add new blog

contents/posts/2024-01-09-readable-unit-testing-for-local-timezone-using-dayjs.md

@@ -0,0 +1,97 @@
+---
+title: Readable unit testing for local timezone using dayjs
+uuid: yyqq6jk
+tags:
+  - TypeScript
+  - dayjs
+  - date
+  - timezone
+---
+
+Unit testing is a crucial aspect of software development, and when it comes to testing functionality dependent on time and timezones, having readable and effective tests is essential. 
+
+In this blog post, we'll explore how to perform unit testing for local timezones using TypeScript and `Dayjs`. We'll walk through a practical example involving a function to identify the state of a resource based on its environment and the current time. Additionally, we'll introduce a `Dayjs` timezone helper to facilitate seamless testing across different timezones.
+
+## Example Code
+Let's start by examining the core function responsible for identifying the state of a resource:
+
+```typescript
+/**
+ * Identify the state of the resource
+ * @param resource
+ * @param date - Passing date for testing purpose
+ * @returns
+ */
+
+export function identifyResourceState(
+    resource: { env: string }, 
+    date: Date = new Date()
+): 'scaled_up' | 'scaled_down' {
+  /**
+   * If the resource is not `dev`, the state is scaled up.
+   */
+  if (resource.env !== 'dev') {
+    return 'scaled_up';
+  }
+
+  /**
+   * Time between 8:00 UTC+7 to 19:00 UTC+7, the state is scaled up.
+   * Otherwise, the state is scaled down.
+   */
+  const hour = date.getUTCHours();
+  if (hour >= 1 && hour <= 12) {
+    return 'scaled_up';
+  }
+  return 'scaled_down';
+}
+```
+
+## Dayjs Timezone helper
+
+To enhance the testing process, we introduce a Dayjs helper to handle timezones effectively:
+
+```typescript
+// Dayjs helper
+import dayjs from 'dayjs';
+import utc from 'dayjs/plugin/utc';
+import timezone from 'dayjs/plugin/timezone';
+
+dayjs.extend(utc)
+dayjs.extend(timezone)
+
+export function createDateFromTimezone(date?: dayjs.ConfigType, timezone?: string) {
+  return (date?: dayjs.ConfigType) => dayjsFromTimezone(date, timezone).toDate();
+}
+
+export function dayjsFromTimezone(date?: dayjs.ConfigType, timezone?: string) {
+  return dayjs.tz(date, timezone).utc();
+}
+```
+
+## Implementation
+Now, let's look at how we can effectively test the `identifyResourceState` function using our Dayjs timezone helpe
+
+```typescript
+import { expect, test } from 'vitest';
+import { createDateFromTimezone } from './dayjs';
+import { identifyResourceState } from './identify-resource-state';
+
+const createBangkokDate = createDateFromTimezone('Asia/Bangkok');
+const dateString = '2023-12-15';
+
+test('identifyResourceState for dev', () => {
+  expect(identifyResourceState({ env: 'dev' }, createBangkokDate(`${dateString} 06:00`))).toBe('scaled_down');
+  expect(identifyResourceState({ env: 'dev' }, createBangkokDate(`${dateString} 09:00`))).toBe('scaled_up');
+  expect(identifyResourceState({ env: 'dev' }, createBangkokDate(`${dateString} 12:00`))).toBe('scaled_up');
+  expect(identifyResourceState({ env: 'dev' }, createBangkokDate(`${dateString} 20:00`))).toBe('scaled_down');
+});
+
+test('identifyResourceState for prod', () => {
+  expect(identifyResourceState({ env: 'prod' }, createBangkokDate(`${dateString} 06:00`))).toBe('scaled_up');
+  expect(identifyResourceState({ env: 'prod' }, createBangkokDate(`${dateString} 09:00`))).toBe('scaled_up');
+  expect(identifyResourceState({ env: 'prod' }, createBangkokDate(`${dateString} 12:00`))).toBe('scaled_up');
+  expect(identifyResourceState({ env: 'prod' }, createBangkokDate(`${dateString} 20:00`))).toBe('scaled_up');
+});
+```
+
+In this example, we've provided a clear introduction, added comments, and organized the content for better readability. Feel free to adapt this to suit your preferences and context!