Merge pull request #259 from naulacambra/master

feat: Added method to retrieve week of the year

Author: iamkun

docs/en/Plugin.md

@@ -144,6 +144,17 @@ List of added formats:
 | `BBBB` | 2561             | Full BE Year (Year + 543)             |
 | `BB`   | 61               | 2-digit of BE Year                    |
 
+### WeekOfYear
+ - WeekOfYear adds `.week()` API to returns a `number` indicating the `Dayjs`'s week of the year.
+
+```javascript
+import weekOfYear from 'dayjs/plugin/weekOfYear'
+
+dayjs.extend(weekOfYear)
+
+dayjs('06/27/2018').week() // 26
+```
+
 ## Customize
 
 You could build your own Day.js plugin to meet different needs.

src/plugin/weekOfYear/index.js

@@ -0,0 +1,12 @@
+import { MILLISECONDS_A_DAY } from '../../constant'
+
+export default (o, c) => {
+  const proto = c.prototype
+  proto.week = function () {
+    const day = this.$W || 7 // Return sunday as 7
+    // Create date at nearest thursday
+    const ins = new Date(this.$y, this.$M, (this.$D - day) + 4)
+    const yearStart = new Date(Date.UTC(this.$y, 0, 1)) // Get first day of year
+    return Math.ceil((((ins - yearStart) / MILLISECONDS_A_DAY) + 1) / 7) // Calculate weeks
+  }
+}

test/plugin/weekOfYear.test.js

@@ -0,0 +1,18 @@
+import moment from 'moment'
+import MockDate from 'mockdate'
+import dayjs from '../../src'
+import weekOfYear from '../../src/plugin/weekOfYear'
+
+dayjs.extend(weekOfYear)
+
+beforeEach(() => {
+  MockDate.set(new Date())
+})
+
+afterEach(() => {
+  MockDate.reset()
+})
+
+it('Week of year', () => {
+  expect(dayjs().week()).toBe(moment().week())
+})