Part III: Implementing Higher-Order Functions with TDD

The three rules of TDD

Robert C. Martin provides a concise set of rules for practicing Test-Driven Development. [?] (opens new window)

1. You are not allowed to write any production code unless it is to make a failing unit test pass.
2. You are not allowed to write any more of a unit test than is sufficient to fail; and compilation failures are failures.
3. You are not allowed to write any more production code than is sufficient to pass the one failing unit test.

Create test case for “filter” function

Create a filter.test.ts file in modules folder, and create a test case for filter function.

import { test, expect } from 'vitest';
import { filter } from './index';

test('filter', () => {
  const actual = filter([1, 2, 3, 4, 5], (v: number) => v > 2);
  const expected = [3, 4, 5];

  expect(actual).toStrictEqual(expected);
});

Run test command.

npm run test

🔴 FAIL  src/modules/filter.test.ts > filter
TypeError: filter is not a function

Implement “filter” function

Update index.ts file in modules folder.

// ...
import filter from './filter';

export {
  // ...
  filter,
};

Create a filter.ts file in modules folder, and implement the filter function.

const filter = (items: Array<any>, callable: Function) => {
  const res = [];
  for (let i = 0; i < items.length; i++) {
    if (callable(items[i])) {
      res.push(items[i]);
    }
  }
  return res;
};

export default filter;

Run test command.

npm run test

🟢 PASS  Waiting for file changes...

Implement more functions

Implement more higher-order functions with TDD:

• map ✅
• filter ✅
• every
• find
• forEach
• reduce
• reject
• some

Finally, run coverage command.

npm run coverage