feat: add a date formatting function for Last-Modified headers

src/functions/dates.spec.ts

@@ -0,0 +1,11 @@
+import {lastModifiedHeaderDate} from '../index'
+
+describe('Dates', () => {
+  it('should generate a last modified date',  () => {
+    const timeString = "Wed Apr 6 2022 09:38:39 GMT+0100 (British Summer Time)"
+
+    const lastModified = lastModifiedHeaderDate(timeString)
+
+    expect(lastModified).toBe('Wed, 06 Apr 2022 08:38:39 GMT')
+  })
+})

src/functions/dates.ts

@@ -0,0 +1,46 @@
+const dateOrStringFunction = <ReturnType>(fn: (date: Date) => ReturnType) => {
+  return (date: Date | String) => {
+    if (typeof date === 'string') {
+      date = new Date(date)
+    }
+
+    return fn(date as Date)
+  }
+}
+
+/**
+ * Takes a date (or string) and returns it formatted as a last modified header date.
+ *
+ * @param date A `Date` or date string to be converted.
+ * @returns The given date in the required format for `Last-Modified` headers.
+ */
+export const lastModifiedHeaderDate = dateOrStringFunction(date => {
+  date = new Date(date.toLocaleString('en-US', {timeZone: 'GMT'}))
+
+  const days = ['Sun', 'Mon', 'Tue', 'Wed', 'Thu', 'Fri', 'Sat']
+  const months = [
+    'Jan',
+    'Feb',
+    'Mar',
+    'Apr',
+    'May',
+    'Jun',
+    'Jul',
+    'Aug',
+    'Sep',
+    'Oct',
+    'Nov',
+    'Dec'
+  ]
+
+  return `${days[date.getDay()]}, ${date
+    .getDate()
+    .toString()
+    .padStart(2, '0')} ${months[date.getMonth()]} ${date.getFullYear()} ${date
+    .getHours()
+    .toString()
+    .padStart(2, '0')}:${date.getMinutes().toString().padStart(2, '0')}:${date
+    .getSeconds()
+    .toString()
+    .padStart(2, '0')} GMT`
+})

src/index.ts

@@ -26,6 +26,7 @@ export {
   numberToVh,
   numberToVw
 } from './functions/css'
+export {lastModifiedHeaderDate} from './functions/dates'
 export {deepIncludesArray} from './functions/deep-includes'
 export {defaults} from './functions/defaults'
 export {diffArray} from './functions/diff-array'