', () => {
+  it('correctly adds two numbers', () => {
+    const result = add(1, 2);
+    previousResult = result;
+    expect(result).toBe(3);
+  });
+
+  it('correctly adds two other numbers', () => {
+    const result = add(previousResult, 5);
+    expect(result).toBe(8);
+  });
+});
+```